Mechanics

GAS MILEAGE SUDDENLY DROPPED BUT GAR

1998 Pontiac Transport

Other Category problem
1998 Pontiac Transport 6 cyl Front Wheel Drive Automatic 206300 miles

I used to get about 700 - 750 km to a tank of gas when using regular grade. I switched to premium a few years ago and have generally gotten around 900 km, sometmes slightly higher, to a tank especially if mostly highway driving. Three tankfuls ago I only barely got to 700 km on a tank. The next tank about the same. I had my garage look at it, run tests etc. They couldn't find any problems with their computer or visual checks but changed the oil, air and gas filters just in case. Tire pressures were right on. This tankful is worse than before as I will likely not make anywhere near 700 km on it, always with premium gas.

What could cause my van to suddenly give such poor mileage and yet run perfectly and show nothing wrong on computer or visual checks? What further tests can be run to isolate the problem? Thank you for your time. I have another question. Not sure if you will answer it here or require me to return to this page.

I have a problem with one of the wheels or tires on my van. It is causing a strange up and down type of vibration. I have had two reasonably new tires exchanged for two brand new ones. The problem persists. People have checked the aliginment, wheel balance, wheels out of round, tire separation etc but not been able to find it. One test drive resulted in someone telling me that my front rotors needed to be machined. Another mechanic said it couldn't be that because there's no vibration while braking. Any suggestions? Again, thank you in advance.
Avatar
Paperpotamus
December 17, 2007.



For the loss in gas mileage it may be time for a tune up, changine plugs, wires, air filter, pcv filter fuel filter. Next check for leaks by going under it and smelling for fresh fuel.

For the tires jack up the van and slowly turn the weel by hand and wiggle it, if play is noted you have bad wheel bearings.

Blackop555
Dec 17, 2007.
Based on what I have paid for, all the things you've mentioned have been done. Tune, up reasonably recently, changed plugs when tune up done, all filters have been done, leaks checked for etc.

As for the wheels, bearings were the very first thing they looked at and did just what you suggested. No problems were observed. Thank you for your suggestions tho.

This particular garage has been very good. Owner/operator has excellent reputation and has serviced this vehicle for some time.

Back to you.

Tiny
Paperpotamus
Dec 17, 2007.
If you have water in the gas and you fill it up with gas the water mixes with the fuel and when ran it requires more fuel to even out the water so the car isnt running too poor a fuel mixture.

Your car is getting up their in the miles and although there is great maitnence on the car it will eventually begin to use up more fuel. You may have a dirty MAF sensor that is just dirty enough for a rich fuel mixture but not tho through a code.

Rotate the wheel on the car and look and see if the wheel is spinning straight.

Blackop555
Dec 18, 2007.