'91 Explorer fuel trim

Yet another post in the ongoing saga of my '91...

The truck gets lousy mileage (12 mixed instead of 16) and shows other
indications of running too rich. Codes are mostly EGO not switching, left
bank lean but occasionally left bank rich. I'm convinced that something,
somewhere is fooling the computer into thinking that it's running lean and
the computer is forcing it to run rich as a result. Computer, EGO sensors,
et cetera checked and / or replaced. The odd thing that the mechanic noticed
last time was that his scanner (or whatever you call the thing that talks to
the computer) showed that the "Fuel Trim" was at 0 and not moving. He said
that this should move up and down (positive and negative) in response to the
voltage from the EGO sensor, but for some reason was not.

What could cause the fuel trim to act this way? What can be done about it?

--
John D. Goulden

leaking upper/lower intake gasket ?

any coolant in oil or high oil consumption ??


"John D. Goulden" > wrote in message
...
> Yet another post in the ongoing saga of my '91...
>
> The truck gets lousy mileage (12 mixed instead of 16) and shows other
> indications of running too rich. Codes are mostly EGO not switching, left
> bank lean but occasionally left bank rich. I'm convinced that something,
> somewhere is fooling the computer into thinking that it's running lean and
> the computer is forcing it to run rich as a result. Computer, EGO sensors,
> et cetera checked and / or replaced. The odd thing that the mechanic
noticed
> last time was that his scanner (or whatever you call the thing that talks
to
> the computer) showed that the "Fuel Trim" was at 0 and not moving. He said
> that this should move up and down (positive and negative) in response to
the
> voltage from the EGO sensor, but for some reason was not.
>
> What could cause the fuel trim to act this way? What can be done about it?
>
> --
> John D. Goulden
>
>
>

Did you replace the FPR(Fuel Pressure Regulator)? Computer told me
basically nothing but over at
the Explorer Forums they said replace the FPR and O2 Sensor and all
would probably be well. They were right except I cleaned the O2 sensor
instead of replacing it.

> Did you replace the FPR(Fuel Pressure Regulator)? Computer told me
> basically nothing but over at the Explorer Forums they said replace the
> FPR and O2 Sensor and all would probably be well. They were right
> except I cleaned the O2 sensor instead of replacing it.

Since my earlier symptoms were indeed consistent with a bad fuel pressure
regulator (at least according to Jeff Singleton's Ford Explorer Maintainence
pages, archived at http://www.explorerforum.com/Singleton/exp.html) I had
the shop check it when it was in time before last. They said the fuel
pressure looked OK, so the FPR wasn't replaced. If someone can convince me
that a bad FPR would cause the computer to show that the fuel trim wasn't
moving, I'll go ahead and have it replaced. Perhaps I ought to do it anyway,
but (a) as I said I'm not convinced that this could affect the fuel trim and
(b) I'm to the point where I'm throwing good money after bad at this
problem. Getting the mileage back up to 16 from 12 will saves me about $500
over the next year, but I've already spent nearly that already on this
problem, to no effect.

The really sad part is during the first few months after the rebuild last
summer I was getting the best mileage ever on that truck - close to 20 mpg
on some tanks. I really would like to know what happened to cause it to fall
like it did.

--
John D. Goulden