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Hey all ,
MY LTFT is still a bit high after fixing a thotle body leak (14.6) when at idle, however it goes down when the vehicle is being driven (9-11). I know it takes awhile to even out after a repair but why is that my LTFT changes as I drive i thought that this value was suppse to remain steady and only change slowly. My STFT is very stable around 0 +or- 4 while driving conditions change.
Your thoughts?
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This may help. http://www.youtube.com/watch?v=oRX2V6_a3dc&feature=plcp
I don’t think theres a problem. has fuel mileage dropped?
don’t forget that the cars air/fuel ratio is changing faster
than an eye blink.The fuel trim will change.I did notice a considerable drop in my mileage prior to fixing a leak around the throttle body and upper intake manifold. With the leak fixed i do believe that my mileage has improved now i am at 240 miles with about 1/4 tank to go still so I should get very close to 300 on a tank. The vast majority of my driving is in town so that puts me at around 22 mpg. I think this is acceptable for the GM 3800 engines. I used to get over 400 per tank but that was entirely interstate driving. Anyway im a bit off topic. I think ill just keep an eye on the ltft and see if it changes over time. Does anyone know of a reason why it changes as I am driving, not just small but fairly large changes. Here is what I am seeing.
Idle 16.4
Regular driving 8-14 (changes frequently and always goes to 16.4 at idle)Does it save memory for different engine RPMs???
The job on the LTFT is to keep the STFT as close to +/-0 as possible.
The video posted above will explain how fuel trims work.
this is the exact opposite of what I have seen in every other post, which is correct
does LTFT control STFT or does STFT control LTFT i thought it was the 2nd
you obviously didn’t watch the video that was posted. It’s a classroom video where students are being educated about fuel trim LT ST.
ok, I found a cracked vacuum fitting, after sealing it up with some RTV my fuel trim at idle made its way down to zero within 5 minutes, however.
At startup the idle is terrible jumps all over the palce ( did not do this prior to fixing leak).
LTFT at higher rpms also is adjusting but probably not done yet
When only the ignition is on the LTFT reads 14
Do I just need to leave the battery un hooked or awhile so the computer clears all the LTFT data??
That may work but I’m not sure. I usually do a reset of the fuel trim with my MT2500 scanner (it’s faster and I don’t have to re program my radio settings which I loose when I disconnect my battery)and let the computer relearn the trim set for my driving, may take a while to do it and a few miles in various driving conditions. I suppose there may be other ways to do it though. Someone who knows may want to post it here.
ok, idle issue is fixed i forgot to plug in the MAF afer cleaning it.. oops
The LTFT did return to 16.4 but I think its still adjusting again with the MAF back in business. I appears to be moving closer to zero as my STFT is now running mostly negative. I have seen some reading where both readings are negative, at this point i should prolly drive for a few days to relearn the Fuel trims as im sure they are totally screwed up with the MAF off and taking a nice long test drive.
Well,
It is looking like I have not corrected the LTFT from fixing the vacuum leak (at least not all the way). At idle my ltft is still 16.4. Im not really sure were to do rom here. I do not have any driveability issues just poor gas milage mainly in town. MY vacuum repair with the rtv should be solid as the crack were small but i may hook up my brake bleader onto a hose and see if it can maintain some negative pressure.
Is there anything else that can cause a lean condition like I am seeing.
LTFT = 16.4 at idle and about 11-13 on the road
Would a small leak around the brake booster cause the LTFT to get thrown off? When I shut the vehicle off I can hear a small hiss from around that area. It only last about 10-15 seconds. Your thoughts
Here is a question why did my LTFT get back to zero with the MAF sensor disconnected. Could I be dealing with an inaccurate sensor?
[quote=”troy5190″ post=32097]Here is a question why did my LTFT get back to zero with the MAF sensor disconnected. Could I be dealing with an inaccurate sensor?[/quote]
Probably because without the MAF the car would run in ‘open loop’ so no fuel corrections would be made.
I think you might be over thinking this. You really don’t base your diagnosis on LTFT especially after a repair. It’s going to need a significant amount of time to ‘normal out’. Keep an eye on your fuel mileage, if you notice a significant change THEN start looking for issues with fuel trim, otherwise call it a win and move on. It’s an old vehicle after all and the fuel trim numbers might just be a reflection of that.
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