mfinkler

I bought a 2004 GT this past spring. It is an automatic with 327 gears, no cats and a spectre air filter. It had about 107K when I bought it. Of course I get a 420 from not having cats or rear sensors so I just clear the code. The car runs great. I drive it 99% city driving. I have checked the MPG's almost after every fill up and consistently was getting 18.4 and on one road trip I got about 20. I do not have a heavy foot, although I did once drive it like a race car for about a week just to see what would happen and I got about 12 MPG. I recently had to have the Battery and A/C Compressor replaced and I did an oil change using using Motorcraft 5w20 and filter. I have no idea what oil was in it when I bought it. I checked the MPG's after the last 4 tanks and now I am only getting 16 MPG. The car seems to be running exactly the same. I always buy my gas at the same place. Could a new compressor have caused this? The A/C is working great and it appears to be cycling like normal. I have no idea if the plugs are original. I now have 111K on the car. It isn't skipping or anything. I cleaned the air filter and re-charged and cleaned the MAF while I was at it. Any ideas? Thanks

99GTvert

Was the decrease in MPG immediately after the new a/c compressor and battery were installed? If so then I wonder if the compressor has a greater (or heavier) load to turn which means more gas is needed to maintain the same speed/accelerate/etc.

Did the a/c work at all before? Because I also wonder if the old unit was gutted or something, meaning it had less drag than a new unit so that it ultimately drained less than the new one.

How about your tire pressures?

Maybe your plugs need to be replaced.

mfinkler

Well, my daughter had to drive the Tang for about 3-4 weeks after the compressor was put on as her car was being repaired so I am not sure if it started right after. The original compressor was working great right up to the time the clutch locked up. I checked tire pressure and all is good. Thanks for the response.

kast1376

Derf00

Sounds like you need to so some regular maintenance as mentioned by Kast, Fuel filter, spark plugs maybe, air filter.... Start with that.

Z28KLR

Get a can of MAF sensor cleaner (there is a specialized product just for this, use no substitutes) and clean the MAF. Install a new fuel filter. Clean and re-oil (not too much oil) that air filter. New spark plugs. Basic stuff....

08'MustangDude

Steps:

Clean MAF
Replace Plugs
Change the oil of at the OCI
Fuel Filter
Air Filter
Check DIFF gear oil, change with appropriate weight if needed.

I got better MPG with new plugs.

Tuning:
I get better MPG with the 93 Octane K&N FIPK tune -vs- 87 Performance Tune,
both using the modified stock air-box.