Adaptive Fuel Strategy mystery

[IanSzgatti]

I have LTFT numbers in the area of -17, while STFT is usually sitting around -08 to 0...but never above, never adding even a little to the mixture... FPR works, actual pump pressure is 36 (down a bit), and my fuel system is clean... also no vac leaks...

im not sure what to make of the evap, could possibly be the source of the rich mixture...assuming the solenoid is opening when it shouldnt.. at operating temp at idle, evap is a 14%... seems at least practical, low idle, low vapor draw?? any ideas welcome.

Another thing to note... pulling the vac off the FPR raises the FP the normal amount.. FPR doesn't leak into manifold... LTFT numbers will rise and stay at -4 with the extra air let in by the FPR vac line... just a note if that helps.

1997 3100 SFI

*new upstream 02
*new LIM
*fuel pressure holds with engine off
*any fuel managment sensors are at least operating within tolerable range, but i dont know the exact values they SHOULD be at... any references to the info greatly appreciated.

main point is that Long Term Fuel Trim numbers are normal kept as close to zero as possible.

(motor&PCM stock)

[Rhedalert]

-what brand upstream O2?
-which octane fuel do you run?
-have you reset the PCM lately?

[IanSzgatti]

-what brand upstream O2?
-which octane fuel do you run?
-have you reset the PCM lately?

02 is made by the NGK company, wish I could give you the exact name

87 octane

PCM has not been reset, and symptom exsisted before the LIM replacement this past sat. I have only owned the car a month though.

[Rhedalert]

I don't have experience with NGK O2's. I know I've seen Bosch O2's screw around with GM computers, causing problems like what you've seen. It might not hurt to reset the PCM at this point after the O2 was changed. Even though it's supposed to "adapt" it's not always perfect.

[IanSzgatti]

02 went in first, then LIM was done, battery was off for the LIM for 6 hours.

i like the other idea about the brand... could be AC delco should go into it... not sure... pretty stumped here. Got the same post over on MP racing forums, hoping the admin would have an idea.

[Rhedalert]

What is the LTFT at idle, while accelerating, and while cruising? Is it always hanging around the high negatives? The evap seems ok based on your description. How accurate do the coolant temp and air intake temp readings seem?

[IanSzgatti]

coolant temps and air temps seem quite normal to me.

at idle, i'll see around minus 17 LTFT, and going up to about 2000 to 2500 rpm will usually see minus 4. I havent gone out on a road test with the scanner on yet, but good idea, I'll do that and post again my findings. So at least when it takes in more air, it adds fuel to keep that 14:1.

let that simmer... on a side note, I understand the rear (or downstream) 02 sensor is a catalyst monitor, but if the cat isn't functioning well enough, would the PCM determine that it should change the mixture slightly to improve the efficiency of that cat?

I might have a little more time in the shop today since it's kinda dead, so i think i'll see if I can pull some IAT and CTS specs off mitchell... unfortunately it's a burned copy, and lot's of info is missing I'd like to double check the sensor values... but I'm thinking something must be up with that cat...

*would e-test results help you? neither my boss or co-worker can help me on this one!!

edit: i should have added that the STFT numbers will go above the 0 point... the sensor data is being used by the STFT, which in turn controls the LTFT... since you appear to have some idea of these systems i'll get you some more definative answers...so this isnt so damn complicated and unorganized

[Rhedalert]

fuel trim readings while driving would be good information to have. It might narrow things down. Before I tuned my car, the trims would go way negative at idle, but would fly positive while driving around, especially under load.

Also see what idle RPM the scanner detects, when the engine is warm. Should be around the 650 area.

[IanSzgatti]

while cruising around the trim numbers go more to positive, but never more than -7. now the short term looks as if it's doing it's job fine... and for rpms, all seems fine... maybe a slight lope in the idle, but really not whats causing this..


i got to thinking though... i read a fuel pressure of 36 psi a couple days ago, and now im wondering if it's a lean condition causing the PCM to richen up the mixture...

i realize that ultimately the negative trim numbers indicate the presence of a rich condition the comp is compensating for... and as confusing a thought I just laid out, im sure you see what im getting at... inverse reactions..

I had a look at my 02 cross counts, and i think it should be switching at least 40 times in ten seconds with a 2500 rpm idle... but it's not switching that fast... i got a cross-count of 5 at 2500 rpm... on a NEW H02S...

Mitchell says MAP voltage should be 1.5v at idle to 4v at WOT, yet at idle im at 0.8v..... now the tricky thing, with key off the baro reading is correct... our computer is hard to navigate though, and missing info... also, im concerned that the info might not apply specifically to my emissions components... even though im in 1997 malibu 3100...

i know this is all over the place... but take what info you can from it... im still kinda stumped... sometimes the symptoms dont match up with the obvious... like the cross-counts... I dont know where to find out exactly how many cross-counts I should expect... but i want to trust mitchell!!!! I've also heard some 02's dont run well in GM's... WHY???

stumped stupid

[Rhedalert]

That is a toughie. I know BOSCH O2's tend to be "lazier" than Delco's and that's why sometimes there'll be issues. For downstream it's ok though. Delco and NGK partnered up with the Iridium spark plugs, but I don't know what that means for NGK O2's.

[IanSzgatti]

Delco and NGK partnered up with the Iridium spark plugs, but I don't know what that means for NGK O2's.

thank-you... im sure i'll find out

heres the brand.... http://www.ngksparkplugs.com/product...gensensors.asp

[IsaacHayes]

I thought I heard Delco O2's are now Denso... I'm running a Denso.

[IanSzgatti]

i've always had great experience with AC Delco parts... especially the brake pads... no squeaks, no fade... excellent stuff