BTC

Oh yeah, replacing the PCM would definitely be last on my list.

It's likely I'll have some follow-up questions. I live in Michigan and the car is currently in Kentucky, so it's going to be a while before I can actually do anything on the car itself.

BTC

Here is another scan. I had a nice formatted spreadsheet, but it was too large even when compressed, so I had to compress the .csv file.

I didn't have a chance to do anything suggested in prior posts, so I'm guessing nothing has changed in regards to what's reflected in this scan. The car has been sitting since Christmas, but I had a chance to drive it when I was home for Labor Day. It still seems to have a high idle on cold start, but after driving it seems to idle normally. The car seems to run good. I've put 200+ miles on it since I started driving it again last Thanksgiving.

Injuneer

Not sure what you want us to do with this data log...... you haven't changed anything, or investigated anything that was suggested. What do you think changed?

It's the largest file I have ever seem.... one hour long, 31,700+ lines of data, 1.8 MILLION data points. No wonder you had to compress the .csv file.

A cursory review shows everything I mentioned before is still the same.... sensor volts jumping around for no reason, low MAP, no attempt by the PCM to drop the IAC counts to drop the 1,000+ RPM idle down to the targeted 800 RPM idle. The idle speed is definitely being affected by the TPS volts (and hence the TPS %) jumping up and down. I can't explain it. Gary offered some thoughts on the erratic readings. It isn't going to fix itself.

BTC

That's plenty. Thanks. I wasn't expecting much, if anything, really. I wish I knew more about the info in the logs so that I could interpret the data. I can figure out what some of it means, but there is a lot of info captured. Is there anything on the web, like some sort of log tutorial, that would shed some light on what everything means? I think I've seen you mention a Scanmaster thing you put together some time ago. Perhaps that would be helpful.

I guess it seems weird to me that the idle is so much different on cold start compared to after the car has been driven for a while, and maybe some of the things mentioned earlier would explain that. I may have some time over Christmas to do some of the things that have been suggested. I need to get the car to Michigan. I did recently have the title transferred and got it registered so that it can be driven legally.

Injuneer

BTC

Thank you.

BTC

I’m gonna have some new scans to post, probably this weekend, that I’m taking while unplugging the MAP & TPS sensors. I’m not sure when I’ll get to mess with the other two sensors that Gary mentioned.

My car has been sitting since January and the battery was dead when I tried to start it a couple of days ago. After jumping it, the car was probably idling around 800 rpm on cold start. Unfortunately, I didn’t have my laptop hooked up at the time, so I don’t have a scan. I let the car run for a little while and afterwards hooked the battery to a charger and let it charge for 4-5 hours. Afterwards, cold start idle is back over 1,000 rpm.

So, with a battery that was pretty much dead, cold start idle seemed normal. After the battery was charged, cold start idle seemed high. Is that relevant and would it point towards anything I should check?

BTC

Attaching a couple of scans.

For the July 15th scan, I unplugged the TPS followed by the MAP sensor, and then reconnected them in reverse order, meaning MAP followed by TPS. The car had been running for a bit and was up to temp when I took this scan, but the idle was still high.
For the July 16th scan, I unplugged the MAP sensor followed by the TPS, then reconnected in reverse order. I believe this scan was taken after the car had been sitting overnight.

I don't how much can be gleaned from this info. If I'm looking at the data correctly, it doesn't seem that unplugging either of these sensors is impacting the voltage readings of the other.

The idle rpms did jump when the TPS was disconnected, which may be normal. Just based upon the way the car sounded, nothing really seemed to change when the MAP sensor was unplugged. I don't know if that means anything or not. To my knowledge the MAP sensor has never been changed and is the factory original sensor.

I've also attached a few photos of some wiring that got up against a header, which probably happened 2-3 years ago. I haven't traced the wires to see where they go, but they are routed on the passenger side behind the alternator & AC compressor. Just based upon some photos I've taken, it doesn't really look like the integrity of the wiring was compromised, but I guess it's hard to say. Maybe that could potentially have something to do with the voltage jitter that has been pointed out.

Injuneer

I'll take a look at the scans. Too hot to go outside today.

Look at Shoebox's harness photo and see if you can figure out what wires are in that bundle:

1995 Z28 A4 Engine Harness

The 94 factory service manual has diagrams of all the harness routings in Volume 2, Section 8A-201

Injuneer

Looked at the July 15 log. When you disconnect the TPS sensor, should read 0 volts. But it is constantly jumping around with readings of 0.00. 0.04, and 0.08 volts. Then you disconnect the MAP, no change to the TPS behavior, but now the MAP should read 0 volts, but it's showing the same range of variation as the TPS, 0.00 to 0.08 volts. But not in synch between the two. For some reason, DTC 34 for low MAP volts did not set?

I don't know what to say. I've never had anyone disconnect senors like that to see what happens, so I don't know if those small variations in voltage are "normal". But the variations with TPS disconnected also appear on the TPS when it is connected, and that is not good. As noted earlier, when the TPS volts jump to 0.59, the PCM says the throttle is 1% open, and that could be what's causing the high idle RPM.

Maybe a problem with reference voltage regulation in the PCM, maybe a bad PCM ground, maybe burned insulation in that harness. I'll ask GaryDoug to look.

The burned harness appears to be the one that goes forward to the A/C clutch, alternator and TPS. You need to verify that. I was concerned that it might have the PCM ground that attached to the block down by the starter, but that does not appear to be the case.