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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.

Reply to
John D. Goulden
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difference -

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.

Reply to
Ulysses

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

Reply to
John D. Goulden

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

Reply to
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

Reply to
chadiy

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.

Reply to
Happy Traveler

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.

Reply to
John D. Goulden

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