Bad gas milage suddenly... What should I be checking?

[hyperzone]

**First, please dont make this a thread just to post you MPG and split. hehe.. I know whats normal and what is not by now with these cars.

I normally get 16-18mpg with around town driving. My last two fill-ups have left me worried because the math shows its now suddenly dropped to 10-11mpg. Even with this problem, the car drives great with no codes or obvious problems.

I have Autotap software but, am very new to using it to diagnose my issues. I was looking at it a few days ago on a highway trip (about 40miles) and from my limited knowledge/experience, things look normal.

MY QUESTION : What are the key things to monitor for this kind of problem and what should the readings be if everything was really in good order vs. bad?

Thanks.

[95LT1Z28]

As far as diagnosing, im not sure what you should look at on the autotap. But I would just check over the simple tune up stuff. Maybe the fuel filter is clogged up a little, or the injectors are getting lazy opening and closing.

[Z28pr0jekt]

Fuel filter/plugs/wires/fuel pressure regulator/ O2 sensors/ clogged air filter

[BLOWNDFIZ]

I was going to say winter formula fuel, but seeing you are in Florida that is probably not the case. Also, your mileage decrease is huge. Mine only decreases 1-2mpg during the winter.

[hyperzone]

^^ Yeah, I have never seen a drop in milage like this. Even in the past when I had bad exhaust leaks, it only dropped a little.

My fuel system is original stock components (have changed the filter before though). As for pressure and injectors, not sure on how well they are, never checked into that but, the car runs great with no hick-ups or any problems so, wouldn't there be some other drivability symptoms if that were the issue?

I dont want to sound ungrateful, I appreciate the input guys. I will be looking into all of this pretty soon when I have some more time off. I mainly posted this in the tuning section to get some info on the scanning software and what readings I should be seeing vs. what it may be telling me right now.

[ksmyss]

i would start with 02s, plugs, wires, fuel filter, air filter, injector cleaner.

[MEAN LT1]

I would mainly look at the o2's and see if there cycling correctly. You could have a dead o2 and the pcm hasnt noticed it yet. Do you still have the factory smog equipment on the car?:cool:

[KissMyWhtSS]

Best start (as stated) is to check the plugs and 02s. Is the exhaust noticable rich/lean? Also when was the fuel filter changed last?

[Ponch95]

Best start (as stated) is to check the plugs and 02s. Is the exhaust noticable rich/lean? Also when was the fuel filter changed last?


Don't forget the PC valve as well.

[LT1RUNNER]

Don't forget about the tires!! I hadn't checked mine in a little while and when I finally did I was about 10psi low on three and 15psi low on one. :shiner: (fricken noob!) :monkey: Made a dramatic change in mpg.

["97z]

most likley causes for that are fuel pressure regulator or o2 sensors. pull the vacume line off the regulator and see if it leaks fuel out. second take your scanner and look at your long term and short term fuel trims they should be at zero +/- no more than 7 percent either way if so make shure o2 sensors are switching properly. hope this helps, adam

[joelster]

O2's

[97ForMuLa]

would the rear o2 (post cat) be doing this too? or only pre cats?