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Falling gas mileage

My wife and I have a 99 jetta 2.0L…we are the original owners…it has 233,000 miles on it …we have kept it in good shape…

But recently we noticed that the mpg droped about 80 miles per tank abruptly…so i replaced the spark plugs that were in bad shape but that didnt help at all…it runs the same we are just wondering what is the problem and how can we get that 80 miles per tank back?


Well - first, don’t judge your gas mileage according to “miles per tank.” There is plenty of stuff that can go wrong there. Fill the car. Drive it. Fill it back up. Divide miles driven by gallons to refill.

Second - your plugs were in bad shape. This suggests neglected maintenance items all around. So do a whole round of basic maintenance and make sure everything is up to date (esp fuel and air filters). A new set of wires to go with those new plugs can’t hurt.

Other than that, pretty much in this order:

  • check that your thermostat is working properly and check your coolant temp sensor.
  • check out your brakes for any signs of dragging from things like sticky caliper slide pins or pistons or whatever
  • be very vigilant about tire pressure
  • get an alignment if this has been neglected too

If you have never changed the oxygen sensor(s) now might be a good time. They don’t last forever and this is the part that controls the fuel mixture. As they get weak, they lose calibration and they tell the computer to squirt in more fuel than is necessary. They have an average life of 100,000 miles. You can’t rely on the CEL to prompt you to change them until they get REALLY bad and by that time have burned up the converter which must deal with the extra fuel…

You may also want to keep in mind that gas mileage will change with variations in outside temps and with “seasonal” gas mixtures. Alot, if not all, of the major gas companies mix different perecentages of alcohol to their gasoline during the summer and winter. kore alcohol content equals less miles per gallon.

Also, is this car an automatic or manual? Transmission issues can greatly affect mileage.

Basically, There are ALOT of factors that can decrease MPG.

basic maintenance is the key.

we judge our mileage per tank because we have kept a log of it ever since we bought the car …so i know what it should be getting and why i know that is suddenly dropped…one tank we got 460 and the next we got 380…and we do keep up on the maintenance i was supprised that the plugs were that bad because we paid a shop to replace them and they didnt…thank you for the suggestions

We have kept track of the gas mileage over the past 10 years…that is the only reason why i can rule out outside factors and little things like tire pressure because we pretty much have a longitudinal study on what this car gets as far as mpg
this is a manual
the mpg dropped suddenly within one week…and it has never done that in 10 years…and it feels like its running the same

The sudden drop in mileage may have occurred if you filled up with a 10% ethanol mix. I watch my fuel economy as well and some brands of gasoline, I suspect, are adding a little more than a 10% mix of ethanol (maybe 15%). Caddyman could be right also as bad 02 sensors can cause as much wasted fuel as ethanol.

Do You Ever Smell Gasoline ? Does The VW Park On Pavement ?

That gas is going somewhere. Sometimes gasoline leaks develop and only leak under pressure when the car’s fuel pump is operating. When it’s parked for a while, it stops.

I’d give it a good smelling after it’s been idling. After shutting the car off, I’d check the pavement and underside of the car for evidence (dampness) of a gasoline leak. Pay attention to fuel lines, especially from the tank to the engine and check the fuel filter area. There could just be a pinhole leak some place.

99 is just about the right model year for age related problems of this nature. We’re talking 11 - 12 years, here. Depending on where you live, that can qualify a vehicle for automotive “Depends” undergarments.


When was the last new air filter installed? A dirty filter will kill mpg.

Everyone has been driving on 10% ethanol for 2 years…

Replace the forward (at least that one) oxygen sensor, $45, and see if that cures it. You are LONG over-due on these parts…

What if your gas gauge is just screwy? Like other things they are no infallible. I’m not saying that your mileage didn’t drop - it probably did. Its just that if you want to sort it out for certain you have to cover all of the bases.

Have you considered getting a mechanic involved in finding out an answer to your concern? (one who can actualy see the car).

Old School, Who Needs A Mechanic Who Can Actually See It ?

Look at all the WAGs he’s getting. I’m sure that almost any one of these gems would fix that baby right up. Do all of them and the MPG would probably double. Besides, a mechanic looking at the car would put us out of business and spoil the fun.


thanks for advice caddyman the website would not let me reply to your answer…is the sensor something i can replace or should i take it in …is there more than one sensor?

maybe last year…that is good thing i can replace that is cheaper…i still wonder why it would drop suddenly…maybe caddyman is on to something with the oxygen sensor

You say you track mileage per tank…have you been running the tank to E repeatedly and refilling it?

I ask because this could be a clue to changes in mileage.

By the way, I recommend that you begin to track mileage in the traditional (and only accurate) way, by dividing miles traveled into fuel used. Tracking it by tankful provides nothing to reference the mileage to and we really can’t tell whether the mileage you’re getting is good, bad, or indifferent.

The jetta has a gas light that will come on when you get low enough on fuel…we base it off of when the light comes on because it is the same every time…plus we have kept track of it for so long we know how much we can drive after the light comes on…well when the light comes on we can fill the car 13.5 gal and efore we were getting 460 miles on that now we get 380…and my wife has been doing that for the past 10 years he same 13.5 gal and same 460 miles this car has been really consistant

so i replaced the spark plugs that were in bad shape

Were they blackened? oily? or just worn out electrodes?
You can tell a lot by the condition of the old plugs:

One of the most common reasons is a thermostat stuck open or a bad coolant temp sensor. Has the actual coolant temp and/or sensor been checked?