Under the hood: Faulty fuel readings can mean a failing pump

Q. On my 2006 Pontiac G6, I think there might be a problem with my gas gauge. Once it starts moving away from the full mark, it seems to move quickly toward empty.

At times I can see the needle moving back and forth. Do you think the fuel pump is bad, or the gauge is defective?

I checked the air filter and it looks new.

A. Thanks for your question. It seems you are on the right track with your diagnosis.

I would suspect a bad “fuel sender” as well. The fuel sender is part of the fuel pump assembly, so you would have to replace the whole thing. There are some diagnostic tests that can be performed to be certain and prevent the replacement of an expensive part that you don’t need.

On rare occasions there could be a problem with the gauge itself, but it usually is a failure in the fuel sender.

Once the mechanic confirms the problem is indeed the fuel sender, your shop will begin the process of replacing it by removing the fuel tank from the car. Once the tank is out they can remove the old pump and install the new fuel pump assembly.

You can make the job a lot easier if you bring your car to the shop low on fuel; otherwise, there is another step the mechanic will have to go through first to remove the fuel from the tank.

Remember it is important to make a habit of keeping fuel in your car. Running around with an empty gas tank is very hard on the fuel pump assembly as a whole, and can lead to premature failure.

Fuel mileage tip of the week: Low tire pressure is a fuel-mileage killer, and a tire killer, for that matter. But let’s stick to fuel economy.

Underinflated tires can reduce your fuel economy by .3 percent for every pound PSI drop in air pressure on all four tires. By keeping your tires properly inflated you will not only save gas, your car will ride better, be safer and your tires will last longer.

You can find the proper tire inflation pressure on a sticker on the driver’s door jamb.

Ÿ Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.