Sudden drop in gas mileage.
 

O2 sensor has only close to 11,XXX miles on it. Spark plugs are about the same as well as air filter. No CEL code. Nothing was tampered with. Fuel/Air mixture is as it was when car came from the factory. I usualy would get anywhere from 280-300 miles on a full tank of gas (city and highway respectively) now I only get 240 miles city. Any suggestions

Any loss in power? hesitation? missing?

no loss in power, no hesitation, no stuttering or misfiring.

winter gasoline,more alcohol? If temps changed because of winter its normal

Rarely get winter get here.....temp is usually mid 60s to 70s during winter months

even in cleveland winters it does not drop that much.

Id check the duty cycle to see if it goes in to closed loop. Also, check you vacuum to make sure you have not sprung a leak.

Obviously duty should be around 55%, Vacuum should be roughly 15hg. 13hg is minimum.

Ismalley, pull the distributor cap and check the condition of the terminals and the rotor ... also, look for any tracking inside the cap. Verify that all ignition wires are securely fastened to the distributor cap and spark plugs.

He states no misfires,or hesitation, so I doubt that would help.

Ismalley, purchase gas from a different source and see if your gas mileage improves.

I don't think you can get an accurate mileage reading from just one tank. Realistically, one should fill up and note the mileage. Then record the gas used over the next few fill ups. Then fill it up to the top and note the mileage. This way, you even out all of the possible problems with a single filling.
Anziani,
'93 E420 128K
'97 S500 coupe, 135K

I agree - I keep a log book and write down every fill-up. It could just be that you didn't get the tank full last time you gassed up.

He said in the first post he usually gets 280-300 per tank, now he got 240. By reading that one would assume he keeps track of fill ups.

Are you guys posting just to increase your post count?

The point is that he shouldn't panic and go tearing into the car based on the results from one tankful, but thanks anyway for your helpfulness...

I never recommend tearing in to anything. I always recommend testing before throwing money around.

If the vacuum and duty cycle check out,(free to check) then I would say it could be a bad tank of gas.

He states a drop of 40-60 miles on a tank of gas. Have you ever under filled your car by 2-3 gallons? Really?

Give or take 1/2 gallon is more reasonable on a fill-up.

I have kept track of my fill ups, which is how I got my averages of 280 miles city and 300 miles highwway. I'm positive that all ignition components are fine, as Lorainfurniture stated, there is no hesitation or misfire. I checked my spark plugs weeks before the issue occurred just because I did a valve stem job and wanted to make sure they were not being coated by oil (this caused me to fail smog)....dry as a bone. What I did do is purchase some seafoam and poured it into the gas tank the other day to see if it would help. Hopefully it was just dirty injectors or something. Thank you all for your input. If seafoam doesn't clean out injectors or if the problem was something else, I will do a search on testing duty as I'm not familiar what that is or how its done.

I tried sea foam and I think that stuff is not good. Try buying a bottle of redline fuel additive. It can be found on amazon. I tried everything first, and the redline was the only stuff that made a dramatic difference on the first tank.

that happened to me in my SD.I set the parking brake one to many times she stuck,and my mileage went down.I spend money before I knew this,then smelled the brakes and knew it was this.Fixed the brakes and mileage came back,then a year later back again.This time it was a stuck caliper,one I replaced by two months.That happens with calipers I see now.You might feel your rear wheels or fronts after a drive.

Thanks for the heads up!