MAF maxes at 400 gps at part throttle
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Joined: Mar 2007
Posts: 362
From: Bend, OR
MAF maxes at 400 gps at part throttle
I just had my car dyno tuned and the tuner managed to fix all of my issues but this new one manifested during the session.
When the throttle is barely cracked open, the MAF reports huge gps numbers up around 400 causing the injectors to spew fuel which makes the car buck horribly and spew black smoke and almost die.
It only happens when the throttle is barely cracked open, doesn't matter if the car is already rolling or not. It's bad under 25 mph and the worst from a stop. The tuner tried a different MAF and it didn't help. He also tried a different PCM and it didn't help. He checked all the grounds at the PCM harness and in the engine bay and they were all good. He checked all the wires for continuity at the PCM harness and they were all good. He couldn't figure out why it was doing this and I was there for 6 hours. He eventually had to give me a speed density tune so I could drive home.
The car throws a dtc 24, vehicle speed sensor, and he thinks that could be related to the MAF issue. We checked the wires and they looked good, I suppose I could try replacing the sensor. I haven't found a pattern for that code yet. So far it seems to show during cold starts consistently, and randomly once warm. I just drove about 230 miles without a code though.
He also suspects the intake could have something to do with it. The way my ram air is set up, the MAF is within an inch of the stock throttle body. He wants me to try a CAI setup with an elbow and see how it does. Here's what the intake looks like:
I had this same intake setup on my stock LT1 with no issues. The dtc 24 didn't show up with the old engine either.
Do you guys have any suggestions, ideas, thoughts, help?
When the throttle is barely cracked open, the MAF reports huge gps numbers up around 400 causing the injectors to spew fuel which makes the car buck horribly and spew black smoke and almost die.
It only happens when the throttle is barely cracked open, doesn't matter if the car is already rolling or not. It's bad under 25 mph and the worst from a stop. The tuner tried a different MAF and it didn't help. He also tried a different PCM and it didn't help. He checked all the grounds at the PCM harness and in the engine bay and they were all good. He checked all the wires for continuity at the PCM harness and they were all good. He couldn't figure out why it was doing this and I was there for 6 hours. He eventually had to give me a speed density tune so I could drive home.
The car throws a dtc 24, vehicle speed sensor, and he thinks that could be related to the MAF issue. We checked the wires and they looked good, I suppose I could try replacing the sensor. I haven't found a pattern for that code yet. So far it seems to show during cold starts consistently, and randomly once warm. I just drove about 230 miles without a code though.
He also suspects the intake could have something to do with it. The way my ram air is set up, the MAF is within an inch of the stock throttle body. He wants me to try a CAI setup with an elbow and see how it does. Here's what the intake looks like:
I had this same intake setup on my stock LT1 with no issues. The dtc 24 didn't show up with the old engine either.
Do you guys have any suggestions, ideas, thoughts, help?
Last edited by AdsoYo; Jul 11, 2009 at 10:03 PM.
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Posts: 71,094
From: Hell was full so they sent me to NJ
DTC 24 is independant of the engine. While VSS readings will affect certain PCM functions, like idle speed, fuel cut on coast down, etc., its not going to make the MAF sensor reading go bad.
The output of the MAF sensor is a frequency. The PCM uses a table entered by the programmer to convert the frequency signal to an air mass flow number (grams/second). Check the output of the MAF sensor using a frequency meter. Should be about 2,600-3,000Hz at idle, and will reach about 10,600Hz at 400 G/S. Just off idle, you should see values in the 3,000 - 4,500Hz range. Many inexpensive multi-meters now include a frequency counter/meter.
One possibility is the MAF signal wire being too close to a high voltage wire - coil wire or plug wire. That can induce spurious frequencies in the MAF sensor wire. The fact your MAF is close to the TB shouldn't be an issue, but to check, move it forward into the outlet of the air box, like the stock WS6 setup, and use the Fernco 3" sewer pipe connector between the MAF and the TB. Fernco is part # 1056-33, available for about $6 at hardware stores, in the plumbing department, near the plastic pipe.
I dont run an MAF, but I do use a WS6 airbox and the Fernco, and you can see where the MAF would fit:
http://www.injuneer.com/images/photo.../DCP03746a.jpg
I don't think the MAF location is a problem. It might be an issue if you have the screen removed, since the proximity of the TB may throw the air velocity distribution across the open section of the sensor off. But not enough to make it read 10X what it is supposed to read.
Are you sure the MAF sensor is facing in the correct direction?
The output of the MAF sensor is a frequency. The PCM uses a table entered by the programmer to convert the frequency signal to an air mass flow number (grams/second). Check the output of the MAF sensor using a frequency meter. Should be about 2,600-3,000Hz at idle, and will reach about 10,600Hz at 400 G/S. Just off idle, you should see values in the 3,000 - 4,500Hz range. Many inexpensive multi-meters now include a frequency counter/meter.
One possibility is the MAF signal wire being too close to a high voltage wire - coil wire or plug wire. That can induce spurious frequencies in the MAF sensor wire. The fact your MAF is close to the TB shouldn't be an issue, but to check, move it forward into the outlet of the air box, like the stock WS6 setup, and use the Fernco 3" sewer pipe connector between the MAF and the TB. Fernco is part # 1056-33, available for about $6 at hardware stores, in the plumbing department, near the plastic pipe.
I dont run an MAF, but I do use a WS6 airbox and the Fernco, and you can see where the MAF would fit:
http://www.injuneer.com/images/photo.../DCP03746a.jpg
I don't think the MAF location is a problem. It might be an issue if you have the screen removed, since the proximity of the TB may throw the air velocity distribution across the open section of the sensor off. But not enough to make it read 10X what it is supposed to read.
Are you sure the MAF sensor is facing in the correct direction?
Thread Starter
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Joined: Mar 2007
Posts: 362
From: Bend, OR
He did address the issue of the MAF wire being too close to the coil wire. It was almost touching and he ended up zip tying it well out of the way. I'm not sure if he measured the frequency or not in the wire, I'll see if my multimeter has that function.
I actually bought one of those Fernco 3" rubber tubes a while back but it was too long. There wasn't enough room to put the MAF back in because the air box was too long and I ended up putting a huge dent in the MAF screen and had to remove the screen. If the MAF's proximity to the TB turns out to be the issue, I'll cut the air box.
The MAF is pointed in the correct direction
I actually bought one of those Fernco 3" rubber tubes a while back but it was too long. There wasn't enough room to put the MAF back in because the air box was too long and I ended up putting a huge dent in the MAF screen and had to remove the screen. If the MAF's proximity to the TB turns out to be the issue, I'll cut the air box.
The MAF is pointed in the correct direction
Thread Starter
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Joined: Mar 2007
Posts: 362
From: Bend, OR
The tuner checked the frequency cells in the tune and they are ok. I also used the Fernco to move the MAF farther from the TB and the problem persists. The tuner suggested I use my old intake elbow so I'll try that tomorrow. My multimeter doesn't have a frequency option and the available options at the parts store were too pricey at this point.
Thread Starter
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Joined: Mar 2007
Posts: 362
From: Bend, OR
Installed the old CAI setup with a 1LE elbow and nothing changed. I can crack the throttle open while idling with no issues, and it idles incredibly well on its own. As soon as I start to pull the clutch out, putting the engine under some load, the MAF reading spikes and the engine dies.
Thread Starter
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Joined: Mar 2007
Posts: 362
From: Bend, OR
Yes I have.
I just talked to the tuner. He wants me to check all the grounds and wires for continuity again. I'm gonna remove the PCM and check the wires there.
I just talked to the tuner. He wants me to check all the grounds and wires for continuity again. I'm gonna remove the PCM and check the wires there.
Last edited by AdsoYo; Aug 3, 2009 at 12:06 PM.
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Joined: Mar 2007
Posts: 362
From: Bend, OR
Just checked the voltage of all the pins at the PCM. There were a lot of numbers that didn't match Shoebox's site. All readings were taken with the key turned on. Here are links to the references I'm using:
http://shbox.com/1/1995_pcm_conn_a.jpg
http://shbox.com/1/1995_pcm_conn_b.jpg
http://shbox.com/1/1995_pcm_conn_c.jpg
http://shbox.com/1/1995_pcm_conn_d.jpg
Voltage at the battery was 12.5. Most of the pins that should match the battery read 11.5, anyone know why? The only exceptions are B15 and B31, the PCM battery feeds which read 12.2.
RED:
A2 PCM ground - .04 (does this mean the ground is bad?)
A3 - A6 (Injectors 1, 4, 6, 7) - 11.5
A7 FP Relay Control - .05 (don't know what this is but it should match bat)
A9 - A11 - 11.5
A13 Tach output - .08
A19 - A22 (Injectors 2, 5, 3, 8)- 11.5
A25 Fuel enable signal - .08 (should be 2.5)
A31 VSS ground - 0
A32 VSS signal - 0 I replaced my VSS sensor because I was getting a SES light with dtc 24 and replacing the sensor didn't fix those issues.
BLACK:
B2 Low resolution signal - 0
B3 Distributor reference low signal - 0
B5 Ignition control - 0
B6 Sensor ground - 0
B8 VSS output - 9.04 (should be 7.5)
B14 Distributor ignition feed - 0 (should match battery)
B15 PCM battery feed - 12.2
B16 Sensor ground - 0
B19 MAF sensor signal - .01 (should be 4-5, should I just get a new wire harness at this point?)
B20 High resolution signal - 0 (should be ~2.6)
B28/B29 5 volt reference - 0 (both should be 5)
B30 PCM ignition feed - 11.5
B31 PCM battery feed - 12.2
GRAY/CLEAR:
C1/C2 IAC coil A high/low - 0
C5/C6 IAC coil B low/high - 0
C7 Right O2 low - 0
C8 Right O2 high - 0 (should be 0.1 - 0.9)
C14 Brake switch signal - 11.5
C15 PNP signal - 0
C19 Left O2 low - 0
C20 Left O2 high - 0.2
C21 IAT sensor signal - 0
C22 TPS signal - 0
C23 MAP signal - 0
C25 ECT signal - 0
C32 PCM ground - .04
BLUE:
D1 PCM ground - 0
D3 PCM ignition feed - 11.5
D20 Output/field service enable - 0 (should be 5, what is this?)
D22 KS signal - 0 (should be 1.5)
D30 Serial data - 4.5
Please Help!!! What does all this mean!?
http://shbox.com/1/1995_pcm_conn_a.jpg
http://shbox.com/1/1995_pcm_conn_b.jpg
http://shbox.com/1/1995_pcm_conn_c.jpg
http://shbox.com/1/1995_pcm_conn_d.jpg
Voltage at the battery was 12.5. Most of the pins that should match the battery read 11.5, anyone know why? The only exceptions are B15 and B31, the PCM battery feeds which read 12.2.
RED:
A2 PCM ground - .04 (does this mean the ground is bad?)
A3 - A6 (Injectors 1, 4, 6, 7) - 11.5
A7 FP Relay Control - .05 (don't know what this is but it should match bat)
A9 - A11 - 11.5
A13 Tach output - .08
A19 - A22 (Injectors 2, 5, 3, 8)- 11.5
A25 Fuel enable signal - .08 (should be 2.5)
A31 VSS ground - 0
A32 VSS signal - 0 I replaced my VSS sensor because I was getting a SES light with dtc 24 and replacing the sensor didn't fix those issues.
BLACK:
B2 Low resolution signal - 0
B3 Distributor reference low signal - 0
B5 Ignition control - 0
B6 Sensor ground - 0
B8 VSS output - 9.04 (should be 7.5)
B14 Distributor ignition feed - 0 (should match battery)
B15 PCM battery feed - 12.2
B16 Sensor ground - 0
B19 MAF sensor signal - .01 (should be 4-5, should I just get a new wire harness at this point?)
B20 High resolution signal - 0 (should be ~2.6)
B28/B29 5 volt reference - 0 (both should be 5)
B30 PCM ignition feed - 11.5
B31 PCM battery feed - 12.2
GRAY/CLEAR:
C1/C2 IAC coil A high/low - 0
C5/C6 IAC coil B low/high - 0
C7 Right O2 low - 0
C8 Right O2 high - 0 (should be 0.1 - 0.9)
C14 Brake switch signal - 11.5
C15 PNP signal - 0
C19 Left O2 low - 0
C20 Left O2 high - 0.2
C21 IAT sensor signal - 0
C22 TPS signal - 0
C23 MAP signal - 0
C25 ECT signal - 0
C32 PCM ground - .04
BLUE:
D1 PCM ground - 0
D3 PCM ignition feed - 11.5
D20 Output/field service enable - 0 (should be 5, what is this?)
D22 KS signal - 0 (should be 1.5)
D30 Serial data - 4.5
Please Help!!! What does all this mean!?
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Joined: Feb 2004
Posts: 1,154
From: VA
My guess is that all the 11.5 V outputs are being fed by the 11.5 V "ignition feed." Don't know how normal an 0.7 V drop is on that, but I don't think you need to worry about that right now.
What I would worry about is all the things that are 0 V that aren't supposed to be. Especially, for instance, 5V reference...
I wonder if you are doing something wrong in your measuring, because for instance you measured basically 0 V at fuel enable, yet your car runs, so that can't be right.
What are you measuring relative to?
What I would worry about is all the things that are 0 V that aren't supposed to be. Especially, for instance, 5V reference...
I wonder if you are doing something wrong in your measuring, because for instance you measured basically 0 V at fuel enable, yet your car runs, so that can't be right.
What are you measuring relative to?
Thread Starter
Registered User
Joined: Mar 2007
Posts: 362
From: Bend, OR
I used a wooden clothes pin to hold the black lead of my multimeter to a ground on the chassis. I set the multimeter to "20" under "V" so the screen read "0.00" then stuck a paper clip in each pin of the PCM harness and touched the red lead of my multimeter to the paper clip.
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Posts: 71,094
From: Hell was full so they sent me to NJ
You've got to read the notes that are included in the chart. Appears you ignored them.
If you look at the chart, there is an "*" next to A1 voltage. The note indicates this means "Less than .5 volt", so 0.04V isn't a problem.
If you look at the chart, there is "(2)" next to A7. The note indicates "Battery voltage for first two seconds". "0" is the normal reading. The PCM only runs the fuel pump for 2 seconds when the key is turned to on and the engine doesn't start.
The PCM system voltage reading is always lower than the battery reading. That's normal. And neither of those measurements will match the dash gauge.
If you look at the chart, there is an "*" next to A1 voltage. The note indicates this means "Less than .5 volt", so 0.04V isn't a problem.
If you look at the chart, there is "(2)" next to A7. The note indicates "Battery voltage for first two seconds". "0" is the normal reading. The PCM only runs the fuel pump for 2 seconds when the key is turned to on and the engine doesn't start.
The PCM system voltage reading is always lower than the battery reading. That's normal. And neither of those measurements will match the dash gauge.