14 mpg. What is wrong?
 

94 Trans Am. 2.73 rear, auto. stock motor, flowmaster muffler is the only add on.

I have been averaging 14-15 mpg even when I drive easy. This was with a mix of slight traffic, hills, and interstate. Best I got was 19.

Tire pressures are good, new fuel filter, clean MAF, new plugs and wires, clean air filter (K&N with stock air box), no codes.

Does have a little bit of a rough idle and a slight exhaust leak on passenger side, sounds like manifold gaskets.

Any ideas? Surely this thing should be getting better mileage.

Compression test? Try cleaning the injectors?

02 sensors only last so long before they degrade.

My 2.73 car got pretty close to 14mpg. Very short drives, lots of stomping. Now it gets 14.5ish with only slightly less stomping and slightly longer drives.

Seafoam it....:)

-Ivan

You didn't state the mileage of your car and I have no idea of the conditition of the engine besides the tune-up parts you mentioned, but from the information you gave..

If you have an exhaust leak at the manifold, it will suck in air making the O2 sensors think it's lean, then it will add fuel it doesn't need. Also like the other guy says, if you have a lot of mileage on them, your O2 sensors probably aren't in the best shape and you could have a bunch of other things like loose intake manifold or bad gaskets and leaks too, contributing to the problems.

How many miles are on the car? there's lots of thins that can cause a car to get bad mileage. Bad driving (not timing the lights, getting on the interstate for 1-2 blocks, etc.), 02 sensors, old distributor (opti), etc.

I'd start by looking at the condition of the 02 sensors, but a tune up might not be a bad idea. Fuel pump, differential fluid, radiator flush, plugs and wires, etc.

you gave very little info about the cars shape. Theres only about a thousand other possibilties that could be causing the bad milage.

Hows the suspension? Tires? Miles? etc, etc.

wow I cant believe I forgot to put down the mileage. 114k on the car, new transmission, suspension seems to be fine, shocks and strutts are still in good shape, tires only have 3k on them (255-45-R17). Engine runs pretty good, doesnt burn oil, doesnt drip oil, underneath looks pretty clean, alittle bit of oil, but not enough to show up in the driveway.

Thanks for the help, that gives me a couple places to start checking. The opti always worries me with these cars. How can you determine if you need new 02 sensors? Or is it just one of the things you replace after awhile? Ive always just let them go until they throw a code.

Anything else I can answer?

One thing that I am starting to think is that it is actually the 2.73 gears. Im sure in flatland with alot of interstate travel, they would be great, but Im thinking since I am more in the mountains, that the car has to shift too many times. That make any since? Just something I have been thinking about. 3.42 swap maybe? just a thought.

thanks for the help guys, Ive had plenty of 3rd gens, but this is my first 4th gen (well except for the wife's v6). Quite a few differences.

Where are you located? Is this a problem in cold weather, or all the time? Cold weather eats up more fuel with richer/extended cold starts. Cold weather blended fuels contain additives for smog control that reduces mileage. The exhaust leak is definitely causing lost mileage, as Kevin pointed out.

What do the BLM's look like on the scanner?

First off, as said before, get the exhaust manifold leak fixed. It is definitely not helping your mileage. The O2 sensors, have they ever been replaced? They have a manufacturers recommended replacement schedule (usually around 60-75K miles IIRC). I replaced mine (even though not throwing any codes) when I did the mid-length headers and some new valve springs and roller rockers. Mods dropped 7/10ths off 1/4 time and I can still get 19.5 mpg with 3.73 gears (A4) on the freeway.

Injuneer: I am in southwest Virginia. Mostly around Blacksburg (atleast for one more semester), and I have no idea what the BLM's are much else what they look like. Sorry, Im new to EFI. Oh and I have only had this car for alittle while, I have no idea if it is just a cold weather thing or not.

Z28Nut: I will definately check out the 02 sensors, thanks.

Everyone: How hard is it to replace manifold gaskets, Im scared thinking about it. Header gaskets on my 88 isnt that bad, Im sure I can do it, it just looks like a pain.

possible culprits would include but are not limited to: bad/dirty maf, bad fpr, bad o2's, vacuum leak, clogged cat, etc

exhaust leaks will introduce air into the exhaust system and will affect the O2 sensors. This will cause the O2s to read a lean condition and automaticly enrichen the fuel mixture which will lead to POOR milage/ per gallon

thanks for the help guys. I done some work on it and now it averages around 20 for me. Thanks for the help