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Fuel Trim Experiments
Posted to Technical Theory Forum on 12/26/2011 78 Replies

It amazes me how Albin and Darrell and others can use fuel trims to diagnose misfires. I am trying to learn these procedures so have been experimenting. To start out, I am not sure I fully understand the difference between LT and ST.

Initially, I thought LTs were larger increments like "Hours" and STs were smaller increments like "minutes". So a Positive LT of 16 (lt +16) and a negative ST of 10 (ST -10) would be greater than a LT +15 but less than a Full LT of +16. (kind of like 10 minutes before 16 o'clock.)

But over the weekend I have been doing some reading and now it looks like a LT increment is the same as a ST increment, just the computer has different ways of getting there. For the sake of ease and clarity, I will say that 1 fuel trim = 1ms of injector on time. If this is the case, than a LT+16 means the PCM will add 16ms to injector On Time. But with a ST of -10, it would subtract 10 ms of injector On Time so in all actuality the Injector would really only receive 6ms of additional On Time, (+16(LT) - 10(ST) = +6)

I also did some experimenting with my 95 Buick Park Avenue with 3.8

My MODIS doesn't list Long Terms and Short Terms on this vehicle. It lists Long Terms and Fuel Trims. So I will use those terms here in this discussion.

Starting out my LT was 132 and the Fuel Trim was 130.

Next I disconnected my fuel injector.

My LT stabilized out at 149 and my fuel trim was at 131. Thus if I understand this correctly, the O2 responded with too much oxygen so the pcm responded with longer Injector On Time to balance out the extra Oxygen in the exhaust stream.

Next I plugged back in my Injector and disconnected my spark plug wire (I hooked the wire to a spark plug checker so that it would not affect the other companion cylinder.) My LT stabilized at 146 and my ST was at 127. It looks to me like the extra air that wasn't burned in the cylinder still resulted in extra injector on time, but not quite as much as with the injector unplugged. So maybe the unburned fuel in the exhaust stream skewed the O2 readings to be just a little lower than if that unburned fuel wasn't there.

Next I plugged back in my spark plug wire and disconnected the vacuum line to the fuel pressure regulator. (I did block off the vacuum line so I wouldnt have a vacuum leak.) My LT stabilized at 136 and my fuel trim at 131. This reading surprised me because I expected my LT to drop below the initial LT of 131 because higher rail pressure should cause more fuel dumped for same injector on time. So I expected it to be rich and the PCM to compensate with shorter injector on time.

Next I plugged back in the fuel rail and dissconected the vacuum line to the Cruise Control Module to induce a vacuum leak. This resulted in a LT of 140 and a Fuel Trim of 131. Again it looks like extra / unmetered oxygen in the exhaust stream means the PCM will correct by increasing Injector On Time.

I noticed that my fuel trim would always stabilize at about 130 or 131. Initially after inducing a fault my fuel trim would adjust rapidly, then the LTs would catch up and adjust themselves until my fuel trims were able to maintain just around 131. So I am thinking that GMs like a fuel trim of 131 and will adjust the LT until it can maintain the 131 on the fuel trims.

Any idea of what experiment I could try that would result in lowering my LT of 131? I really expected the fuel pressure regulator being disconnected to do it but it didn't.

Thanks for any replies in helping me to learn this. The next time I get a ford product in the shop I am going to try this same experiment.

Patrick from Virginia

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