I will try to keep this concise and to the relevant information because this has me a little stumped. I have an M6 99 Z28 I picked up August of 2011. I brought it to Futral Motorsports and had them delete the EGR, AIR, CAGS, VATS and the rear O2 sensors shortly after buying it. Periodically it sets a P1415 code for the bank 1 air system but if I clear it it takes months to come back and I can get an inspection done.

Last year in September the T56 finally started popping out of gear (bought it with rough 1-2 syncros) and was going to get it rebuilt. I happened across a great deal on a low mileage 4L80 and decided to go that route instead. I brought the PCM to Futral and they had HP Tuners install a 4L80 segment in the PCM. Now, because of some financial and health issues, I got the car finished two weeks ago so it has been sitting since September of 2012, 6 months give or take. The car runs and drives pretty good but I get 3 recurring codes.

P0506 - Idle speed lower than requested
P0507 - Idle speed higher than requested
P0803 - Skip shift (1-4) circuit

The CAGS connector is unplugged but since I had the system turned off, I didn't think I'd have to worry about it, but I can guess that the 4L80 segment had nothing to do with eliminating this part of the M6 programming.

The car idles at 450 in neutral/park and 650-800 in drive and does not idle up when the AC is turned on now. Also periodically it will idle at 1000rpm in park when I blip the throttle and see if it will settle higher than 500.


The only engine modifications are a lid, BBK longtubes and 2.5" dual exhaust. The headers and exhaust were installed with the transmission so the car has NOT been tuned for them yet, but I don't want to try to get it tuned with DTCs present.

I also kinked/mangled the C2-2 TCC wire pretty good when trying to insert it into the PCM connector (stupidity about not realizing there was a rubber insert blocking the space).

The PCV system busted last year and I ran new hoses with a new PCV valve across the engine.

I have cleaned the IAC and throttle body trying to see if that would help. The IAC worked fine when the car was parked 6 months ago, but I thought maybe someone would have some insight into what I'm looking at here. If it was just a PCV air leak, I'd think the low idle wouldn't be a problem and that doesn't explain the lack of idling to match the AC load. Is there something I'm missing here?

 

I tried 2 IAC motors in addition to the one that was in the car and have still not changed the situation. The last IAC I installed is a new one. In all three cases I have the same idle issue. A new development is a P0155 oxygen sensor code and the car's mileage has been plummeting. The first tank after the swap it was a poor 18.3mpg. The second tank it was 16.7mpg. Today I topped off after 55 miles just to see where it was and I am at a dismal 14.7 mpg. Since the headers are new, I expect them to fry an O2, which is why I put the old ones back in. I am going to try and change them out for the other two that were left after the CAT delete and see if that changes anything.

If anyone has any other feedback or ideas, I'd be welcome for additional suggestions. I need to get this thing into decent running shape so I can get it tuned.

 

Quick update. The car has not really settled in for an idle yet, but now when I turn on the AC it begins to accelerate on it's own. I can turn the AC on at 35 and within a half mile I am already nearing 50. I hooked up an advanced scan tool and watched my live data and I'm getting some strange information:

Z28 in gear: MAF 1.4lbs/min in park:MAF 0.64lbs/min

So for comparison, I plugged it into my Mom's 00 GMC Sierra, My dad's 07 NNBS and my 05 VHO

GMC in gear: MAF 0.9lbs/min in park: MAF 0.91lbs/min
VHO in gear: MAF 0.95/lbs/min in park: MAF 0.90lbs/min
NNBS in gear: MAF 0.99lbs/min in park: MAF 0.97lbs/min

So testing 2 5.3L motors, one DBW one cable, and one DBW LQ9, I found that my MAF numbers are very different. My car is reading too low in neutral and too high in gear. With 3 samples that run and drive fine all within a few hundreths of each other, I am pretty confident in those values. I tried cleaning the MAF but that didn't change anything. I am debating another MAF but I don't want to throw a $100 part at it. I'm not getting a code for anything but idle, so the MAF isn't far enough out of range for the PCM to see a problem.

 

Has the car been tuned after the swap? Can you post the tune and a log?

 

The car has had PCM tuning done, but only for the 4L80 segment swap and the above mentioned alterations. I had those done at Futral over a year ago and they only deleted CAGS, Air, EGR, VATS and my secondary O2s. No alterations were done to the engine tune and it was running fine for a year until the 4L80 swap. They had HP Tuners swap the 4L80 segment into the M6 PCM for them but the engine is stock so they were supposed to leave the base tune alone. I added headers and dual exhaust to it and it has not been tuned for them yet, but I can't get it to idle properly and stop throwing codes to get them to tune it for the exhaust work. I don't have any logs for it or any copies of the tune from them.

The AC acceleration is a new development so that may be related to the new IAC not being relearned yet. I am going to pull the negative this morning and come back 30 mins later and take it for an hour drive through town and see if that portion straightens itself out.

 

It may need to be tuned for the mods. Your converter may weigh less than your old clutch setup and causing it to need idle air adjustment. Without seeing a log or tune its hard to give a good direction without knowing what you have.

 

I'll have to contact Futral tomorrow and see if they will try and tune it with the low and high idle codes. The converter is a stock 4L80 converter but it feels like it weighs twice what the clutch did. The handheld scanner gives me a load index parameter that shows that when I put the car in gear, my load supposedly doubles. On all three trucks, when you put the shifter in gear the load parameter moves 10% or less. That is reflected in the MAF volume readings.

I thought maybe the longtubes and true duals may be interfering, but my VHO has LTs and true duals and without a tune, it's never done this. Thank you for your input though, that gives me a new direction to start in and makes more sense than replacing sensors that are not reading faulty right now. If they will give me a copy of the log and information, I'll see about posting it for your review. Thank you for the help.

 

you might want to try just flashing the PCM to an automatic cal and start from there.

 

Wouldn't that have been handled when Futral and HP Tuners installed the 4L80 control code into the M6 PCM tune? The PCM controls the 4L80 automatically like it should. This mysterious idle issue is the only hiccup.

 

Yes they flashed your PCM to a A4 tune now.

That heavy converter may be the issue as the engine is working harder at idle. The stock 80e converter weighs a freaking ton.

 

Yeah. I swapped back to the stock IAC, pulled the negative and reset everything. After a drive I have eliminated my mystery acceleration but my MAF numbers didn't change. The car idles at 480-520 hot in neutral and 650-800 hot in drive or reverse. I have an Autozone filter in the tranny so I'm going to change it for a Wix and see if that helps some. Since the transmission sat for 6 months after I got it, the filter may have caught some debris and/or its just a badly designed filter. I've heard of some issues with the transmission being resolved by putting a Six or Delco filter in. I am still contacting Futral as well, but since my sticker isn't due till August, I'm more concerned with the possible mechanical failures than eliminating the idle and CAGS codes.

 

Nothing in that Trans filter will cause your idle issues. I would contact them to see their advise.

 

10-4 I need them to remove my CAGS code and remove my AIR code again anyway so its not as if I wasn't going to be giving them a call anyway. Hopefully there's something in the tune they can sort out. Thank you for the help, it is greatly appreciated.

 

Just doing a 4L80E "segment swap" is not the right way to do that. You simply end up where you are now.

 

Futral told me they had HP Tuners themselves do the alterations to run the 4L80. I will ask them about that tomorrow but I would assume that if they did have HP Tuners program my PCM for the 4L80 and it's wrong, then one of the biggest LS tuner support companies has it wrong. I'll have to find out more information on how they programmed the PCM but I have just restated what they did as it was described to me when I asked if they could handle the reprogramming.

Could you detail what it is that I must be missing from the PCM's tune that would cause this issue? Every aspect of the changeover seems to be working fine except the variation in idle speed between neutral and in gear.

 

No, I won't detail how I do it.
I will tell you that is what happens when you put automatic "segments" (that is not what GM calls them) in an M6 calibration. Your idle issues are due to using this method. I don't need to have hptuners, or anybody else do it for me. I don't use hptuners to do this.

 

I was simply asking what it is they must have missed. I will have Futral look into it since they already tuned it twice. I already contacted them and I'll wait to hear back from them before I proceed. I'll see about putting the A4 calibration onto the PCM. I really don't care who you are or what you use to do this, if it is a known occurrence/glitch with trying to edit an M6 pcm this way, then it should be something you can handle or you shouldn't offer the service. At least, that's how I've always handled my business.

 

Probably not known to somebody that hasn't done that swap before. Just pulling up an A4 calibration as a compare file, and selecting "Copy differences" may handle your idle issues. There is some idle control code in other "segments", I don't know if hptuners will get it all. The M6 idle control code is much different than automatic vehicles. It shouldn't even be looking for the CAGS solenoid.

 

That makes a lot of sense. I was wondering how I was getting a P0803 if the PCM had been set up as an automatic. The 4L80 swap stickies all say HP Tuners can handle the calibration but they seem to be going from 4L60 to 4L80 and nobody added that you should recalibrate an M6 to an A4 then add the control code for a 4L80. Hopefully the "copy differences" will work, but as many different code portions that effect idle control over different conditions, I may have to pull the PCM again and have it overwritten (if that's possible) for a stock A4 and restart my changes. The only good thing is that I haven't done anything drastic to the engine yet so it should be easier to correct.

 

This is along the lines I was thinking as well.

The manual cal also has no idea what 'in gear' is, so I would imagine this will help a bunch.