FZJ80 running rich

[dansawyer]

The vehicle is a stock '97 FZJ80 with poor gas mileage that is running rich. Local mileage is 12 to 13, mostly 10 mile trips on local free ways. Running a OBDII meter with graph display shows a voltage plot that is a series of inverted 'U'. The majority of time is spent in the 'rich' condition.

The computer operation appears to adjust the mixture slightly leaner in increments until it goes lean and then to 'snap' back quickly to rich. It seems to over adjust to the rich condition, and adjusts the leaner steps in slow increments. The result is it spends too much time in the rich state.

1. what have other observed.
2. is this normal
3. is there a way to 'reset' the the computer to 'relearn'.

Thanks - Dan

 

[mjl132]

You can reset the computer to 'relearn' by pulling the positive battery cable off for about 5 minutes.

And 12mpg around town isn't out of normal range at all.

 

[elhombre]

How old are your O2 sensors? The computer may be getting crappy data. The ECM always seems to error on the side of rich so that a lean condition will not burn up the engine. Is your CEL on?

 

[dansawyer]

I will reset the computer. The front O2 is new, I need to drop the exhaust to get at the rear, it is pretty badly rusted.

It is the shape of the curve that is most disturbing. It would seem that a less aggressive 'move to rich' algorithm would more appropriate. The system literally spends over 90% of the time on the rich end of the curve. It is more of a step function to rich and a sloping return to lean. The rich step is large and short in time, the steps to lean are smaller. They take 8 or 10 steps to reach lean.

Is there an output from OBDII that shows the injector timing? This would help diagnose what is going on. There appears to be a percentage change function, not sure what it is showing.

 

[Cosman]

Not sure if this helps, but what I do know of Toyota ECUs in a lot of performance applicatons (have owned a Celica All-Trac, briefly owned an MA70 Supra Turbo, own an AE86) that the factory programming errs on the rich side. Having installed an ECU upgrade on the All-Trac, turns out the A/F ratio was tuned to be almost stochiometric. The vendor told me that running rich stock (at least on the turbo cars) was a bit of a safety net...a way of keeping exhaust temps low, avoiding preignition/detonaton/etc. So by slightly leaning out the fuel map (in addition to altering the boost and timing maps) gave some pretty impressive performance gains

Giving the extremes that the '80 can see, running rich could be a bit of the same safety net..

Just my 2 pesos...