I have a 1992 two door 4x4, Vin Z so TBI. I've had pretty shatty mileage for a few months now.
I was pulling a code 13 and 44, I plugged my exhaust all the way, and eliminated 13. I'm still pulling 44, which is telling me lean. From what I've gathered, with constantly getting a lean code, the engine will start running constantly rich trying to compensate. I found a loose plug wire, that resolved some noise issues and such, but not much else.

But my exhaust is plugged, so I'm not too sure as to why to it's still throwing lean. I can't figure anything out. I was getting about 17 to the gallon, now I'm getting a -LOT- lower. Like 8-10ish right now it seems like. Granted it could probably use a tune up, it could use one when I bough it (January) and it still delivered 17. There's been an oil change, all new tires, seafoaming, and leak repair taking place across the whole truck. Nothing has seemed as if it could be causing this.

Any ideas?

 

How long did you leave the battery unplugged for? You could check the output of the o2 sensor yourself, is it a one or two wire? Grab a multi meter and hook posi lead to wire (if it's one wire) and neg to the exhaust pipe right next to sensor, engine running it should be dithering between .100-.900 for a healthy sensor. If it's staying low and not getting up as high then grab a propane torch and bleed some through the intake, the sensor should go high and kinda stay there till you stop the propane then it should drop low again. p.s this called a system performance test and this will determine if the sensor is in fact reading a constant lean condition. On the bright side they usually fail rich. I would also check the continuity from the sensor to as close to the pcm as possible ,wiggle the wires as you check. GL!

 

first thing that popped into my mind for low fuel mileage :

thermostat stuck open,and computer temp sensor is faulty/ either or will tell the computer the engines cold,and cause it to run rich.

 

I left the battery unplugged for a good half hour or so while I was mowing. The O2 sensor is only about a month or so old, not to say that it isn't faulty, I do still have the old one though. I guess I could switch it back out and see if that works out.

Any way I can check the t-stat?

 

That would be good if he was just experiencing poor fuel economy, but he's getting a constant lean code meaning the pcm can't throw enough fuel at it to get the o2 sensor back within range. A plugged fuel filter can restrict flow, kinked line, restricted injector. I'm not to familiar with the gm tbi but I would still make sure the pcm is getting the correct information then you could check fuel pressure if there's a test port. Edit: You're checkin the o2 for proper range and faulty wire, high resistance in the wire would equal low voltage or a showing lean condition.

 

It's a two wire sensor, but I don't have a multimeter. Never had many tools at my house, and what little we have is what I've accumulated on my last few birthdays. (I'm 17.) Would the local oreilly's loan one? Or should I just pop my old one on? It was throwing 44 as well, however, it was throwing 44 before I plugged the exhaust. I can see how that could be a really stupid question, but, I'm gonna ask it if I got it. lol. Fuel pressure has never been an issue. It could probably use a new fuel filter either way, honestly. It sat for a good year before I got it.

The tstat still might be an issue, my gauge never moves. Always rock bottom.

 

Get the exhaust cleared! The back up of exhaust gasses is going to reek havoc with the 02 sensor etc.
After the exhaust is cleared then try to reset the PCM & do the tests as Newb has suggested.
As for the thermostat I'd look into the wiring to & from the coolant temp sensor to the gauge.

 

If i were you,id go ahead and give it the tune up and since you said it sat up,and i bet it probably needs it. If the temp gauge is flat bottom,its either the sensor,wiring or the tstat. tstat is pretty important. without it the engine will never get up to temp,and things wont work like they should. they are only like $5 too. They usually stick open when they go bad,which allows coolant to constantly turn through the engine which keeps it from heating up to operating range.

You need to get the engine in good running health before we can fix this lean issue.
get plugs (ac delco) ,wires,fuel filter,air filter.

 

Have you verified that the engine still in spec for timing?

 

Alright, it's going to be a process, low gas mileage and a commute to college kill my paychecks readily now. I'll get some at a time, but it can't be all at once, unfortunately. What about cap and rotor? All cylinders fire, so should I worry about that?

How does one clear the exhaust? I kinda thought that it cleared as it sat, because it's supposed to flow. No?

I in fact have not. I need a timing light though, right? And I would need to sand down the degree marks on the harmonic balancer, no? (or is that just ford?)