Negative LTFT [Archive] - Forum


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11-12-2013, 07:27 PM
So I'm looking for some opinions/thoughts/ideas here.

My GAGT is showing some dramatically negative long term fuel trim, most pronounced at idle once the vehicle has been driven for 15+ min or longer.

I am leaning towards suspecting a leaky fuel injector, if not multiples.

In the last week, it has become prone to going to -29% LTFT if left to idle on its own, eventually stalling out and then taking some effort to get restarted. Once I get it running, I have to keep the RPMs around 1200-1300 to keep it from stalling out when sitting at a red light. However in gear, and rolling, it seems to be able to compensate and at RPMs over 1500, the LTFT settles between -8 and -5.

I have replaced the MAF sensor.

I have replaced the MAP sensor, the old one and the new one were giving different readings. The new sensor seems to have helped some, but the idle trims still go way negative over time.

7x Crank Sensor and the Camshaft Position sensor are both new.

I have replaced the IAT and the IAC.

Both O2 sensors have less than 10k on them.

EGR valve is new.

FPR is new and does not appear to be leaking fuel into the vac line, and functions properly when the engine is running. Removing vac line adds 10 PSI to fuel pressure, putting vac line back on drops back down 10psi.

Engine vacuum is reading around -18psi at idle, if I let off the accelerator at speed, the vac reading goes up into the low/mid 20s.

I have replaced the upper and lower intake manifolds with ported versions, intake gaskets, head gaskets, heads were resurfaced, exhaust manifolds were ported and new gaskets put in, and a MMS catted downpipe.

11-13-2013, 04:18 PM
I had this and it mysteriously went away when I changed the battery... now it's rock steady around 0 fuel trim but still drops to -7 or so when slowing down. My battery was good for one crank.. if I turned the lights on for a couple minutes and attempted to crank... no go.. so it was pretty bad. You've done a lot more than I was willing to do.. after owning the car 13 years I was going to buy another one rather than deal with intake gaskets... even accessing the fuel rails requires removing the upper intake gasket. Barring the battery, the last thing I did before that was the fuel pressure regulator...

The only other stuff I did before that was PCV Valve, spark plugs and wires, 02 sensors, and MAF sensor. Also checked for exhaust backpressure (none detected) .. these were all done over a period of a month or three and well before the battery was done.

Mine never stalled out or anything. The symptoms 'suggest' a leaky fuel injector (fuel trim worse at idle, better under load) and that's what I thought I had... I had no confidence changing the battery would fix anything... I just did it because it needed it.

I have sense taken it on a 450 mile trip (each way) with no issues.

11-13-2013, 06:09 PM
I think I need to make a thread on fuel trims as this keeps popping up.

Any fuel trim reading within 5 values from 0 (-5 to 5) in LT are generally accepted as normal and will have no effect on the car's performance or driveability.

The narrowband oxygen sensor and closed loop system in this vintage GM car cannot ACTUALLY read the air/fuel ratio. The fuel trims are the way that the car guesses and checks itself to try and get as close to "stoich" as possible.

Running a maxed out fuel trim either positive or negative means that the car is either adding tons and tons of fuel to compensate for it thinking its running lean (think if you took the oxygen sensor out of the exhaust stream and into open air, the car wouldn't even think its running!) or running rich (skew the MAF reading so the car thinks more air is entering the combustion chambers than actually is).

If your car runs fine, and the fuel trims aren't 0, don't worry.

If you have done lots of work on the car, you need to reset the fuel trims as they are a learned value in the PCM. IIRC a battery cable pull for more than 10 minutes will reset fuel trims, but I always did it with HP Tuners. Some simple PC based OBD2 readers should also be able to reset fuel trims as I think the function is SAE defined.

11-13-2013, 07:31 PM
Good info hoys.

11-17-2013, 04:19 PM
The car does not run fine. It stalls at idle when warm.

I replaced the fuel pump because I was slowly losing pressure Key On Engine Off. It ran fine for a day after replacing the pump, still had very negative LTFT at idle once warmed up but did not stall.

It holds pressure now, losing 1-2psi over the course of an hour and then pretty much holding stable. However as of today, it will not start at all.

I've got the battery disconnected right now going to try resetting the PCM.

The car cranks nice and strong, the spark plugs are all healthy looking and gapped properly. The ignition coils are similar resistance 5600-6300 OHM.

The injectors are all 12.3-12.7 OHM

The car just cranks but never even acts like its trying to catch. I have not had a single missfire code thrown, it is currently not throwing any codes for the no start.

When it is running, it runs well, I'm getting 26-27MPG on the highway @ 70mph. I just can't figure out why it has stopped starting reliably and why it won't idle when warmed up, and why the LTFTs go beyond -29 at idle. LTFTs when driving, accelerating, cruising, at various speeds and loads are typically -7 to -3, but at idle it bottoms out and then stalls if I don't force it to idle at 1300+ rpm.

Can anyone offer any suggestions advice before I go dump it on a shop and probably get reamed for labor.

11-17-2013, 04:57 PM
Battery Disconnect did not change anything.

Cranks strongly, but does not even try to catch