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MPG's are dropping quick

drive a 2004 Jetta, and currently has 48000 miles but my MPGs for the city has dropped from 26-27 down to 14mpg. Has not missed any services. The symptoms started in the summer and the car had only 37000 miles on it. I took it to VW for a tune up and had the injectors cleaned, new fuel filters, new spark plugs, and air filter and added a can of 44k. That worked for one tank and now I am back down to 14mpg. ANY IDEAS?

Have you had the Jetta checked for sticking brakes/parking brake? Have you checked the air pressure in your tires? These problems can cause a plummeting fuel economy in a hurry. Have you changed the way you calculate mileage?

Brakes are fine, tire pressure is spot on, did not change my calculation method.

Thermostat is ok? Gets up to correct temp quickly?

Smell gas anywhere? Exhaust look or smell different?

Thermostat never came into question, but that could be it. It has had multiple diagnostic done with nothing being found, other than a bad O2 sensor which was replaced.

On a daily driver I like to change the thermostat every 5 years or so, along with a coolant change.

Could also be time for a new MAF/MAP sensor, and PCV valve.


Any check engine light on?

check engine light has never turned on.

coolant was changed when I did the tune up

I cleaned it, but never replaced it. VW told me it is good.

What exactly is your calcultion method?

It certainly sounds like the problem could be with that but I’ll reserve judgement for now.

Take the number of miles driven, divide that by number of gallons it took to fill the tank.


200 miles/20 gallons = 10 MPG’s

That’s the best way to calculate it but the EPA estimate for this car shows 21 in city driving. How did yours manage to best that by so much?

Had the car scanned for any codes?

At this point with limited info it would seem to me that if the engine were really running that rich there should be some codes stored. As to the CEL not being illuminated it’s debateable whether it would come on or not. In theory, yes. In practice, maybe or maybe not.

Drivetrain slippage at work? (Auto trans on the way out or clutch slippage with a manual transmission.)

To answer you EPA estimate for the car; the window sticker used the old estimate before the new estimate came into play. 25 city/30highway. Oddly enough the highway mileage has never been affected.

I have had numerous scans and VW told me everything was in working parameters; check engine light never came on and the only code found was a faulty front O2 sensor which was replaced at the time of the tune-up and fuel injectors cleaned, and 44k was added [love that stuff]. That worked for 1 tanks of fuel which got me back to 26mpgs. I had tried 44k prior to the tune-up and it didn’t do the trick.

Drivetrain is not slipping, but I think maybe the injector heads might be out of alignment, not positive.

Thanks for your feedback.