355 STFT/LTFT questions

I have been chasing fuel trim issues for a few weeks. Readings were relatively consistent over a period of time/measurements, and imbalanced from bank to bank. Bank 1 was +7%, and bank 2 was +14% suggesting a lean condition with ECU adding fuel to compensate. First smoke test showed exhaust leaks which I corrected. Next smoke test revealed both plenums leaking at the seams, one of the oil breather hoses with a small gash, and smoke billowing out of the purge valve adjacent to the charcoal cannister.

Replaced those components and new smoke test shows no leaks. Took a few drive cycles to get trim readings to change, and now they are very different......banks are now balanced with similar readings:
LTFT bank 1 is -2.3 and bank 2 is -1.6. But STFT are bouncing from +11 to +25 on each side?? and at times, they will settle down into numbers under +2.0. In addition, I threw codes 1124 and 1126 which are LTFT additive air, system too lean. I thought that LTFT would follow the positive STFT?

Can anyone make sense of this? Since things are balanced, is it possible I have a MAF issue?. One more thing, prior to fixing all the leaks the car idled smooth and spot on; now, there are times when the idle will, ever so slightly surge a few hundred RPMs, like you might have with an air leak......???? crazy

Another observation is that my car seems to be running cool. at 45mph, temp is 166'. Could a stuck open thermostat cause low temps, and rich condition?

 

Wouldn't that show up with smoke test?

 

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I thought those LTFT numbers would be good? If my STFT numbers were 0.0, would these numbers be good? Should those numbers "always" be positive? I did throw CEL for lean condition, but aren't negative numbers indicative of rich condition? (I recognize my high STFT when added to the LTFT shows lean)

 

Amateur mistake and a bit of egg on my face: IAC had detached from the air delivery tube. Crazy numbers gone, but I still don't know how to interpret fuel trim readings. Changes are not immediate, and require a few drive cycles;

After a few drives and stops this AM, I thought everything was perfect with LTFT at 0.8, and 1.6%. A few more stops and starts and readings are at 3.1, and 9.4%. What should I expect and what exactly does this mean. Online searches say to add ST and LT numbers to get a total, but STFT changes all the time?

 

great explanation, John, thanks. I finally have an idea of what its all about....at least, I think so....LOL

I feel very confident there are no more air leaks in the system. I smoke tested twice more today with a friend's help, in case my eyes missed something. Its over! Smoke free.

Highway driving, consistent at about 80mph, no crazy up and down accelerating, the numbers for Bank 1 were at +0.8 and Bank 2 at +5.5%. No idea what that might mean, but numbers were lower on the highway. With "regular" in town driving, (prior to the highway) numbers were at +1.8% and +10.2%, and after the highway drive, returning home, those exact numbers returned. So it seems, that things have settled in for fuel trims.?

I think we can agree that Bank 1 is as good as it gets and new plenum gaskets and other leak fixes corrected that side. Bank 2 is another issue: Agree, we can eliminate common parts like MAF, ISV, IAT, etc. The issue is only on Bank 2, so nothing shared is of concern. Choosing my words carefully, I feel very confident that air is not the issue. So that leaves fuel and spark. It has been a few years since the car was used much, and even prior to that PO only used a few hundred miles a year.

Is it possible that I could have a slightly balky spark plug or a slightly dirty injector on that bank? An issue so minute, that those components would not exhibit any overt behavior, but just enough to throw LTFT out of sorts? Not that big of a deal to get new plugs, and/or send out the injectors for cleaning.

Car runs great and ran great before all the leak repairs. Would have been nice to have a tangible benefit, but I can't see any difference; no regrets, certain its
best to have the car in its best health

 

Is the secondary air pump and related vacuum lines only fed to bank 2? Wouldn't smoke testing show any issues with those vacuum lines?

I am on my third tank of injector cleaner.

 

I did see some benefit, but not "big changes." I am not sure off the shelf bottle cleaners can solve all injector issues. Could it be a poor spray pattern that requires ultrasonic cleaning?

 

a ha.....good point.....will go back in there and investigate. I could pull hoses related to the secondary and check directly?

 

yes...all "looks" good.

good advice., will do

spoke to Dan from Bradan yesterday. Surely you have seen some of his threads. nice guy and he does nice work. He told me he was chasing the exact issue for a customer with higher LTFT on bank 2. He said the resolution was to replace a micro chip inside the ECU specifically for trim?? Bosch CJ125. not sure if there are 2 or more of those chips inside or only one? Do you have 5.2 ECU schematic?

 

Was excited, in anticipation, to find a leak with this test, but unfortunately (really...lol) there are no leaks in the secondary air system. I have asked before: can a spark plug be half pregnant? Are plug failings binary, or is there a grey area and perhaps one plug is just not be firing at full capacity?

 

No I have not. How would I do that?

 

^^^^LOL

 

Spent a few days driving and bank 1 is consistently at or near 0.0, while bank 2 stays around 10%. I don't think its a big deal, but I don't like the imbalance, and 10% is borderline; I would like to clean this up. I was able to speak with a local seasoned tuner and will be visiting his shop this week. He suggested putting the car on his dyno and using wide band O2 sensors to check things out. He felt the best attack was a 5 gas analyzer, but is not in his tool kit. Like Bradan, he has seen ECU micro chips at issue, but thought (over the phone) injector cleaning would be first avenue of pursuit.

In the meantime, I would like to Mityvac my bank 2 FPR but have no idea where the vacuum connection is? I felt around in the front of the plenum with no luck.

One oddity, is that at highway cruise, bank 2 number drops to 6.3, while engine temp drops to 165'? I would have thought the reverse with cooler engine temps forcing ECU to dump fuel? Could my higher in town fuel trim be related to the kicking on of the cooling fans? I think the cooling fans cause both bank fuel trims to elevate and the fan loads trump the cooler highway temps?

 

Yesterday I was able to record a bit more. No highway driving, a bit of stop and go and cruising, at times, around 50mph: Bank 2 LTFT was between 9.4, 10.2 and more often at 11.7. While sitting, the LTFT did not change at idle, 1,500, 2,000, 2,500, 3,000, and 3,500. While driving I got that same results while holding RPMs at these same numbers. Does this suggest a fuel delivery issue?