Problem with '91 Eddie Bauer

'91 Eddie Bauer 4x4 4.0L, 220K but with engine rebuild about 10K ago.
Rebuilder is not my regular mechanic.

This has been driving me (and my mechanic) crazy. Check engine light comes
on after about 10 minutes; codes are EGO not switching, left bank lean (hope
those make sense, going by memory of what they told me here). However, other
indicators are that the engine is running rich: poor mileage (12 mpg instead
of my usual 16 in mixed driving; black soot in tailpipe).

Computer is relatively new and checks OK. Last time in they replaced the
thermostat (which wasn't closing and they thought part of the problem was
that the engine wasn't reaching normal operating temp), oxygen sensor, and
fuel filter and checked the fuel pressure regulator (symptoms were
consistent with stuck fp regulator according to Singleton's pages so I had
them check it - they said it was OK). The thermostat did make a difference -
at least the temp gauge comes up pretty quickly now - but mileage didn't
improve. Catalytic converter is not clogged. Coolant level stays pretty
constant (fairly new radiator) and no hint of coolant in exhaust.

Any idea on what I / we should look at next?

I almost hate to add this because it's so weird, but shortly after the
rebuild last summer it was having the same problem (check engine light on,
poor mileage, black soot / smoke). The rebuilder did a fuel injector
treatment and the truck ran like a dream - no check engine light and an
unprecedented 20 mpg mixed (I'm happy to get 16). Took it back in to the
rebuilder for unrelated things six weeks later and he noticed that a vacuum
line was off. He plugged it back in and the problems immediately returned.
After much head-scratching, his speculation was that the resulting vacuum
leak was fooling the computer into running extra lean. The line in question
was on a connector on the passenger side and runs across the firewall to the
topmost connector on the octopus on the driver's side of the engine. If
anyone wants to take a stab at that one, great. I'm almost ready to unplug
it again and see what happens, but I'd like the truck to run correctly
without resorting to such voodoo.

--
John D. Goulden
who needs to coax two or three more years out of this truck - then it's a
new...something, not sure what yet. Some kind of regular bed, regular cab
pickup, perhaps.

"John D. Goulden" > wrote in message
...
> '91 Eddie Bauer 4x4 4.0L, 220K but with engine rebuild about 10K ago.
> Rebuilder is not my regular mechanic.
>
> This has been driving me (and my mechanic) crazy. Check engine light comes
> on after about 10 minutes; codes are EGO not switching, left bank lean
(hope
> those make sense, going by memory of what they told me here). However,
other
> indicators are that the engine is running rich: poor mileage (12 mpg
instead
> of my usual 16 in mixed driving; black soot in tailpipe).
>
> Computer is relatively new and checks OK. Last time in they replaced the
> thermostat (which wasn't closing and they thought part of the problem was
> that the engine wasn't reaching normal operating temp), oxygen sensor, and
> fuel filter and checked the fuel pressure regulator (symptoms were
> consistent with stuck fp regulator according to Singleton's pages so I had
> them check it - they said it was OK). The thermostat did make a
difference -
> at least the temp gauge comes up pretty quickly now - but mileage didn't
> improve. Catalytic converter is not clogged. Coolant level stays pretty
> constant (fairly new radiator) and no hint of coolant in exhaust.
>
> Any idea on what I / we should look at next?
>
> I almost hate to add this because it's so weird, but shortly after the
> rebuild last summer it was having the same problem (check engine light on,
> poor mileage, black soot / smoke). The rebuilder did a fuel injector
> treatment and the truck ran like a dream - no check engine light and an
> unprecedented 20 mpg mixed (I'm happy to get 16). Took it back in to the
> rebuilder for unrelated things six weeks later and he noticed that a
vacuum
> line was off. He plugged it back in and the problems immediately returned.
> After much head-scratching, his speculation was that the resulting vacuum
> leak was fooling the computer into running extra lean. The line in
question
> was on a connector on the passenger side and runs across the firewall to
the
> topmost connector on the octopus on the driver's side of the engine. If
> anyone wants to take a stab at that one, great. I'm almost ready to unplug
> it again and see what happens, but I'd like the truck to run correctly
> without resorting to such voodoo.
>
> --
> John D. Goulden
> who needs to coax two or three more years out of this truck - then it's a
> new...something, not sure what yet. Some kind of regular bed, regular cab
> pickup, perhaps.
>
>

Hmmm. Did you check for automatic transmission fluid in the vacuum lines,
particularly the one that fell off? This might indicate a bad vacuum
modulator on your transmission. I think, however, that it would take an
awful lot of ATF to do what you described.

Something to check anyway.

> Hmmm. Did you check for automatic transmission fluid in the vacuum lines,
> particularly the one that fell off? This might indicate a bad vacuum
> modulator on your transmission. I think, however, that it would take an
> awful lot of ATF to do what you described.

Sorry, should have mentioned that, too - ATF fluid levels are steady.

--
John D. Goulden

> Sorry, should have mentioned that, too - ATF fluid levels are steady.

And yes I know I just said automatic transmission fluid fluid...post in
haste, repent at leisure...

--
John D. Goulden

I had the mass air flow sensor go out on my 93explore.It gave me black soot
and bad mileage also,but mine ran really bad warm.Just one more thing to
look for.GOOD LUCK

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.

"John D. Goulden" > wrote in message
...
> '91 Eddie Bauer 4x4 4.0L, 220K but with engine rebuild about 10K ago.
> Rebuilder is not my regular mechanic.
>
> This has been driving me (and my mechanic) crazy. Check engine light comes
> on after about 10 minutes; codes are EGO not switching, left bank lean
(hope
> those make sense, going by memory of what they told me here). However,
other
> indicators are that the engine is running rich: poor mileage (12 mpg
instead
> of my usual 16 in mixed driving; black soot in tailpipe).

> 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