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Diagnose faulty gas gauge

| Saturday, Nov. 10, 2012, 12:01 a.m.

Q I have a 2005 Scion XA with 45,000 miles. I am the original owner. My problem is my gas gauge. In the past year, it has failed twice to indicate the amount of gas in the tank. This goes on for about 4-5 days and then it clears itself and operates like nothing happens. Is there an additive that I can put in with my gas to clean the sensor in the tank? Is there a mechanical fix I could do? The gas I use is Sunoco 89 Octane.

A Your Scion employs an integrated fuel pump/gauge sending unit within the tank to deliver fuel to the engine and indicate fuel level. You didn't mention what the fuel gauge indicates when it's misbehaving, so we'll look at all three possibilities.

If the needle reads too low or drops to empty, this could be caused by a faulty electrical connection — such as an open circuit or high resistance — either within the sending unit or somewhere between the sending unit and the instrument panel. If the needle rises to full or beyond, a short to ground in this same circuit could be the cause; it's rare but possible. If the gauge continues to indicate perhaps one-quarter full, yet you run out of fuel, the sending unit's float arm may be hanging up on the fuel pump's strainer sock; it may be mispositioned or has gotten distorted.

It's unlikely a fuel additive would help restore gauge function. During one of these episodes, a quick test of the gauge circuit can be made by removing the back seat lower cushion and accessing the fuel pump/sender electrical connector, beneath the fuel tank access hatch. If opening the gauge circuit (unplugging the connector), then jumping the violet to brown wires of the car-side of the connector together results in an empty, then full gauge reading, the fault is within the sending unit.

Renewing a faulty sending unit/fuel pump assembly is easy, thanks to the access hatch. Vehicles without this feature require tank removal. If the sending unit or fuel pump is to be serviced or replaced, be sure to first disconnect the battery negative terminal and open all doors to vent dangerous fuel vapors.

Q I bought a new 2010 Acura TL and have enjoyed it very much except for the road noise, especially on rough freeways. A service tech told me a change to a different tire could make a difference. I have Michelin Pilot HX MXM4 tires. Does this make sense? I know you don't recommend aftermarket undercoating but would it help in this case?

— Rudy

A Some all-season tires are quieter than others, and noise often increases with age due to tread wear irregularities and rubber hardening. After checking many tire reviews, it seems folks prefer the Michelin Primacy MXM4 over the Pilots for a quieter ride when opting for a similar tire for replacement. If considering a choice of tires, you'll want to contemplate summer or all-season, performance (traction and handling), tread life, rolling resistance and comfort (firmness and noise). The choices are dizzying, and a recommendation from a tire expert, addressing your concerns, would help you make the best choice.

I don't see a problem with applying a spray-can underseal product to the wheel wells to dampen tire noise. Clean the wheel house first and avoid sealing areas where water needs to drain.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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