Some vehicle repair issues need help of a certified technician
Q: My 2002 Chevy Suburban has 116,000 and has been giving me trouble with service codes for random misfires and the knock sensor. The Chevy dealer informed me that the Suburban's catalytic converters were clogged and causing problems. They put in new catalytic converters and oxygen sensors, but I'm still getting the same trouble. I had a different mechanic change the fuel regulator, and put in new knock sensors, new wires and plugs, but I still have the problem of random misfires. (The knock sensor code is resolved). The SUV runs very rough when stopped. I'm at my wits end and running out of money. Please help.
Mike A: For the catalytic converter issue, a simple backpressure test will identify if there is a blockage in the exhaust. A qualified technician with a scan tool can monitor all vehicle inputs and analyze them. The sensor input and values will also help the technician know what area to look into. A cylinder balance test will identify the cylinder causing the rough idle. A qualified technician will also be able to check the engine. With the ignition system disabled, crank the engine over and listen to hear if the engine is cranking over evenly. A visual inspection of the spark plugs and compression test can also tell the story. It sounds like there are a lot of inaccurate diagnoses. As an ASE Master and L1 certified technician, I would suggest finding one in your area. We take great pride in the American Society of Engineers certified program.
Q: I have a 1995 Mercedes S500 coupe with 62,000 miles and it's in great shape. For the last six months when I turn the key to start the vehicle I get no response at all. This happens only occasionally. All the dash lights come on fine and stay on when I turn the key to start the engine, and it will not start in neutral or park. The engine does not crank; I just get silence. If I keep trying, eventually it starts normally. I left the car with my mechanic for several days, but it always started perfectly.
Leo A: I cannot pinpoint the source of the problem without knowing if the starter motor is getting power when the ignition key is turned to the start position. Have your mechanic hook up a test light to the S terminal on the starter in view of the driver. This small bulb can be located at the bottom of the windshield area. If the light does not go on when the key is turned to the start position, then you will know the problem is not the starter. (If the light goes on, then the problem is the starter). The next step is to trace the circuit back to all relays, neutral safety and the ignition switch.
Q: I own a 2001 Nissan Maxima with a 6-cylinder engine. The vehicle has 165,000 miles on it and I had a tune-up in 2004. The airbag warning light comes on and stays lit. A mechanic suggested that the problem could be a faulty wire or a fuse. I need your opinion on this problem. My bigger problem however is a stalling engine. While the car is stopped either at the traffic light or in the parking lot, the engine stalls. Sometimes it's after a quick hesitation, and sometimes just as soon as the vehicle comes to a full stop. I have suspected it was the battery but the body shop re-charged the battery. Any suggestions would be helpful.
Nicholas A: The airbag system has many electrical safety sensors that all need to work together. Each time the ignition key is turned on the system does a self-test. If a problem appears in the system, then the airbag light will illuminate. A technician will need to connect a scan tool to the connector under the dash. The computer system will show the fault codes. The drivability concern will also need to be checked with a scan tool and the technician will observe the computer readings. When we get a car such as yours we will first road test the vehicle, hook up a scan tool and fuel pressure gauge and observe readings and compare them to the factory specs. Our next step is to check the Identifix database for similar complaints on the model. Dirty throttle body carbon deposits often cause stalling as will a lazy EGR valve. In some automatic transmission vehicles a lockup torque converter failure can cause stalling when slowing down.
Q: I have switched to synthetic oil. I normally purchase the synthetic oil by the case for the discount. My car only requires 5 quarts of oil and the case contains 6. Now I have left over one quart of brand "C," one quart of "V", one quart of "M" and one quart of "P". All are 5w-20. Can I mix the different brands on my next oil change?
Mike A: I would like you to use the same brand and viscosity. The oil manufacturers each have their own secret blend of additives, which differ from one brand to the next.
Junior Damato is an ASE-certified Master Technician. E-mail questions to info@motormatters.biz. Mail questions to: Auto Doctor, 3 Court Circle, Lakeville, Ma., 02347. Copyright 2008, Motor Matters. The Auto Doctor