I am attempting to resurrect my 94 Z-28 M6 Camaro that has been stored outside for 10+ years. I parked it in 2010 with a worn out clutch. Engine was still running fine, had good oil pressure, no major problems.

I decided this year I would try to put it back on the road. The fuel system was in terrible condition. I cut a trap door, drained the tank, replaced the fuel pump, filter, FPR, removed and flushed the injectors. That got it cranked. I flushed the cooling system, and have changed the oil/filter twice. Oil pressure is good, there is a very slight random miss probably due to fouled plugs/old wires. I will replace those as soon as possible.

Now, my main issue is since I got the car running again my idle is off the charts high. 1500 to 2000 rpm. It just WILL NOT COME DOWN. I know my PCM was never tuned for the Hotcam, but it ran and idled perfectly fine for 10 years like that. I drove this car a LOT of miles in the past with this current configuration with no issues.

I have read posts on forums, relentlessly hunted vacuum leaks, checked throttle blade settings and linkages, cleaned the connectors on the IAC, TPS, and MAF with electrical contact cleaner, removed and cleaned the throttle body passages, replaced the IAC, and TPS, done the IAC 're-learn' steps, removed and cleaned the stock MAF, new air filter.

The IAC counts are around 100-105. It showed it idling at 1525 rpm at the time. My TPS sensor % is fluctuating between 0% & 4% without my foot on the pedal. My TPS voltage is within acceptable range ~.55 with my throttle stop adjusted to minimum throttle blade opening (screw barely touching stop then 1/4 turn more) with no improvement in the idle.

I have considered an internal intake leak. I really don't want to pull the intake to check until it is a last resort. When I did it for the Hotcam install years ago I used Fel-Pro and Permatex products and made sure I did a good job. It has no oil leaks.

The injectors seem to be operating at least ok. The intent is to replace them of course. I wanted to get it back on the road and run some volume through the fuel system and run the gunk out of the lines before considering new injectors. Could injector idle fueling issues be driving the idle up high like this?

I am preparing to get a log run of 'cold crank through warmed up' without touching the gas pedal. I will post it here when I get it done. (See Injuneer's post below for log) If anyone could take a look at it, I would sure appreciate it.
Can anyone offer any advice meanwhile as to the path to take to troubleshoot/resolve this high idle issue?

 

I have been learning more about the tuning side, but will likely be somewhat different than what you are working with because I have swapped to a Holley ECU.

That said, your IAC counts seem high from what my ECU expects. On warm idle, my IAC counts are between 8-16. The other thing that is crucial is the TPS. The ECU needs to see 0% for the idle to function correctly.

I would start with the TPS. Verify the voltage is constant when there is no pressure on the pedal. If you see variation, consider replacing the sensor. If the sensor is constant, check the wiring for potential issues.

For the IAC, what throttle body are you running? To lower the counts, you need more air into the intake. If you make adjustments to the throttle blades to allow more air in, you’ll need to reset the TPS back to 0%.

 

With the STOCK PCM, you want closed throttle idle IAC counts in the range of 20-40.

With the STOCK PCM, you can change the throttle stop screw and you do NOT have to do any “reset”, as long as the TPS volts remain in the range of 0.30-0.90 volts (limits vary slightly by year). In fact there is no external way to “reset” to “0%”. The PCM looks at the TPS volts as the key is turned to “START” and sets that value as 0%. It then adds about 4 volts to arrive at a WOT (100%) value, and prorates TP% between those values.

I've been helping him via PM, but suggested he start a thread so others could contribute. I've also directed him to my STOCK PCM scanning guide:

https://www.camaroz28.com/forums/com...-guide-886891/

 

I should have been more specific about the reset as I didn’t give any detail. I was talking about rotating the TPS while measuring voltage on the signal (blue if memory serves) wire.

If the TPS is bouncing between 0-4% while while the car is trying to idle, I can see that causing an issue.

Thanks for the extra detail Fred, though now I have another question: how would the ECU assign TPS if the throttle was held open while starting?

 

If he has a stock TB and TPS sensor, there are no slots in the TPS to allow rotation. No adjustment. The PCM process takes care of that.

Also noticed in your other post your ECU seems to report IAC position as a %,. The stock PCM operates on “counts”. The stock IAC valve is a “stepper motor” The PCM tells is how many steps (counts) to open. Max is 160 counts. Some aftermarket ECU's use a “pulse width modulated” (PWM) IAC control, where the valve is opened and closed rapidly, varying the % of the time the valve is open, vs. the % of the time it is closed. Same method the stock PCM controls the EGR and EVAP purge solenoids.

My MoTeC ECU is an older model that can only operate a PWM IAC. I wanted to keep the factory stepper motor IAC on my Holley 58mm TB, so my idle is still controlled by the stock PCM. But the TPS does need to be set in the MoTeC ECU, Both closed and WOT volts get entered by moving the blades against the stop and hitting a button on the screen to “set” closed, then moving blades to full open and hitting the “set” open volts.

 

Downloaded the two files you sent me. Problem is there are no labels on the columns. I can guess what each column is from the rough numbers, but that's not going to help much. It appears that the PCM is operating in Cell 3, and that is not the "idle" cell. Should be in Cell 16. So something is "off" but that is just a guess.

I don't see numbers I would expect that might represent IAC counts, TPS volts and TPS %. Unfortunately, it's 20+ years since I used pcmcomm, and I don't remember anything about how to set up the log, and what data it actually is capable of producing. If you can't figure out how to show the labels on the columns, may be best to try using Scan9495. Free software download, and the author - GaryDoug - is always ready to help out if needed with the install and setup.

https://www.camaroz28.com/forums/com...95-lt1-874306/

 

The PCM will not accept the closed throttle reading if it is not between ~0.30 - ~0.90 volts.

 

Hi, thanks for taking a look and weighing in Injuneer.

I 'think'
RPM is column C
TPS % is column J
TPS voltage is column W
IAC counts is column X
Column 'T' values are 16 - 18 the whole run- could that be the operational cell?
​​​​
I wondered about the labels for that data in the log. I cannot be sure what is what myself or I would label it. So the newer scan programs automatically provide the labels for the data?

I will order a USB ALDL cable so I can download and use the Scan9495. My old cable goes with the pcmcomm on a Win 98 laptop and is the serial port style cable.

It's been so long since I did anything with this car that I don't even know for sure what tune was last loaded on it.

I'm considering dumping it and reloading my original stock file that I pulled from the PCM years ago and saved.

 

Reviewed the Scan9495 file. Every sensor that relies on a good ground FROM THE PCM, AND a 5 volt reference voltage FROM THE PCM has readings that are jumping up and down for no reason at all - CLT, IAT, TPS, AC Temp, AC Pressure, MAP (although MAP tends to jump around anyway). Appears you have a problem with either the grounds for the PCM, the grounds internal to the PCM, or the internal regulation of the reference voltage by the PCM.

The high RPM and the high IAC counts are the direct result of the TPS volts moving up and down, producing a TPS% jumping from 0% to 5%. When you open the throttle, the PCM reacts by raising the IAC counts to keep the engine from stalling if the blades suddenly close. Initial thought is there is mechanical movement of the blades or the TPS sensor, but once I saw so many sensor readings jumping around, it has to be an electrical problem inside the PCM, or passing through the PCM.

I will ask GaryDoug (electrical engineer) to take a look at this, since he knows the internal electrical workings of the PCM way better than I (mechanical engineer) do - LOL

 

Hi Injuneer, Thanks for taking a look at my logfile. I will take your observations and look at the logfile to learn to identify them. I did notice the very thick jagged traces on the chart views, but not being familiar with the logs I didn't think much of it and considered maybe it was normal for certain sensors.

Based on what we have learned, I think I will inspect and deoxit the PCM plugs and connections.

I also discovered yesterday that my alternator does not seem to be charging properly. I wonder if bad rectifiers could be letting some AC hash through to the pcm that could upset things? I would imagine it would be well-filtered and protected though. I have been just putting it on a trickle charger between sessions to keep the battery topped off. I do remove the battery charger any time I attempt to scan it crank it.

​​​​​​​I suppose I could take my fluke multimeter and see if I can identify any AC on the charging system.

As luck would have it, it is raining here as soon as the work week is over and I would have time to look at it. I should receive a new radiator for it tomorrow so I will be trying to install it this weekend. Hopefully Gary can chime in and possibly give some troubleshooting ideas.

 

The "ignition volts" as seen by the PCM is jumping around rapidly from 11.3 volts to 12.0 volts. Some sort of electrical problem. Again, I'll ask GaryDoug to look.

For some reason, your posts are going in the moderation queue" and I have to "approve" them before they become visible to anyone other than the moderators.

 

Just in case anyone else wants to look at this file (is there anyone out there?), I'll upload the file. I've modified it by hiding columns, relabeling the columns, etc., but all the original data is still there.

 

Thanks for uploading the logfile to the forum. Not much progress on car this weekend due to weather.

Drove around town with intentions of buying a new alternator. Every single place would have to order it. I may order a rebuild kit and fix the original instead of put an unknown remanufactured one on it.

The plan is to replace the radiator where I can run it more than 5 mins at a time, and get the charging system working again, then loop back to this idle problem.

 

Use your multimeter to measure the voltage at the under-hood terminals on the passenger side fender well. If it is a steady 12vdc, then the problem is with the PCM, ign switch, or wiring. How does it look on the voltmeter in the gauge cluster? Then check the voltage at the battery. If it is wavering about the same as the PCM readings, investigate the alternator. If the alternator is working ok, the readings should be at least 13.5 volts.

 

I just went out and cranked it and probed with the multimeter. I'm not seeing a variation in voltage at the under hood terminals. I used clip leads and once they got a good bite it seemed pretty solid. BUT, the alternator isn't putting out anything. All I was seeing was battery voltage-. 11.9V - 12.1V. I shut the car off and the voltage was the same.

Gauge in the dash is not showing charging. That was what led me to look at alternator after I saw battery voltage levels in the scanlog. I have put a new battery in it last week and have just been leaving it on a trickle charger when I am not messing with it. That's why I hadn't noticed that it wasn't charging. I can't drive the car because no insurance on it and it has a slipping clutch. Don't want to invest in those until I'm sure the other bugs are worked out and the engine is going to be ok.

I have checked to make sure all my fuses under the dash and under the hood were good (actually replaced them all with new). I'm going to go back out and pull the alternator connector plug and clean those terminals.

Is there a test I can do to check the alternator sensing wire to see if it has the proper voltage?

Anything else I should check before tracking down either a new alternator or an alternator rebuild kit?



​​​​​​​