Letters: Fuel level unit culprit in fuel gauge needle bounce

• Reprints

Updated: January 8, 2013 2:39PM

Q: The fuel gauge in my 2002 Buick Rendezvous jumps between empty and full. I’d really like to know how much fuel is in my tank. Sometimes after I put around $20 worth of fuel in the tank the low fuel message comes on. This problem has been there ever since I purchased this vehicle. What do you think could be causing this? — Eric, Chicago

A: The fuel level sending unit in the fuel tank is your likely culprit. The fuel level sending unit consists of a float, a linkage arm, and a variable resistor. There could be something wrong with the resistor part of the sending unit. But since there’s no access hole provided to test the sending unit, the fuel tank will need to be removed. Once the tank is removed, the fuel level sending unit can be inspected and tested.

Q: My 2000 Nissan Altima (2.4-liter) stalls out when coming to a stop. When this happens, the engine will start up again right away. I don’t know what the problem could be. What do you think? — David, Chicago

A: It’s possible that the throttle bore and/or idle air control valve could have an accumulation of dirt or sludge on them. If that’s the case, they should be thoroughly cleaned. If that’s not the problem, there could be a vacuum leak or there could be something wrong with the air flow sensor.

Q: My car’s check engine light stays on. I took it in to have the code read and was told that my car has a slow vapor leak. I’d like to know if this is costly to repair. — Burt, Chicago

A: That depends on what the leak is. If it’s just a leaky vapor hose or gas cap, it shouldn’t cost too much. To find out what’s leaking, though, you’ll need to have the evaporative emission control system tested with a smoke machine. A smoke machine allows the technician to pinpoint exactly where the leak is originating and saves the expense of replacing unnecessary components.

Q: Sometimes there’s a clunk when the transmission in my 2008 Ford Taurus SEL shifts from first to second. Nobody has been able to diagnose this. What could be wrong? — Daniel, Cicero

A: The first thing that should be checked is the automatic transmission fluid level, as well as the condition of the fluid. If the fluid level is low, make sure the proper type of fluid is added. If the fluid looks dirty, or is possibly the wrong type from a previous service, replace the fluid. If the clunking isn’t the result of low fluid or worn or improper fluid, the transmission’s line pressure control (LPC) solenoid should be checked. This solenoid is responsible for regulating the line pressure within the transmission. Other possibilities include a problem with one of the shift solenoids or a problem within the valve body.

Questions for Ira Siegel? Call the Auto Advisor Hotline at (708) 633-6839 or email ira.siegel.is@gmail.com. Siegel is an automotive instructor and an automotive service excellence-certified master auto technician.