Does this sound like a bad PCM??

[vetteman3]

I have been chasing erratic tuning issues in my '01 supercharged Vette for a while, and I'm starting to think it may be the PCM. Indications -- When the RPM is brought up to 2000 (+/- 200), the right bank goes so lean the engine stumbles. This occurs in neutral, or under light to moderate acceleration, and only occurs if the engine is hot. And the hotter it gets the worse it gets. Today after many minutes of testing while parked the coolant temp got up to 220. At this point I also noticed that during the 2000 RPM stumble,the A/C compressor would disengauge!! Above or below that RPM the A/C compresser would re-engauge.
Does this sound like a bad PCM??

[EC_Tune]

What kind of supercharger system?

[vetteman3]

Thanks for the reply. Its a non-intercooled Magnacharger set up for 7lbs of boost. It's a Lingenfelter engine with a 220/226 cam and forged internals. After the car cooled off the A/C compressor worked normally. I'm thinking this is related to the location of the PCM and as the engine heated up with the car stationary so did the PCM.

[EC_Tune]

Take a look at the STFT's at around 1600-2000 RPM. I think there may be a harmonic in the intake with the blower. I have seen this on I/C maggies as well.

The only thing you can do is bias things up in the VE table & MAF.

[vetteman3]

I have spent consierable hours twiking the fuel trims and they are pretty close. Would this harmonic only effect the right side injectors? And what about the a/c compressor disengauging at the same time as the lean-out when hot? I assume you're thinging the PCM is ok - is this correct?

[EC_Tune]

Yep. PCM is fine & doing it's job. Not sure what to do about the harmonic. I see it more pronounced on the IC'd MagnaChargers though especially on C5 Z06 Vettes. It's only in that 200 RPM band as well. Probably a combination of air movement in the intake system (harmonic) and the exhaust system. It seemed to be more pronounced on cars with free flowing exhaust systems.

[vetteman3]

Ok, but what about the a/c compressor disengauging at that RPM???
And as I said earlier, it was only when I got the car really heated up. I figured the PCM, like most solid state circuits are heat sensitive, and high heat would make an internal "short circiut" really show itself.
So, what are some of the indications of a defective PCM?
As far as exhaust, the car has stock manifolds, gutted pup-cats, x-pipe, and b&b mufflers - no headers. So, not really a 'free flow' system.

[EC_Tune]

PCM's have self diagnostics and would set a code telling you there was something wrong.

As to the A/C dropping out ??? Dunno

Your exhaust is now "different than stock" which could (maybe, maybe not) change those dynamics and make the lean bank leaner.

[Rob's 73]

EC Tune

You may have read my similar problem with IC Mag Z06. Mine happens at 1900-1950 rpms at low MAP. So should I just fatten those cells up and let Bank 1 go way rich. If I set the cruise control to stay at 1900 rpms I will see a 30% fuel trim (shorts and longs added) difference from bank 1 to bank 2. At 1950 the problem goes away. If I fatten it up I will be real fat from 1950 to 2200. The higher the IATs the worse it gets. I do have LG Street LTs and factory Ti exhaust.

Thanks

Rob

[vetteman3]

After reading some posts about O2 switching voltages it occured to me this could be a solution for my bank 2 lean out at 2000 RPM. I first examined my scan file to determine the airflow value associated with the R bank lean-out. I then pulled up the OL/CL page and first spread out the mode numbers on the 'mode vs airflow' table from 1-16 in order. Then when to the bank 2 'O2 rich/lean vs airflow' table and plus'ed up the switching voltages (made richer) for those modes cooresponding to the airflow where the lean out occurs. Guess what, that pretty well fixed it. I will occasional notice a slight lean at 2000 RPM, but nothing like before!! Tune attached, check it out.