Odd LTFT behavior on 01 GS

[Trannyman95]

A friend of mine bought an 01 GS that had quite a bit of work done to it and had a bad tune on it and fuel pressure problems to boot. After sorting out the fuel pressure it was time to start tuning and get the fuel trims back where they should be. After an initial maf tune and VE tune I drove the car for a while only to find out that for no rhyme or reason the pcm will go to +16.4 lftf. Again this is after maf and ve tuning and fuel trims were near zero at all times, except when the pcm goes into its fits. This can be just after startup, cruising down the highway, coming to a stop, it really doesnt matter when it does it, it just will and is random. I tried a complete pcm rewrite with a stock file, went out and did a quick maf and ve tune and it still acts the same. No codes and all sensors are doing what they are supposed to. The 02 sensor fluctuates, plenty of cross counts when this problem pops up, then obviously the car runs pig rich and will stay that way for an unknown time but normally no longer than a minute and then things go back to normal. Anybody have any ideas or have run into this before? Our next step was to try a different pcm but I dont have any of the correct code pcms on hand to try so until we find another one the car is being driven as is.

[louvered97gtp]

I'm going to assume you don't have a scan of this because you didn't post it up. Well, I'll throw something out there. Have you checked for vacuum leaks? I understand a vac leak can be interpreted as a lean condition which will make the PCM add fuel. Exhaust leak could also contribute.

[Trannyman95]

Its not a vacuum leak as all of that has been looked over. I tune these cars very frequently and do a lot of custom engine/transmission builds and know them up and down. The 16.4+ fuel trim is out of the blue. It doesnt gradually climb from 0 to 16.4 to cover up a lean condition, its an instant change. As I mentioned above the 02 sensor is plenty active and there are plenty of cross counts- rich lean activity. It never once showed signs of being lean to require this sudden spike. I also had my wideband on the car during all of this and was running very close to 14.7 +/- a the small range that it will cycle up and down, and when it goes to +16.4 it is around 11-12 afr. I dont recall saving any of the scans but when I would scan the car and drive it for 20-30 minutes and watch the file in replay mode there was no aparent reason for what it was doing. Its been about a month since I had a chance to mess with the car but it would be nice to find a solution which I feel will be a different pcm but again I dont have one handy to try out to verify.

[BlackGS]

The obvious things that come to mind are a bad O2 connection that shorts on occasion, a bad injector(s), or vac leak problem. If it is a short in or near the PCM, that will be a real pain to hunt down without replacing the PCM.

[Trannyman95]

After trying a different pcm, 02 sensor, rear 02 sim in and out of car and various other changes the problem still exists. This car has no cat or rear 02 sensor. I have talked with a few people about this problem that has had this same exact problem and here is what I learned.... It appears that some 01+ pcms have a cat diagnostics test that runs and basically the pcm wants to see that the cat is doing its job. With no rear 02 sensor and the code turned off the pcm keeps dumping fuel to see a change but it wont see the change with no rear 02 hooked up as the voltage is going to hang at its referrence voltage all the time unplugged or fluctuate all the time with a sim which is not what it wants to see. With the rear 02 codes off you wont ever know and it wont fail-out so to speak from the pcm continously dumping fuel. One guy that I talked to that had an 01 GTP ended up flashing the 01 file onto a 98 pcm and the problem went away, but because HPT does not like a mismatched OS and Vin I cant make changes to the file but I can at least read it. Without really knowing what causes this for sure a good local friend of mine that uses a different tuner was working on an 05 Grand Prix and there was no cat being the customer just installed an offroad downpipe and wanted the rear 02 codes turned off and guess what!!! LTFTs climbed to +16.4 and ran pig rich exactly like this 01 Regal I have been working on. He requested the tuner programmer to check into this and look for a cat diagnostic turn on temp and related parameters and earlier today they sent him an update to try and low and behold these parameters DO exist in the pcm and it does want to run the cat diagnostic test to verify it is working properly. This with an efficient cat you will never likely run into this condition, but I really dont understand how any 01+ car with no cat does not have more complaints of this condition because it is real and does exist. With his updated software he bumped the diag enable temp pretty high as well as a few other changes and went out for a drive and it fixed the +16.4 LTFT and running rich and wanted to kill the car! This is a REAL fix and really needs to be added to our tuner to address this problem.

[Perforator]

Who else makes a tuner for the 3800 besides HP?

[Perforator]

Oh and not to throw a monkey wrench in your theory Dave, I have a 2005 Grand Prix with an off road pipe and do not have any problems with my LTFT's.

[IndeedSS]

Dave, since I've gotten and Aeroforce gauge and watch the LTFT's all of the time, I see this type of thing as well. Only mine, will go to -16.4 and other times +16.4. The odd thing is the WB doesn't seem to react to the short periods of high or low LTFTs.

[Trannyman95]

TunerCat has tuning support for the V6 cars. Unfortunately the only way to get a TC cable is to find a used one as they started licensing the cables through JET and dont have the freedom that the TC software has and any time one is up for sale it is normally sold within hours! The only way to my knowledge to get a new cable is with a RoadRunner emulator pcm that is used with the V8 cars and is pricey. A good friend of mine has used TC for roughly 10 years as they are a big OBD1 supporter and came out with OBD2 support a few years back and he is normally right on top of anything new that is supported by them. They also just released 96 support which is great since nobody else seems to have it.

[Trannyman95]

Lee this isnt a short-term problem. When you are cruising down the road and this happens it does not go away until you get into PE mode and the car will start running pig rich indefinately until you jump on the gas. It will do it for miles if you let it. I realize this may be isolated to only some peoples cars but regardless this is something that really needs to be supported because it causes a LOT of grief for the owners of the cars that dont understand what is happening and is not something that can be tuned out without turning off the cat diagnostics.

[Perforator]

Dave, do you have the DTC's for the CAT tests turned off?

[Trannyman95]

It has nothing to do with turning DTCs off from what we both tried and that only would mask how it works. He found IIRC that leaving the codes turned on for the rear 02 would cause them to trip and then it would not run the diag test.

[Perforator]

Just a suggestion, wasn't sure if it would help. These PCM's do some strange things. I wish we could see how it's coded. Would make problems like these much easier to fix.

[IndeedSS]

Lee this isnt a short-term problem. When you are cruising down the road and this happens it does not go away until you get into PE mode and the car will start running pig rich indefinately until you jump on the gas. It will do it for miles if you let it. I realize this may be isolated to only some peoples cars but regardless this is something that really needs to be supported because it causes a LOT of grief for the owners of the cars that dont understand what is happening and is not something that can be tuned out without turning off the cat diagnostics.

That would be quite frustrating and much worse than what mine does. V6 additions are far and few between, as we all know.

[Sinister Performance]

I'm TrannyMan's friend being referenced in this thread. I do have one correction to make, the car I was working on is a 05 Monte SS w/ L67 engine not a 05 GP which uses a different PCM.

I used to work at a GM dealer and took some classes/tests on OBD2 emissions protocols. After I encountered this "fuel trims locking full rich" issue, I did some thinking and remembered reading about how GM was running CAT and post-cat O2 sensor diagnostics in their "new cars" (at the time). Several times during a drive cycle, once time and engine coolant temp, and other qualifiers are met, the PCM will command the fuel delivery to the engine rich on purpose to determine if the CAT and the post-cat O2 sensor are functioning. The fuel needs to be commanded rich in order to saturate the cat so the computer can accurately run these tests.

When the PCM runs these tests, it expects to see the post-cat O2 sensor output voltage go high. If it sees this, it will issue a PASS flag on the test, discontinue the test, and wait until the next test loop to run the test. If the PCM does NOT see the post-cat O2 sensor go rich (as indicated by high voltage), then it will set the appropriate DTC which disables future tests from running -- that is if you have it enabled. If the DTCs for the cat and post-cat O2 sensor are disabled in the programming, then these tests may never cease from running as TrannyMan described. On the O5 Monte I tuned, I noticed the AFRs running (and locked) in the 10.5 - 11.5:1 range as indicated by the WideBand O2 sensor continuously unless you subjected the engine to full throttle operation (or during decel fuel cutoff).

The first time I tuned this car, the tuning software I had available to me (TunerCat and HP Tuners) DID NOT gave me the ability to disable these diagnostic tests. So the only thing I could do is set these DTCs to option2 = No MIL so at least the code could set (in my case the first/only code that would set was a P0140) which suspended the cat/post O2 diagnostics. The only problem is this car had one of those driver message centers that displayed "service vehicle soon" once the P0140 code set even though it did not turn on the check engine light.

I have a long working relationship with the gentleman behind TunerCat so I contacted him about this issue and he got me an update in a very short amount of time to include parameters that allowed me to disable these CAT test loops (coolant temp qualifiers, min/max number of times allowed to run these tests, other param's, etc). I set the min/max no. of times allowed to run these tests to 0/0 and set the coolant temp qualifiers to 125 deg C, disabled the cat and post-cat O2 sensor diagnostic codes, and went on a few more test drives. These tests never ran again (at least the fuel trims never maxed out and the AFR's never got stuck rich) for the amount of time we test drove the car. So I think we can call the problem solved at this point.

I haven't had the opportunity to contact HP Tuners directly to ask for this ability to be added to the V6 cars yet so somebody may want to do that. I have tuned a couple LS4 cars using the HP Tuners software and in those platforms, HP Tuners DOES give you the ability to disable the cat and post-cat O2 sensor diagnostic tests. So I'm sure these param's can be added to the V6 platforms.

Just FYI: These diagnostics we are talking about here are NOT the same as the CAT OVERTEMP diagnostic which HP Tuners currently has the ability to disable in V6 apps.

[Perforator]

There must be something else in the tune that is causing the PCM to continually run the CAT tests because I know for a fact I have the rear O2 DTC's turned off and don't have this problem. I am also sure there are dozens, if not more, of other Grand Prix owners who do the same thing on ClubGP and also don't have the problem. I do agree it would be nice to have access to this parameter though.

[Sinister Performance]

There must be something else in the tune that is causing the PCM to continually run the CAT tests because I know for a fact I have the rear O2 DTC's turned off and don't have this problem. I am also sure there are dozens, if not more, of other Grand Prix owners who do the same thing on ClubGP and also don't have the problem. I do agree it would be nice to have access to this parameter though.

I don't know what else to tell you other than what I've already posted. I've gone thru the GM training that I think pertains to this subject so I can only tell you what GM taught me. In addition, I know what happened with the '05 Monte SS I tuned and what I had to do to correct the issue. And simply disabling the DTC's did NOT prevent the diagnostics from running that created this problem.

TrannyMan said earlier he has heard of this NOT happening on every 01-up V6 car. Furthermore, we haven't determined the reason why this diagnostic doesn't run when a newer calibration is used in a 98 PCM (at least this is what we were told by a 3rd party). Could be an invalid function in the earlier hardware so that PCM might simply ignore it. And in your case, your PCM could be simply using a different OS that doesn't run this diagnostic. Who knows.

Just telling you what WE found.

[garyd02gtp]

Oh shi* I have this exact problem. 2002 GTP, with no cat. I have been chasing my tail to fix this, including changing PCM. I now have a 1998 PCM flashed with 2002 bin- and it STILL does it. Si it IS a REAL problem and we need a fix. Anysuggestions new since last post????

[dtabet2]

interesting in deed.. i read about this several times.. in alot of forums... we need access to theses regardless of the HUGE effects on some.... i dont want my car to go rich even slightly unless i say so..... this could throw off tunes..... and im good on that....

Dan

[Sinister Performance]

Oh shi* I have this exact problem. 2002 GTP, with no cat. I have been chasing my tail to fix this, including changing PCM. I now have a 1998 PCM flashed with 2002 bin- and it STILL does it. Si it IS a REAL problem and we need a fix. Anysuggestions new since last post????

For now all you can do is re-enable DTC P0140 but set it to option 2 (No MIL) and uncheck the box next to it. What this will do is this: The PCM will run the post cat O2 and cat check diagnostics I described earlier. But as soon as it does not see the expected signal return from the post-CAT O2 sensor, it will flag the PO140 DTC (but since you will have it set to NO MIL, the SES lamp won't turn on). As soon as this DTC is set, these diagnostic test will no longer run, for that ignition cycle at least. But even if they do run again it will be brief until the PCM realizes there is no post-cat O2 sensor there.

You can have all of your other cat and post-cat O2 related DTC's disabled (option 3) so the SES light doesn't come on for those.