LT1 Rough Idle, Bad PCM?
09-08-2016, 04:05 PM
#1
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LT1 Rough Idle, Bad PCM?
I have a 37 Chevy that I purchased from a deceased mans widow. He hadn't finished the car, but it was 90 percent complete. It has been my project off and on for the past 2 years. It ran fine and then the fuel pump failed. I replaced it in the spring and it has not run right since. It runs real rough and stalls after about 30 seconds. The fumes are so bad it burns your eyes.
The engine is an LT1 from a 1995 Impala. However, the VIN info for the PCM is for a 1994 Z28.
I have replaced the fuel pump, injectors, fuel pressure regulator and put new fuel in. New plugs. I removed the OPTI and it was very rusty, so I replaced that, too. Nothing I've done seems to get this running right. I have checked and tested the MAP, TPS, ECT, IAT, coil and ICM, too.
The company that programmed the PCM has sold that part of their business (Street and Performance). My understanding is the MAF sensor/input has been disabled as well as VATS and EGR.
I have attached the SCAN 9495 file from this afternoon.
Is it possible the PCM is defective? I was under the impression that the PCM was either a GO/NOGO. I am getting tired of "Easter egging" this car.
Any help/comments/ideas are welcome.
Thanks.
The engine is an LT1 from a 1995 Impala. However, the VIN info for the PCM is for a 1994 Z28.
I have replaced the fuel pump, injectors, fuel pressure regulator and put new fuel in. New plugs. I removed the OPTI and it was very rusty, so I replaced that, too. Nothing I've done seems to get this running right. I have checked and tested the MAP, TPS, ECT, IAT, coil and ICM, too.
The company that programmed the PCM has sold that part of their business (Street and Performance). My understanding is the MAF sensor/input has been disabled as well as VATS and EGR.
I have attached the SCAN 9495 file from this afternoon.
Is it possible the PCM is defective? I was under the impression that the PCM was either a GO/NOGO. I am getting tired of "Easter egging" this car.
Any help/comments/ideas are welcome.
Thanks.
09-08-2016, 05:16 PM
#2
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Re: LT1 Rough Idle, Bad PCM?
When they disable the MAF sensor (MAF sensor does show "0"), the PCM defaults to calculating mass air flow using the speed-density mode. The two key inputs to speed-density at the inlet air temp (IAT) and the manifold absolute pressure (MAP). Your MAP sensor readings seem excessively high. IAT seems OK.
The other input that seems incorrect is the throttle position sensor. The volts and percent open are jumping around all over the place. In 0.12 to 0.15 second increments, the reading are 45% - 10% - 20% - 64% - 4% - 69% - 17% - 65% - 12% - 65% Were you actually pumping the accel pedal 7 times per SECOND? I doubt it.
Your coolant temp is 84* to 87*F. Doesn't seem right. That's lower than the air temp (87*F and never changes).
Right side O2 sensor seems very sluggish.
Since the MPH input is not there (confirm PCM is not driving the speedometer - DTC 97 indicates a vehicle speed sensor failure or no input), I can't tell if the car is moving, and changing speed when the throttle position changes. The RPM is unusually steady, not changing in response to changing throttle position. Were you driving the car, or was it just sitting and idling? Were you holding the throttle down?
What if any are the mods to the engine? Does it have a more aggressive than stock cam?
Have you ever actually measured the fuel pressure before or after replacing all those fuel system components?
My guess would be that there are major wiring problems with the sensors - but you indicate it ran fine before the fuel pump failed. If that's not the problem the PCM becomes suspect.
The other input that seems incorrect is the throttle position sensor. The volts and percent open are jumping around all over the place. In 0.12 to 0.15 second increments, the reading are 45% - 10% - 20% - 64% - 4% - 69% - 17% - 65% - 12% - 65% Were you actually pumping the accel pedal 7 times per SECOND? I doubt it.
Your coolant temp is 84* to 87*F. Doesn't seem right. That's lower than the air temp (87*F and never changes).
Right side O2 sensor seems very sluggish.
Since the MPH input is not there (confirm PCM is not driving the speedometer - DTC 97 indicates a vehicle speed sensor failure or no input), I can't tell if the car is moving, and changing speed when the throttle position changes. The RPM is unusually steady, not changing in response to changing throttle position. Were you driving the car, or was it just sitting and idling? Were you holding the throttle down?
What if any are the mods to the engine? Does it have a more aggressive than stock cam?
Have you ever actually measured the fuel pressure before or after replacing all those fuel system components?
My guess would be that there are major wiring problems with the sensors - but you indicate it ran fine before the fuel pump failed. If that's not the problem the PCM becomes suspect.
09-08-2016, 08:03 PM
#3
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Re: LT1 Rough Idle, Bad PCM?
Injuneer, thanks for responding.
The interesting thing is this car had NO IAT sensor. I had to purchase one and install it. I forgot to mention the car has a Ron Francis wiring kit. From everything I've read, I don't understand how the car ran without the IAT sensor. Any idea what would cause the high MAP sensor reading? The voltages checked out and changed when I applied vacuum. I will replace it if you think that might be part of the problem.
I was not pumping the throttle that fast. I had the pedal to the floor in an effort to keep the car running.
I did not install the IAT sensor into the air cleaner (yet). I connected the wires and had it laying on top of the intake. I thought that would be ok for my Troubleshootng purposes.
Regarding the coolant sensor, I found some info on what the resistance should be at various temperatures. I can remove it and apply heat and see if it the resistance changes. Even if it's "bad", if it's telling the computer its 87 degrees, which was the temp here in Virginia today, would that make the car run that rich?
As far as the O2 sensor, I read that in Open Loop mode, the PCM does not look at O2 sensor inputs. Regardless, if it's bad, I need to fix it.
The car is not moving. It is sitting in my garage. I have a feeling the VSS is not even connected. I am going t have to jack the car up and get to the transmission. Does that have to be connected for the PCM to act right?
I never measured the fuel pressure before replacing the pump and injectors, but I did after. I can't find my notes, but I believe is was 45 PSI. I do remember that it was within the specs I found on the web.
The interesting thing is this car had NO IAT sensor. I had to purchase one and install it. I forgot to mention the car has a Ron Francis wiring kit. From everything I've read, I don't understand how the car ran without the IAT sensor. Any idea what would cause the high MAP sensor reading? The voltages checked out and changed when I applied vacuum. I will replace it if you think that might be part of the problem.
I was not pumping the throttle that fast. I had the pedal to the floor in an effort to keep the car running.
I did not install the IAT sensor into the air cleaner (yet). I connected the wires and had it laying on top of the intake. I thought that would be ok for my Troubleshootng purposes.
Regarding the coolant sensor, I found some info on what the resistance should be at various temperatures. I can remove it and apply heat and see if it the resistance changes. Even if it's "bad", if it's telling the computer its 87 degrees, which was the temp here in Virginia today, would that make the car run that rich?
As far as the O2 sensor, I read that in Open Loop mode, the PCM does not look at O2 sensor inputs. Regardless, if it's bad, I need to fix it.
The car is not moving. It is sitting in my garage. I have a feeling the VSS is not even connected. I am going t have to jack the car up and get to the transmission. Does that have to be connected for the PCM to act right?
I never measured the fuel pressure before replacing the pump and injectors, but I did after. I can't find my notes, but I believe is was 45 PSI. I do remember that it was within the specs I found on the web.
09-08-2016, 08:56 PM
#4
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Re: LT1 Rough Idle, Bad PCM?
Too many sensor are not moving, IAT, ECT, MAP all stuck. The pedal to the floor might explain the MAP being near barometric pressure all the time, but probably not that high. I would test those 3 sensors while connected to the wiring. Light a match under the removed IAT sensor and see if the voltage changes appreciably. Check the pcm grounds by running a jumper cable from the pcm case to engine ground and see if anything in the scan changes.
Missing VSS wont keep the engine from running well.
Missing VSS wont keep the engine from running well.
09-08-2016, 09:21 PM
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Re: LT1 Rough Idle, Bad PCM?
It's pouring in fuel mostly because of the high MAP reading. The speed-density starts with calculating the volume air flow (as cubic feet per minute). That's the displacement of the engine X RPM/2 X volumetric efficiency (VE is read by the PCM directly from a table in the program). So far, minimal room for error.
Now the PCM has to calculate the density of the air, to determine the mass air flow. Volume air flow in CFM X density in #/cubic foot = #/minute (or other consistent unit like grams/second). At this point it has replicated the measurement the MAF sensor produces directly.
To calculate the air density, the PCM uses the prefect gas law. Gas density is directly proportional to absolute pressure, which is what the MAP sensor measure. The higher the MAP, the denser the air. The denser the air, the greater the mass flow. The PCM divides mass air flow by the target A/F ratio (typically 14.7:1, except at/near WOT), and that determines the required mass fuel flow. The PCM sets the injector flow rate to deliver that much fuel. Let's leave IAT for later.
So... at idle, with a stock cam, your MAP should be around 30-35 kPa. Your MAP is up at 90-100kPa. As a result the injectors are spraying almost 3 times the normal amount of fuel. The high MAP is due in part to the open throttle, and in part due to misfires. That's why I asked if it has something other than a stock cam. The more aggressive the cam, the higher the MAP at idle.
I didn't discuss IAT, because your reading is rational - even if it's not plugged into the air duct, it's still probably very close the the temp of the air entering the engine. Additionally, degrees-F is not an "absolute" referenced temperature. To get the absolute temperature in degrees-Rakine, you have to add 459* the the degrees-F. So a 10*F error in IAT doesn't have a great effect on density. And as long as it's plugged into the harness, the PCM recognizes the readings. In effect it isn't running without an IAT sensor.
The MAP sensor also measures barometric pressure at key on, before the engine cranks. You barometer reading looks fine, assuming you are near sea level, and not in Denver at 5,500-ft elevation.
If you have checked all the sensors, did you check them by measuring the resistance with the harness connector off the sensor? Did you also check them by measuring the voltage with the harness connected? I ran across a TPS sensor that had the correct resistance, but the voltage was not correct - 0.67 volts at closed throttle, near 5 volts at WOT. After staring at it long enough I figured out the signal and reference wires were switched when the pigtail had been spliced to the harness wires. Your TPS readings would seem to indicate that there is an intermittent circuit that is causing the voltage to jump around due to the shaking engine.
Is the harness connector for the MAP sensor "old"? Again from my experience the stock connectors dry out and crumble, screwing up the MAP readings in an intermittent manor. Does the MAP sensor have the correct rubber seal where it pushes into the manifold, and are the screws holding the sensor tight?
I only pointed out the O2 sensor so you know it isn't healthy. The PCM still reads the O2 sensor in open loop, but "learning" is not enabled, so the PCM can adjust the short term (integrators) or long term (BLM's) fuel trims to correct for the massive rich condition. The low coolant temp is preventing the system from going into closed loop. Additionally, at low coolant temps the PCM richens the fuel mixture, and elevates the idle RPM, just like the choke on a carb.
Fuel pressure should be 43.5psi with the vacuum compensation line off the pressure regulator. GM spec accepts 41-47psi.
Now the PCM has to calculate the density of the air, to determine the mass air flow. Volume air flow in CFM X density in #/cubic foot = #/minute (or other consistent unit like grams/second). At this point it has replicated the measurement the MAF sensor produces directly.
To calculate the air density, the PCM uses the prefect gas law. Gas density is directly proportional to absolute pressure, which is what the MAP sensor measure. The higher the MAP, the denser the air. The denser the air, the greater the mass flow. The PCM divides mass air flow by the target A/F ratio (typically 14.7:1, except at/near WOT), and that determines the required mass fuel flow. The PCM sets the injector flow rate to deliver that much fuel. Let's leave IAT for later.
So... at idle, with a stock cam, your MAP should be around 30-35 kPa. Your MAP is up at 90-100kPa. As a result the injectors are spraying almost 3 times the normal amount of fuel. The high MAP is due in part to the open throttle, and in part due to misfires. That's why I asked if it has something other than a stock cam. The more aggressive the cam, the higher the MAP at idle.
I didn't discuss IAT, because your reading is rational - even if it's not plugged into the air duct, it's still probably very close the the temp of the air entering the engine. Additionally, degrees-F is not an "absolute" referenced temperature. To get the absolute temperature in degrees-Rakine, you have to add 459* the the degrees-F. So a 10*F error in IAT doesn't have a great effect on density. And as long as it's plugged into the harness, the PCM recognizes the readings. In effect it isn't running without an IAT sensor.
The MAP sensor also measures barometric pressure at key on, before the engine cranks. You barometer reading looks fine, assuming you are near sea level, and not in Denver at 5,500-ft elevation.
If you have checked all the sensors, did you check them by measuring the resistance with the harness connector off the sensor? Did you also check them by measuring the voltage with the harness connected? I ran across a TPS sensor that had the correct resistance, but the voltage was not correct - 0.67 volts at closed throttle, near 5 volts at WOT. After staring at it long enough I figured out the signal and reference wires were switched when the pigtail had been spliced to the harness wires. Your TPS readings would seem to indicate that there is an intermittent circuit that is causing the voltage to jump around due to the shaking engine.
Is the harness connector for the MAP sensor "old"? Again from my experience the stock connectors dry out and crumble, screwing up the MAP readings in an intermittent manor. Does the MAP sensor have the correct rubber seal where it pushes into the manifold, and are the screws holding the sensor tight?
I only pointed out the O2 sensor so you know it isn't healthy. The PCM still reads the O2 sensor in open loop, but "learning" is not enabled, so the PCM can adjust the short term (integrators) or long term (BLM's) fuel trims to correct for the massive rich condition. The low coolant temp is preventing the system from going into closed loop. Additionally, at low coolant temps the PCM richens the fuel mixture, and elevates the idle RPM, just like the choke on a carb.
Fuel pressure should be 43.5psi with the vacuum compensation line off the pressure regulator. GM spec accepts 41-47psi.
Last edited by Injuneer; 09-08-2016 at 09:29 PM.
09-09-2016, 07:29 PM
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Re: LT1 Rough Idle, Bad PCM?
Today I used a heat gun and applied heat to the IAT sensor. The scan tool showed an increase in temperature from 85 to 169 degrees F and then I turned off the heat gun. The temperature slowly decreased back to 85, which was the ambient temp. Test performed with key on, engine not running.
Next I observed the TPS voltage with the scan tool. Closed throttle was .65 volts and 0 percent TPS position. Half throttle was 2.5 volts and 50 percent TPS position. Full throttle was 4.6 volts and 100 percent. Test performed with key on, engine not running. I heard an intermittent clicking from the TPS. It sounded like a solenoid or relay type click. Is this normal?
Now for the MAP sensor. The sensor is not mounted to the engine and I assumed this was normal, but found a picture on the web that shows where it is supposed to go. There is red RTV in the spot where the vacuum part of the sensor would plug in. I have no idea why it's like this. It is getting its vacuum from a nipple on the intake between numbers 4 and 6 injectors. The vacuum line come out of that nipple and there is a "T". The top of the "T" is connected to the MAP and the other side goes to the throttle body right under the hose for the PCV valve. I disconnected the vacuum hose from the nipple and tried to apply vacuum, but it would not hold, so I removed the "T" and applied vacuum directly to the MAP sensor. No vacuum reads 4.88 volts. 5 inches vacuum reads 3.69 volts. 10 inches vacuum reads 3.00 volts. 15 inches vacuum reads 2.14 volts and 20 inches vacuum reads 1.14 volts. Barometric pressure was a constant 28.26. All tests were run with the key on, engine not running. A few questions here. 1. Should the barometer reading have changed as the vacuum increased? 2. Should I bypass the "T" and connect the MAP directly to the intake? 3. Any idea why the "T" is connected that way? 4. Does having the MAF removed/disabled have anything to do with the way the MAP is connected on this car?
I noticed the PCM case is not grounded. It is just sitting in the trunk near the battery. Also, the main wiring harness is in the trunk. I measured resistance from the PCM case to battery ground and wiring harness ground and get 0 ohms. I measured 12 volts from the PCM case to the battery positive terminal. It appears that the PCM is getting ground from one of the wires in the connectors. Should I worry about physically grounding the case?
I did not take any voltage readings using a multimeter at the sensor connections because I thought the scanner readings would suffice. Let me know if I should still try measuring with a multimeter.
Any more thoughts or things to check?
Thanks.
Dan
Next I observed the TPS voltage with the scan tool. Closed throttle was .65 volts and 0 percent TPS position. Half throttle was 2.5 volts and 50 percent TPS position. Full throttle was 4.6 volts and 100 percent. Test performed with key on, engine not running. I heard an intermittent clicking from the TPS. It sounded like a solenoid or relay type click. Is this normal?
Now for the MAP sensor. The sensor is not mounted to the engine and I assumed this was normal, but found a picture on the web that shows where it is supposed to go. There is red RTV in the spot where the vacuum part of the sensor would plug in. I have no idea why it's like this. It is getting its vacuum from a nipple on the intake between numbers 4 and 6 injectors. The vacuum line come out of that nipple and there is a "T". The top of the "T" is connected to the MAP and the other side goes to the throttle body right under the hose for the PCV valve. I disconnected the vacuum hose from the nipple and tried to apply vacuum, but it would not hold, so I removed the "T" and applied vacuum directly to the MAP sensor. No vacuum reads 4.88 volts. 5 inches vacuum reads 3.69 volts. 10 inches vacuum reads 3.00 volts. 15 inches vacuum reads 2.14 volts and 20 inches vacuum reads 1.14 volts. Barometric pressure was a constant 28.26. All tests were run with the key on, engine not running. A few questions here. 1. Should the barometer reading have changed as the vacuum increased? 2. Should I bypass the "T" and connect the MAP directly to the intake? 3. Any idea why the "T" is connected that way? 4. Does having the MAF removed/disabled have anything to do with the way the MAP is connected on this car?
I noticed the PCM case is not grounded. It is just sitting in the trunk near the battery. Also, the main wiring harness is in the trunk. I measured resistance from the PCM case to battery ground and wiring harness ground and get 0 ohms. I measured 12 volts from the PCM case to the battery positive terminal. It appears that the PCM is getting ground from one of the wires in the connectors. Should I worry about physically grounding the case?
I did not take any voltage readings using a multimeter at the sensor connections because I thought the scanner readings would suffice. Let me know if I should still try measuring with a multimeter.
Any more thoughts or things to check?
Thanks.
Dan
09-09-2016, 09:48 PM
#7
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Re: LT1 Rough Idle, Bad PCM?
Go to this site. It has pictures and wiring diagrams for a 95 LT1. That will show you things like the MAP sensor and the required seal.
4th Gen LT1 F-Body Tech Aids
The vacuum nipples on the passenger side between #4 and #6 injector are for the fuel pressure regulator vacuum compensation line (lower nipple, but doesn't matter which), and the vacuum line for the HVAC system flapper door actuators. Logically your setup wouldn't need the HVAC line. So it would be OK to connect the vacuum line for the MAP sensor there (although the further the sensor is from the manifold, the slower the response).
What I don't understand is the tee. Are you saying that one side of the tee connects to the smaller (lower) of the two nipples on the passenger side of the throttle body? In a stock application, that is the vacuum source for the EVAP canister purge solenoid. Do you even have the EVAP canister to control fuel tank vapor venting? That should also be a full vacuum port. The nipple dumps into a groove in the rear face of the throttle body, which forms a passage with the face of the intake manifold-to-throttle body gasket. That passage ends in two fairly small openings that vent into the two air streams flowing through the throttle bores. Maybe that setup is throwing the MAP reading off, because the voltages vs "Hg readings you got are good.
http://shbox.com/1/1995_evap.jpg
When you said you couldn't pull a vacuum that held on the line you took off the nipple between the injectors, the extra line run to the throttle body would explain it. Is it also possible that one of the lines connected to the tee had a leak?
As far as barometer, the MAP sensor can only read ambient air pressure when the engine is not running and there is no vacuum. The PCM reads the Map sensor before the engine starts to crank, and sets that as barometric pressure. It will stay constant.
I would put the MAP sensor where it belongs, and cap the nipple between the injectors and the lower nipple on the throttle body.
http://shbox.com/1/map.jpg
http://shbox.com/1/map_seal.jpg
The IAT sensor is fine. The reading it gave for the data log was rational, and it was fine that it didn't change because it was sitting on the intake. I didn't really question that.
TPS readings are fine, but did you try shaking the wires to try and find the intermittent connection? You saw the huge variations in the throttle position % that I posted. It was changing every 1/6th of a second. Can't explain the "clicking" unless the PCM was moving the pintle in the idle air control motor in the throttle body base plate, right below the TPS. It's a stepper motor.
4th Gen LT1 F-Body Tech Aids
The vacuum nipples on the passenger side between #4 and #6 injector are for the fuel pressure regulator vacuum compensation line (lower nipple, but doesn't matter which), and the vacuum line for the HVAC system flapper door actuators. Logically your setup wouldn't need the HVAC line. So it would be OK to connect the vacuum line for the MAP sensor there (although the further the sensor is from the manifold, the slower the response).
What I don't understand is the tee. Are you saying that one side of the tee connects to the smaller (lower) of the two nipples on the passenger side of the throttle body? In a stock application, that is the vacuum source for the EVAP canister purge solenoid. Do you even have the EVAP canister to control fuel tank vapor venting? That should also be a full vacuum port. The nipple dumps into a groove in the rear face of the throttle body, which forms a passage with the face of the intake manifold-to-throttle body gasket. That passage ends in two fairly small openings that vent into the two air streams flowing through the throttle bores. Maybe that setup is throwing the MAP reading off, because the voltages vs "Hg readings you got are good.
http://shbox.com/1/1995_evap.jpg
When you said you couldn't pull a vacuum that held on the line you took off the nipple between the injectors, the extra line run to the throttle body would explain it. Is it also possible that one of the lines connected to the tee had a leak?
As far as barometer, the MAP sensor can only read ambient air pressure when the engine is not running and there is no vacuum. The PCM reads the Map sensor before the engine starts to crank, and sets that as barometric pressure. It will stay constant.
I would put the MAP sensor where it belongs, and cap the nipple between the injectors and the lower nipple on the throttle body.
http://shbox.com/1/map.jpg
http://shbox.com/1/map_seal.jpg
The IAT sensor is fine. The reading it gave for the data log was rational, and it was fine that it didn't change because it was sitting on the intake. I didn't really question that.
TPS readings are fine, but did you try shaking the wires to try and find the intermittent connection? You saw the huge variations in the throttle position % that I posted. It was changing every 1/6th of a second. Can't explain the "clicking" unless the PCM was moving the pintle in the idle air control motor in the throttle body base plate, right below the TPS. It's a stepper motor.
Last edited by Injuneer; 09-09-2016 at 09:51 PM.
09-09-2016, 10:01 PM
#8
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Re: LT1 Rough Idle, Bad PCM?
I noticed the PCM case is not grounded. It is just sitting in the trunk near the battery. Also, the main wiring harness is in the trunk. I measured resistance from the PCM case to battery ground and wiring harness ground and get 0 ohms. I measured 12 volts from the PCM case to the battery positive terminal. It appears that the PCM is getting ground from one of the wires in the connectors. Should I worry about physically grounding the case?
09-10-2016, 06:34 PM
#9
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Re: LT1 Rough Idle, Bad PCM?
I went to those sites and looked at the pictures and diagrams. They are a big help.
My configuration has about a 6 inch hose coming out of the lower nipple and then the tee to the MAP and then to the EVAP vacuum suppply port of the the throttle body (the lower one). I do not have the EVAP canister, so should I just plug that hose?
I will connect the MAP per the diagrams and see what happens.
Ref your question about the possibility of a vacuum leak, no, I checked the hoses and the tee and they all hold good vacuum.
The only way I could get the MAP to hold vacuum was if I disconnected/bypassed the tee/hose for the EVAP system. I'm not sure if I understood you or not, but should it have held vacuum the way my car is configured (with the tee)? Either way, like I said, I am going to get rid of the tee. I just need to confirm that I should plug the EVAP hose to throttle body.
I did not try shaking the TPS wires/connector. Are you saying the only way that the voltage would change like that is: 1) If I pumped the pedal that fast (impossible to do), or 2) The TPS or its wiring is faulty? Like I said, I held the throttle to the floor just to try to keep it running. As far as the clicking, it is a very soft click and definitely intermittent. I never noticed it before, but then I don't recall ever being under the hood with the key on. I wondered if it could have been the IAC motor, but the sound went away when I disconnected the TPS. I will check everything again, though, because it may have just decided to stop clicking at the same time I disconnected the TPS.
Thanks for all the help, and I will keep plugging along with this.
Dan
My configuration has about a 6 inch hose coming out of the lower nipple and then the tee to the MAP and then to the EVAP vacuum suppply port of the the throttle body (the lower one). I do not have the EVAP canister, so should I just plug that hose?
I will connect the MAP per the diagrams and see what happens.
Ref your question about the possibility of a vacuum leak, no, I checked the hoses and the tee and they all hold good vacuum.
The only way I could get the MAP to hold vacuum was if I disconnected/bypassed the tee/hose for the EVAP system. I'm not sure if I understood you or not, but should it have held vacuum the way my car is configured (with the tee)? Either way, like I said, I am going to get rid of the tee. I just need to confirm that I should plug the EVAP hose to throttle body.
I did not try shaking the TPS wires/connector. Are you saying the only way that the voltage would change like that is: 1) If I pumped the pedal that fast (impossible to do), or 2) The TPS or its wiring is faulty? Like I said, I held the throttle to the floor just to try to keep it running. As far as the clicking, it is a very soft click and definitely intermittent. I never noticed it before, but then I don't recall ever being under the hood with the key on. I wondered if it could have been the IAC motor, but the sound went away when I disconnected the TPS. I will check everything again, though, because it may have just decided to stop clicking at the same time I disconnected the TPS.
Thanks for all the help, and I will keep plugging along with this.
Dan
09-28-2016, 08:48 AM
#10
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Re: LT1 Rough Idle, Bad PCM?
I am still troubleshooting this car. I have not had any success to date. Since my last post I have replaced the TPS and the PCM. I have attached 2 scans to this post. The scan from 16 Sep was prior to replacing the TPS and PCM. The engine never started, just tried to fire and stumbled. I have question reading that scan. Look at position 203 and you will see the speed is 244 mph and the run time is 50832. How could that be possible. Also there are what I'm assuming are fault codes "95&97&98". I can only find info on code 97, which is VSS. What are the other 2 codes?
The 27 Sep scan is after I replaced the PCM and TPS. The engine started and ran at idle and then just seemed to "load up with fuel" and stalled. I tried to give it some throttle, but it backfired and then stalled. I looked at the scan and have a few questions. Please keep in mind that the PCM is supposed to have the MAF, EGR, VATS and AIR pump disabled.
1. I see the A.I.R. pump relay changes to "1" periodically. Is this normal if the A.I. R. is disabled?
2. I also see the fuel enable (VATS) is "1". Is this normal if VATS has been disabled?
3. The scan indicates 1 reset has occurred. Any idea how or why that happened?
I am really at a loss as to why this car won't run right? Any/all comments and ideas are welcome.
Thanks,
Dan
The 27 Sep scan is after I replaced the PCM and TPS. The engine started and ran at idle and then just seemed to "load up with fuel" and stalled. I tried to give it some throttle, but it backfired and then stalled. I looked at the scan and have a few questions. Please keep in mind that the PCM is supposed to have the MAF, EGR, VATS and AIR pump disabled.
1. I see the A.I.R. pump relay changes to "1" periodically. Is this normal if the A.I. R. is disabled?
2. I also see the fuel enable (VATS) is "1". Is this normal if VATS has been disabled?
3. The scan indicates 1 reset has occurred. Any idea how or why that happened?
I am really at a loss as to why this car won't run right? Any/all comments and ideas are welcome.
Thanks,
Dan
09-28-2016, 10:10 AM
#11
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Re: LT1 Rough Idle, Bad PCM?
Being disabled means you won't get any trouble codes for those items, not that there is no indication of the status. Actually I don't think the VATS (fuel enable) works in the app. A Tech 2 scantool doesn't even have that reading.
The AIR pump will normally turn off when you increase the throttle within the initial few minutes of operation.
The AIR pump will normally turn off when you increase the throttle within the initial few minutes of operation.
09-28-2016, 11:24 AM
#12
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Re: LT1 Rough Idle, Bad PCM?
Also there are what I'm assuming are fault codes "95&97&98". I can only find info on code 97, which is VSS. What are the other 2 codes?
DTC 97 is for the PCM output to the speedometer, not the VSS input to the PCM.
DTC 98 is for the PCM output to the tachometer output circuit. Is your tach driven by the PCM, or off your coil?
Use Shoebox's LT1 code list:
http://shbox.com/1/Dtcs.htm
I'll take a look at the data logs, but I'm a bit backed up, and going to be out of town until Tuesday of next week.
10-01-2016, 06:42 AM
#13
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Join Date: Jun 2016
Location: Gloucester, Virginia
Posts: 8
Re: LT1 Rough Idle, Bad PCM?
Injuneer,
Sorry for the delay in getting back to you. The tach was not hooked up when I got the car and it is on my list. Is it better to connect it to the PCM or the coil?
I want to get the car running before I worry about accessories.
Thanks.
Dan
Sorry for the delay in getting back to you. The tach was not hooked up when I got the car and it is on my list. Is it better to connect it to the PCM or the coil?
I want to get the car running before I worry about accessories.
Thanks.
Dan
10-01-2016, 10:05 PM
#14
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Join Date: Jul 2010
Location: Born on the Florida West Coast, now where can I retire?
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Re: LT1 Rough Idle, Bad PCM?
DTC 95 is for a fault in the Oil Life indicator ("Change Oil" Lamp), which the Impala has in the instrument panel, but is not in a Camaro or Firebird. Usually it means the bulb is burnt out; but in your case, it's just not connected. Perhaps the pcm is actually from an Impala. Scan9495 shows that info on the DTC tab.
10-03-2016, 09:17 AM
#15
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Thread Starter
Join Date: Jun 2016
Location: Gloucester, Virginia
Posts: 8
Re: LT1 Rough Idle, Bad PCM?
The old PCM was from a Camaro. The new one is from a 1995 Corvette. I got the VIN from your scan program. Since I have not noticed any difference in the way the car runs, do you recommend I put the old PCM back in?
Dan
Dan