BTC

Some background on my car, I'm the original owner and the car was put into storage in October of 2000. The car wasn't prepped for storage. I got it out of storage in late 2006/early 2007 and started working on getting it running again.

There is more background info here, but it hasn't been updated in a long time.

Over Thanksgiving, I drove the car a little bit for the first time in 17+ years, and I drove it a little bit more over the Christmas/New Year holiday, which is when I captured these scans.

I'm just curious if anything looks "abnormal". With the exception of headers, exhaust, springs, shocks & a CAI, the car is stock. AIR & EGR have been deleted. The only sensor I'm certain hasn't been replaced over the last ten or so years, at least not by me, with a new OEM sensor is the MAP sensor.

The car seems to idle high. The IAC and TPS are new. One thing we did do, which in hindsight was a mistake I'm sure, was to back off the set screw on the throttle body, closing it down, hoping that would lower the idle RPMs. I suspect at some point I'll have to try resetting that back to wherever it was from the factory.

The car is running the factory tune.

The car seems to run very good when driving it. However, the last time I drove it, which was just a couple of days ago, it seemed like it might be missing a bit under load in the 2000-2500 rpm range. I don't have anything to base that on other than the way it felt and the way it sounded. When accelerating from a stop, and under load at higher RPMs, I did not get that impression. Considering I've driven the car very little in the last 17 years, and that it now has headers & exhaust, it may be normal/nothing.

Scans, which were captured with Scan9495, are attached below. I have them in Excel format if it matters. The software won't seem to play back the logs if they are not in .cvs format.

BTC

This is the first scan and is the car idling for about 13 minutes from a cold start.

BTC

This scan started shortly after the one above and is approximately one hour. I turned the car off and restarted it just to see if the idle RPMs would drop, which they usually seem to do after the car has run for a while and is then restarted. At some point during this scan, I actually drove the car. The scan ends when I pull into a gas station to get some gas. It looks like I stopped the scan before actually turning the car off.

I had to compress this file as it was larger than 5mb. I hope that doesn't present a problem.

BTC

This scan was captured as I'm leaving the gas station and runs for about 14 minutes.

If there is any helpful info I might be able to provide, let me know.

Thanks.

Injuneer

I'll try and take a look over the next few days.

The problems that concern you are high idle (which sometimes corrects on restart), and the general roughness/possible misfire ar 2000-2500 RPM under moderate load?

BTC

Correct, and really just anything that might point to something worth investigating/fixing.

I'm not sure that the engine is missing/misfiring and I don't really recall thinking it might have been when I drove the car over Thanksgiving. That was a Christmas observation. Engine wise, I didn't do anything over Christmas other than change the oil/filter and check the header & exhaust bolts for tightness. Maybe I loosened a plug wire or something. I'm not sure whether the scans would even identify something like that, but it would be awesome if they do.

My headers are ceramic coated and I'm not entirely sure when I put the current O2 sensors in the car. I'm pretty sure it would have been post header install and after having let the car run quite a bit because I had heard that brand new ceramic coatings could potentially foul the sensors. I'm pretty sure I have another new set if something indicates there might be issue with the current set.

Other than the perceived potential miss/misfire, the car seems to run pretty darned good.

It will be months before I have a chance to mess with the car again, so there isn't any rush to get info. I do very much appreciate it.

Injuneer

In the first (idle) log, it's idling at 1,400 - 1,600+ RPM, IAC is open a bit at 86 counts and the PCM doesn't seem to be responding by closing down the IAC. Appears you blipped the throttle at least once to try and bring it down.

The throttle is closed and voltage is in the range of 0.53 - 0.59 volts. That's a bit lower than the typical unmolested value of 0.60 - 0.67 volts. So is doesn't appear the throttle stop screw is too far off, maybe in just a bit. The PCM will accept anything in the range of ~0.30 - ~0.85 volts as acceptable for closed throttle.

Here's where things get messy. The TPS volts should be rock steady, but they are jumping around over that narrow range. At the low end, the PCM sets 0.53 volts as closed throttle, calling it 0%. But with the TPS volts jumping around, as the volts increase, the PCM raises the throttle percent to 1% or more. Once the PCM sees greater than 0% throttle, the idle RPM/IAC strategy changes. Could be the reason the idle is not under control.

Then there's the coolant temp sensor (CLT). That is also jumping up and down a few degrees. It actually caused the PCM to enter and then drop out of closed loop, because the range of 138 - 142degF was flip flopping on either side of the minimum temp that allows the PCM to enter closed loop. Again, the coolant temp usually makes a very steady rate of increase, with no jumping around. Another possible problem.

Looking at the PCM wiring diagram, the TPS and the CLT sensors share a common ground, supplied by the PCM on pin B6. Maybe there's a bit of corrosion on the terminal, maybe at some point theblack sensor ground wire is bare and shorting to chassis ground, which might cause the small, but problematic variations in voltage. Or something in one sensor is affecting the reading of the other. Not positive the above would cause a high idle, but I would definitely recommend trying to figure out why the two sensor readings are jumping around more than they should.

Right now, that's about all I see. The MAP sensor reading at idle is a bit low, indicating an abnormally high manifold vacuum. Again, might be a wiring problem. Or the sensor calibration is off. But that one should not be causing the high idle.

I tried to see if there was a direct correlation between the TPS volts and the CLT temp swings, and it's sort of there, but not a direct 1:1 relationship. This gets complicated by the fact that all the data on a single line is not take at the same instant. The PCM is cycling through the data one step at a time, so there is a time lag between any two points.

And that just made me go back and look at the IAT.... again, jumping around. And that in turn shares a PCM provided ground from pin B16 with the MAP sensor. Now maybe it's a sign of faulty voltage regulation in the PCM. Or maybe the whole PCM connector is screwed up. Let's see if I can get our resident electrical engineer (GaryDoug), with significantly more knowledge of the internal electronics than I have (I'm only a mechanical engineer), to read through what I wrote. Maybe he can add to it, or maybe correct it.

BTC

Thank you. When you refer to pin numbers, those are on, or part of, the PCM, correct?

Also, I want to say that we backed the set screw off to the point it's not even touching the throttle body blades, but it's been a couple years since we did that, so I'm not certain.

GaryDoug

It looks like anything using the 5v supply internal to the pcm has a lot of jitter. Could be that a sensor is failing or a 5v wire is shorting and causing the 5v to be unstable or could be a bad pcm. I would unplug the sensors one at a time and watch the 5v level. Or do a scan while you unplug sensors and see if anything causes it to clear up. The sensors to try are the AC pressure, MAP, TPS, and AC evaporator temp (very difficult to access). I am not sure what the effects of unplugging some of those sensors would be.

Other sensors make use of the 5v but that 5v does not go directly to them so I would doubt they would have any effect. Those sensors are the ECT and IAT.

Injuneer

The PCM pinouts are the same on a 95, and Shoebox has those on his site:

4th Gen LT1 F-body Tech Articles

4th Gen LT1 F-body Tech Articles


The connectors are labeled A, B, C, D, and are also color coded. The PIN numbers are shown on Shoebox's diagram of each connector, and in the table that opens when you click on the diagram of the connector.


If you want to download a 1994 factory service manual, GaryDoug has that covered:

https://www.mediafire.com/?40mfgeoe4ctti

BTC

I really appreciate the input.

This sounds potentially much more involved than I was anticipating and may be above my pay grade/skill level. I'm not super handy dandy when it comes to diagnosing/troubleshooting electrical issues, but I did build a multi-meter when I was in high school. I guess there is no time like the present to learn. Scanning and unplugging sensors sounds simple enough though.

If it did turn out that the PCM were bad, can those be replaced anymore? I'm not finding the PCM listed on GM Parts Direct. Are they repairable? Perhaps a salvage yard would be the only option if it were necessary to replace.

BTC

I eventually did find the PCM listed on GM Parts Direct, as well as Rock Auto. I apparently just overlooked it when surfing the GM site earlier. I guess I just needed to look to the right and down at the bottom of the Powertrain Control page on the GM site. I noticed pretty much every other part listed down there, but missed the control module. Getting older is a *****.

It seems everything available is remanufactured, which isn't surprising. Any idea how one would go about getting your own PCM rebuilt, assuming that were to ever become necessary? This may be a stupid sentiment, but I guess if I were to eventually need a PCM, I would rather repair the one that came on my car from the factory.

terminal_crazy

Hi
I’d try unplugging all the sensor plugs a couple of times and spray some electrical contact cleaner in them.

HTH
Mitch

BTC

I would like to dip the car, along with myself, into some restoration sauce.

GaryDoug

The pcm is probably the least likely item to fail in all of this. I would try everything else before resorting to replacing the pcm, which would have to be programmed to your VIN information (usually included in the price).