This is driving me nuts!

After my H/C swap I left the factory PCM in to get things running. Once things seemed OK, we swapped in a programmed PCM and it seemed to run better, but when trying to drive it the engine started to fast idle first at 2k, then 3 and then onward so I shut it off, tried a few times and it kept doing the same thing. It would start and idle fine, but as soon as you started driving it would just idle away. 0% TPS, 8-15% engine load and 3-4k RPM!!!

I swapped back to the stock PCM and never had a similar problem, actually no problems on the factory PCM other than not wanting to idle on a hot start and I'm sure it could be tuned better.

I swapped the programmed PCM back in and it seemed to run great. No sign of the gremlin until yesterday. It came and then it went. Tonight it came back again with a vengence. I turned down the highway and leaned on it hard while it was doing this and it just laid down and then backfired. I turned for home and it kept trying to idle above 3k!!

I put the factory PCM back in and went for a drive...a hard drive. It runs great. O2 readings are in the +8 at 2800rpm on cruise in 5th gear, but both banks are reading 8.25 exactly. The other PCM gave me readings of +6.24.

To me, it sounds like there may be a hardware issue with the PCM. I don't think it has anything to do with the tune as it is too extreme and random. Tuning would be a consistent difference like the 6.24 reading versus the 8.25 readings.

All of the connectors have been checked, cleaned and greased with dielectric grease. The car is dry (it's never been in rain in its life) and the air is cold, but no no water, snow etc.

Any ideas?

BTW, it's a Wait4me PCM and when this first happened Jesse said he'd send me a new one, but I elected to give it another try which looked like it was going to work until last night. I'll call him Monday, but I'm curious as to what anyone else thinks about it?

One last thing, do O2 readings mean anything under acceleration and deceleration like in traffic and around town? When I log them under constant load (2,800 @10% engine load) I get almost identical readings on the two banks. Driving in traffic on and off the gas sometimes one is way high while the other is real low. They always even out under constant load though???

 

Are the PCMs from the same year model cars?

 

I don't know. I ordered it from WFme and gave him all of my car specs. I don't know how to tell the year of the PCM. My car is an 02 SS, wont any 99-02 Fbody PCM work?

 

Not 100% sure, I do know that there is some variation between some years. But your on the right track by letting WFme take care of it, since he is the one you are dealing with. And you have proven that it is related to that PCM....

 

Hmmmm...it started doing it (albeit not as bad) on my factory PCM today. I finally pulled the IAC plug and it runs a lot better. Do IACs go bad? I've cleaned it, should I lube it with anything?

I've checked for vacuum leaks and so far I can't find anything. I emptied an entire can of starting fluid around the intake, vacuum lines an brake booster and no change in idle.

This is driving me nuts!!

 

Found the throttle body was loose. I put it on late at night and didn't tighten it as I thought it was coming back off. The next morning we kept on reassembling and I forgot about it. Could be the cuase of a vacuum leak which would help explain the problem. If so, Jesse was right all along. I need to go for a 30 mile drive right now for my wife (Walmart run) if the problem is gone I'll swap back to Jesse's PCM and see how it runs.

 

lol, I am glad you found the problem before going through the hassle of sending your pcm back...

 

That wasn't it. The throttle body wasn't torqued down, but those O-rings do seal well. It runs like crap still with the IAC plugged in. It starts fine, but driving is miserable as it hangs at 2k between shifts and when cruising it just does whatever it wants. I was going down the highway at 50mph without cruise control and without touching the gas!! Long story short, I unplugged the IAC and it runs great. Of course it won't idle right without it, but under all other conditions the car runs great without the IAC plugged in. My LTFTs are usually right around 0 and it's a blast.

What is the failure mode of the IAC? What worries me is it seems that this would more likely be an input driving the IAC and not the valve itself. It looks like the IAC is just a linear electric motor that would fail and get stuck in one position, not take on a personality

As I think back the car's idle or off the gas rpm always hung more than my other SS, but now with all the engine mods it's much worse and with the louder exhaust it is more noticeable. It's unsafe the way it is though, I wouldn't let my wife drive it if it may decide to cruise at 3krpm through town...which is 40-50mph in 4th or 5th gear.

Any thoughts on maybe a bad TPS switch too? My scanner reads 0 TP angle while this is happening so I kind of doubt it, but I'm not sure how these systems work together? I might be able to swap the IAC off of my Suburban and see what happens?

 

Looks like this was it. Again this is what Jesse told me as soon as I said the idle hung on me. I hate vacuum leaks and this one was no different. The good news is I was so paranoid about leaks the two on the rear of the intake manifold are good to go. It was just my SNAFU that I forgot to torque down the throttlebody. While it may have been frustrating, it turned out to be easy and very educational. I'm learning a lot more about the control circuits on an LS1. When I eventually get a copy of EFI Live this will help me understand tuning parameters.