So I have spent a long time trying to figure out my fuel trims of the long and short variety.

Background: 96 SE7. My initial issue was horrendous mileage and codes for o2 sensors.

I looked at the outputs from the o2s via ultragauge and they seemed reasonable, they were equal in range for both sides and generated voltages that were appropriate for the conditions at hand.I replaced the leaking y-pipe and cats and got no change (aside from a quieter truck).

Moving forward, I began to have a hot start problem and it started to run hot. After investigating, hot problem was viscous coupling on the fan (YEAH $35 chevy astro fan clutch!) Hot start was a little tricky to diagnose but it turned out to be a combo of bad fuel pump/filter clogged tight.I replaced those as well as a cracked tank vent hose.

Now, here is my question. During this time of bad running, short term fuel trim was reasonable, and positive for the most part, indicating mostly lean conditions.This would be expected; no fuel flow means lean conditions and the ECM is trying to compensate by richening it up at the injector pulse.

HOWEVER, during this period the long term fuel trim remained a nearly constant and equal -17.5% for both banks. This would be exactly the OPPOSITE of what I would expect. The negative indicates that long term the engine was running RICH and the ECM was trying to lean it out. Hmmm.

Now that it is all back together, its running great. After four days of driving my short terms have dropped to better levels, and the long term hasn't moved off of 0% for both banks.

Any thoughts?

 

What's back together...?

Well guess what? I have a CEL for long term fuel trim - rich and I'm getting 330+ per 18/19 gallons of fuel which averages over 17 mpg with mixed driving. Do I care if my long term fuel trim is rich? Nah!!!!! I don't feel like messing with a gift horse.

Here's my issues and we'll compare. My IACV is somewhat not what it should be, I get 700 rpms no matter what the temp is so when it's cold I use my foot to initially warm up the motor.

I don't have well functioning rear 02's. I have no cats, so they are lifted out of the direct exhaust stream.

My base idle is not factory any longer. I messed with it to lean out the mixture at idle and then adjusted my throttle which might have something to do with the cold weather starts not automatically bumping idle up to 1k for warm up. When I initially bought the truck it did automatically bump up to 1k and then settle to 700. I dropped my IACV on concrete....right on the pintle......good thing I'm creative.

 

I figured you would be the only one to reply.

Fuel tank/pump was what I was referring to being back together.

My idle is spot on. Starts at 1000, settles down to 700. In fact, it really runs flawlessly now. It just starts, touch the key and pow!

My mileage is way up as well. It seems counter intuitive that a lean condition would drastically lower fuel economy.

I'm with you though, I could care less what the gauges say if it is running well.

Appreciate the reply.

 

Your scenario sounds counter intuitive to me too, but maybe there's some logical explanation. My Disco is running like a clock now, although my long term fuel trims tend to creep up into the positives and occasionally give me the OBDII code for max enrichment (can't recall the code right now) - maybe once in six months. But as long as it's running good and passes emissions, I'm happy.