John D. Goulden

'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.