Looking for some advice from the more experienced members here. 89 4.3 TBI (I think Z code) non "vortec" engine. Since I bought the truck a couple months ago I've done a seafoam through the intake, replaced distributor (set timing with ESC disconnected etc), plugs, wires, fuel filter, air filter, synthetic oil change (no cat as that was removed before I bought it). The truck runs pretty good, idles fine no major issues, just very poor mileage. The truck seems to run rich and the exhaust smells. I want to figure out what to check, test, and possibily replace if needed. Where should I start? Thanks

Jason

 

I would be somewhat concerned about the cat being gone. Im not entirely positive on this as i havent had an opportunity to crawl under a 1st genbut there should be a post cat o2 sensor. Whoever removed the cat probably either put a dummy sensor in or probably just has it dangling somewhere under the truck. The engine could be trying to compensate for that which could be part of your issue. Definately though having no cat is accounting for the smell.

 

OBD1 s-series trucks (pre-95) did not have any catalytic monitoring O2 sensors, IIRC.

 

Hmmm, i stand corrected then. For whatever reason i thought the OBD1's had those sensors.

 

I think what you are thinking of is the pre-cat sensor that is found on some of the later '95 trucks and on the '96-'97 trucks. All '95-present have a post cat sensor that monitors the catalyst. Even in an OBD2 vehicle, if the cat were not present, it would trigger an SES light. It would not affect the operation of the engine at all.

 

Nope... OBD1 vehicles usually only had one O2 sensor...

Maybe the injector leaks a bit..

What kind of bad mileage are we talkin' here?

 

Well when I fill up tomorrow I will have a better idea. Roughly I was getting about 250km's to about 3/4 tank or so. There does appear to be an O2 sensor just after the y-pipe in the exhaust. Kyle do you have any insight? Should I check the ECM coolant sensor, 02 sensor, injectors, fpr? I just don't want to through to much money at it unless it's required.

 

That's like 13MPG... For city driving in the winter... That's pretty close...

 

It is quite difficult to come up with a mileage with the information given. You should fill up until the pump clicks off and go no further. Drive it until you are ready to fill up again. When you fill up again, write down the distance travelled and the quantity of fuel to fill it up to the same point. Use the same pump if you can. Then let us know the distance and quantity of fuel required to fill your tank back up.

This is a TBI motor so you don't have an FPR, atleast not in the sense of one that is internal to the engine. Lets get an accurate baseline for the mileage and we'll move forward from there. But it is quite possible that the O2 sensor is to blame. It shouldn't be all that expensive for your truck...

 

I wrote down the mileage last time I filled up, which is basically all highway this tank. So tomorrow after work I will fill it up and will have that information. What I have noticed if that I am down over 100km's or so between fill ups. I know not accurate, but it was quite noticeable.