cuengineer

Recently noticed that my gas mileage has dramatically decreased, as in less than 1/2 what it has been being. I checked my fuel pressure and it shows a little low but engine still runs good. Wondered if low fuel pressure could be causing this by not working the fuel injectors right? Would this really make sense though or is it more likely that it is one or more of the fuel injectors? Or does anyone have other ideas that I can check out? BTW, I checked and no code errors. I am looking to figure out the problem and not just throw parts at it. Thanks!

RustyCFI

Could be the cat, depending on the years it won't throw a code either.

cuengineer

How do you check the cat? Backpressure somehow or disconnecting?

RustyCFI

Check out my other post in this same forum section that is labelled "backfiring."

What year is your car?

cuengineer

Forgot to mention, the car is a stock 1988 with L98 engine.

RustyCFI

No worries. Read up on that post I wrote but the position of your cat may be slightly different from what I personally experience.

cuengineer

Read the cat post. Dread the idea of taking it loose but may come to that yet. Thanks.

RustyCFI

Yea, but hey it could be something else! Just hope for the best. Could be something simple like a bad o2 sensor. It might be a good idea to check the cat anyways just for general maintenance and to see if it needs to be replaced or taken out.

ddahlgren

It seems hard to believe the mileage is 50% and still runs well. I would think a quick run through basic tune up and a scan tool to check sensors in operation.

antfarmer2

Original o2?

ex-x-fire

Is the engine running cool while going down the road? It should be at 190-200* on the road. If its running 180* or less you'll see it effect your mpg.
Your Maf sensor could be dirty too.
When you do the fuel pressure test, does it hold pressure after shut down?
Check all of the grounds, esp. the ones behind the heads. Those ground the ecm.

cuengineer

Not sure how old the O2 sensors are so am thinking about replacing as part of maintenance anyway. Temps run around 200F so should not be that. The dropoff was pretty sudden so I am thinking it could be either sensors, MAF (but that should throw a code, I think) or Cat. Thanks for all of the suggestions. It will be a couple of weeks before I can get back to it as I just had a colon resection to hopefully prevent colon cancer. Quick aside: be sure to get your routine colonoscopy - could save your life or at least extend it significantly!

JimLentz

Sounds like you have a fuel injector sticking open to some degree. Hope you heal up quickly.

cuengineer

Thanks, Jim. While laid up, I am trying to come up with a plan of attack to check out the possibilities with as much of a logical sequence as possible in order to avoid throwing parts at the problem.

vader86

Nice to see another Clemson guy.

If the engine runs good and there are no codes, and your mpg really sucks, suspect the oxygen sensor. Drivers' side exhaust manifold. It can fail long before it ever sets a code. There are O2 testers, and you can hook up the laptop and check its voltage readings to see how they oscillate, but that one is likely the problem.

Check the FP while running and after shutdown, that will point out an injector leak, but I'd bet on O2.

JimLentz

Post questions here as you think about the best way to troubleshoot this issue. There are a lot of great C4 troubleshooters on this forum.

vetteLT193

could also be missing a couple of cylinders. The old GM computers did a very good job masking misfires. My brother drove a 1500 chevy truck for a couple years on 6 out of 8. when he found out and fixed it he was pretty blown away by the power.