Fuel Trim Questions?

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
519
Right now I have no SES light or pending codes at all but I am a little concerned about my STFT and LTFT on my handheld scan tool. Ive read that in order to get an actual DTC, the fuel trim has to be +/- 25. Im not that far...yet.

I noticed that at cold start, my LTFT is around +18 and my STFT is around +12. Once I start driving, the STFT jumps around a lot, sometimes its in the negative, but most the time its in the positive.

I dont know if I have an vacuum leak or not. This morning I went out and put the clamp back on my brake booster to intake manifold hose. No difference. I bought a TB gasket b/c Ive had the TB off 4-5 times and never replaced the original gasket. Im replacing that tonight.

When I press the gas, I get a stutter. Its like the idle dips first before it accelerates. My TB wasnt very dirty at all, I just cleaned it maybe 8000 miles ago. So Im leaning towards my fuel mix being leaned out somehow. When I re-installed the intake manifold the other day, I had a real hard time getting it to line up- it seemed like something was in the way that wasnt allowing it to go flush with the block. I dont know if thats normal or what with all those wiring looms down there but now I sit here wondering....

Does LTFT of 13.2 sound too high? The LTFT sometimes goes down to 3 something, and sometimes up to 17 but generally stays around 13. My STFT changes a lot, almost like an O2 sensor reading. What are you guys with handheld scanners seeing with your fuel trims?
 

AtlWrk

Member
Dec 6, 2011
674
I checked mine a few months ago and was seeing LTFTs around +7 to +8%. This seemed a bit high but it would be nice to know what others are getting...
I've read that double-digits is when you need to start looking for a problem.

What does your upstream 02 sensor data look like? Rapid swings from ~100mV to ~900mV?
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
519
AtlWrk said:
I checked mine a few months ago and was seeing LTFTs around +7 to +8%. This seemed a bit high but it would be nice to know what others are getting...
I've read that double-digits is when you need to start looking for a problem.

What does your upstream 02 sensor data look like? Rapid swings from ~100mV to ~900mV?

I replaced the upstream O2 in December with a BOSCH unit. It appears to be performing well, according to the live data. It switched from low numbers (mv?) to high and back again and all over the place. Although Im not going to rule it out as a suspect, I am more suspect of the recent work I just did on the engine. :undecided: Unfortunately, I dont recall the exact FT numbers I had BEFORE the work. All I know is the last 6-12 months, Ive gotten really, really crappy mpg's (14 city/16.5 hwy).

I just did a bunch of work that included dis-assembly of some key components (intake gaskets, vacuum lines, fuel regulator, etc) and was thinking maybe an intake gasket slipped while I was reinstalling it :confused: All in all, I suppose if a gasket slipped, the FT's would be a lot more than 13.2 :confused:

I bought a new TB gasket on my lunch hour, the one thats on there now is original.
When i was on my lunch hour today I noticed the fuel trims changed. At times, the LTFT was in the negative now, but nothing big, it was like -2.3 but then would bounce around and still generally settle either at +3.2 or +13.2. The STFT is 50/50 in the positive and negative. Those darn numbers move around like the O2 readings do! :crazy:

Curious to see what others FT's are. If you got a scanner, check them out while driving and post back for Atlwork and myself, and others to interpret and compare...
 

Chickenhawk

Member
Dec 6, 2011
752
Where is your coolant temp gauge pointing when warm?

The absolutely most common cause of rich mixtures when warm is a malfunctioning thermostat.

Also, others have not had good luck with any oxygen sensor except AC Delco.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
519
Chickenhawk said:
Where is your coolant temp gauge pointing when warm?

The absolutely most common cause of rich mixtures when warm is a malfunctioning thermostat.

Also, others have not had good luck with any oxygen sensor except AC Delco.

I "just" replaced my thermostat last month, my ECT reads about 195-201*f when running. I also replaced my ECT at the same time.

I read about the O2 sensor sensitivity. So far, my mpg's haven't changed from the old O2 to the new one. I replaced my old O2 sensor only b/c I smooshed the wiring harness when I was replacing the motor mounts :lipsrsealed: Otherwise, I didnt have any codes or anything.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
519
Well, sorry we couldnt troubleshoot more but I found the reason to my increased fuel trims... :redface:

*Somebody* forgot to tighten the two lower TB bolts when he finished putting everything back together the other day :bonk: I replaced the TB gasket today b/c I didnt like the way it looked the other day, and well lets just say taking off the TB was realllly easy today with the two lower bolts loose...lmao! Took it for a test drive, my hesitation at take off is gone and my STFT and LTFT are now back under 10 and going into the negative numbers a lot more.

I went outside and checked my exhaust manifold bolts. If air gets sucked in, it will skew the front O2 sensor into thinking its a lean mixture and the PCM will richen it up. Turns out 5 of my exhaust manifold bolts (cylinder 1-2-3) were really, really loose again!!! To its credit, I havent tightened the bolts since probably 2008. But still... :hissyfit: Went on a 20 minute drive with the scantool hooked up, LTFT now hovering down in the 0 to 7.1-8.3 zone! Thats a lot better than the 10.9-13.1 I was dealing with earlier. STFT alternates between +3.7 to -12.
So far, the loose TB bolts and old gasket contributed to some leanness and the exhaust manifold bolts did too.
 

Forum Statistics

Threads
22,611
Posts
629,195
Members
17,327
Latest member
maciek_ni

Members Online