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Can anyone shed some light on a sudden drop in gas mileage.
Car runs great, no diagnostic codes.
Everything is stock
Replaced fuel filter, wires, plugs....
Cleaned the MAF & IAC
Just over 100k miles
Literally overnight it dropped from a consistent 20mpg (mostly in town driving) to 14 mpg.
Baffled as the car still runs very well....this has been going on for some time now

Any thoughts are appreciated.

What about your 02 sensor? You haven't mentioned anything about replacing that. If so, when was the last time? If you tell us all the maintenance that you've done since owning the car, it would probably be easier to narrow the possible causes down. Also, have you changed gas stations that might use the ethanol mix?

have not replaced the O2 sensor
for the most part in my area I use Speedway gas
any recommendation on a particular brand of O2 sensor?
As far as maint more or less just the routine maint other than changing the plenum recently but the mileage issue was present long before that issue
guessing the O2 sensor could be just enough out of spec to affect the mileage but not enough to throw a code?

Delco or Denso for the sensor. I prefer Delco, as my Denso went on me in 20k miles. So far so good with a Delco that appeared better-made than the Denso it replaced. Don't do Bosch, no matter what any parts guy tells you. At least not on these cars, since they chew up Bosch plugs and O2's for breakfast.

Add Borg-Warner O2s to the list of don'ts. I went through 2 of them in four months.

I forgot that I did also replace the EGR years ago - probably 5-6 yrs

I'll try and O2 and see what happens

THX

I'd check for vacuum leaks. Listen for a hissing, and spray TB cleaner in general areas of gaskets between parts. If the idle changes/hiccups when you spray a certain area, there is a leak hiding.

Let us know what you find out with your car.

did you check your temp sending unit.. the pcm controls the fuel / timing with one of the factors is temp... also note that happened to me ended up being the temp sensor even though my temp gauge worked it was off enough to cause huge fuel reduction.. just my two cents

That may be worth a try. I had an issue on our 02 Rendezvous (3400) where the Coolant Temperature Sensor was flakey. When it heated up after the car had been running for a while, it stopped reading accurately. I couldn't tell from the temp gauge on the dashboard. I finally figured it out when somebody suggested I check the sensor in real time with a scan tool. I was having other strange driveability issues besides reduced gas mileage though. I'd get a fast idle when running the A/C and reduced engine power resulting in constant downshifting on the highway.

jbugz wrote:did you check your temp sending unit.. the pcm controls the fuel / timing with one of the factors is temp... also note that happened to me ended up being the temp sensor even though my temp gauge worked it was off enough to cause huge fuel reduction.. just my two cents

The thermostat could have failed into the open position or just become so weak that the water pump easily pushes it open.

I've noticed my fuel economy will drop 2-3 mpg when I use an ethanol blend. It was even worse with my Aurora and my Deville, which both had Northstars that preferred premium. Even if you're filling up at the same stations, the gas formulation could change on you without your knowledge. This is the time of year when many refiners begin switching over to summer blends. But its usually the oxygenated winter blends that produce fewer emissions but worse mileage, so maybe things will improve once you're running summer gas again.

By formulation you mean the chemical make up and not whether it contains ethanol or not right? I ask this because I thought they had to post on the pump by law if the gas contained any percentage of ethanol.
Sorry not trying to highjack this thread, just curious.

The thing is, the pump says it may contain up to 10% ethanol. It could go from 6 to 8%, or whatever, at any given time. They change the additive packages starting about now, and that affects mileage. Even on E85 pumps, it says contains at least 70% ethanol. It could be anywhere in between...

Ok that makes sense to me. Festpace have you had the time to check anything out? Hope your able to get it dialed back in again.

hadn't thought about a temp sensor - thermostat has been replaced
tried checking for vacuum leaks "before" but who's know may have missed something
working on the O2 part now - it's only drivern 3,000 miles a year so may take a little while to figure it out

Gas price is just getting to the point I'd like to get it figured out even though it's not driven much

Will advise - Thanks for all the ideas...............

I have a similar issue too. Although my car is garaged until I fix the manifold issue, it drives fine otherwise yet the fuel guage drops dramatically in only a half hour of driving (granted, I have a pretty heavy foot, but it seems way too thirsty). Coincidentally, my car is throwing a code and last scanned, it was a bad O2 sensor......I suppose I should be replacing that soon, lol.