John D. Goulden

> It's not surprising that the engine is running rich -- the computer thinks
> it's lean, so it keeps throwing more and more fuel at it, until reaching
> the
> end of the trim adjustment. Sounds like you have a bad oxygen sensor on
> the
> left bank, or (much more likely), the sensor is unplugged, disconnected
> from
> the computer, or shorted to ground. Oxygen sensors rarely fail, but your
> rebuilder might have damaged it by using silicone sealants, or, perhaps,
> coolant got into the exhaust.
> Have you or your mechanic run the on-demand diagnostic (KOEO, KOER)? On a
> '91 the only tool needed for that is a paper clip. Those should confirm
> the
> always-lean reading.
> One way to start troubleshooting this condition may be to unplug the
> sensor
> in question, hook up a DVM to it, and force a rich condition (unplugging
> the
> coolant temperature sensor typically does the trick). If you see black
> smoke
> billowing and the sensor is still at 0V, you have a bad sensor. Otherwise
> check if the harness is intact and not shorted to ground somewhere.

EGO sensor is new and tests good. However, I did get more data when I took
it by last Friday. The engine-running test reports that the computer shows
the EGO sensor giving both lean AND rich conditions. However, the "Fuel
Trim" reading on the console they were using to monitor the computer just
sat at zero. The mechanic said that this reading should be going up and down
to compensate for whatever condition the EGO sensor was reporting, but it
was not. They replaced the computer (it was in warranty and didn't cost me
anything, but this is the third computer in twelve months) but the check
engine comes on as always.

Now, it could be that the computer isn't commanding the fuel trim correctly.
However, I doubt that the last two computers would have the exact same
failure (the notes on the back of the ticket from the last computer said the
same thing - EGO reporting both lean and rich, and fuel trim sitting at
zero). What could be preventing the fuel trim (whatever that is) from doing
its job? You mention the trim in your post, so I hope you can shed some
light on this.


--
John D. Goulden