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Q&A: Premium gas may clear up knocking noise

QUESTION: I have a very clean 1994 Cadillac DeVille with only 70,000 miles. It was my mother-in-law's car from Florida, and it has developed a problem. While under load and light acceleration, it has an engine knock. It does not have a knock before the car warms up and does not knock while accelerating in neutral. What do you think the problem is? Could you give me a rough estimate of what the repair cost would be?

ANSWER: Maybe as little as a tank or two of premium unleaded fuel. Most likely, you're hearing a spark knock. The low mileage for this 16-year-old vehicle means it wasn't driven much and most times just for short trips. Thus, the spark knock could be the product of carbon deposits in the induction system and combustion chamber. Higher-octane fuel might help quiet the knock, and a thorough induction cleaning and decarbonizing might eliminate it. On a do-it-yourself basis, SeaFoam works well for this job, and the service is also available on a professional basis from most shops.

If the knock is mechanical -- unlikely, but a possibility -- it would mean a serious internal engine problem, and repairs could cost more than the car is worth. A mechanic's stethoscope would help pinpoint the source of the noise.

Q: I have a problem with our 2003 Expedition when I have the air conditioner on and set to a low temperature. When I accelerate, the fan keeps blowing, but the dash vents close and the defrost and floor vents open. If the temperature is turned up so the heater comes on, this does not happen.

A: Assuming your vehicle is equipped with a climate control system rather than a manual heating, ventilation and air conditioning (HVAC) system, the most likely cause for this is a loss of engine vacuum from the vacuum-operated mode doors during acceleration. With the air conditioner set to a colder temperature, the compressor adds to the burden on the engine, which may cause lower vacuum levels during acceleration.

A shop can check for HVAC fault codes with a scan tool and physically test for vacuum leaks at the check valves, vacuum reservoir, vacuum control motors and vacuum lines.

Q: I own a 1991 Lincoln Town Car. In the past month I've spent $600 with four mechanics, and it still has a problem. It runs great but then sputters and quits every time it sits in drive at a traffic light. They've replaced plugs, wires and positive crankcase ventilation hose, and cleaned the exhaust-gas recirculation and idle-control systems. It shows no codes from the computer. Everyone has their guess, no one's been right, and I can't keep putting money into it and it not getting fixed. I love the car and want to keep it.

A: Has anyone checked the coolant temperature sensor? It may not have completely failed, but if it does not accurately signal the computer that the coolant is fully up to temperature, the engine can run quite rich. Even a thermostat that doesn't regulate coolant temperature in the normal 180-220 degree range can cause this symptom. In either of these scenarios, the engine will start fine cold, run reasonably well under load but not idle well. Fuel mileage would also suffer.

A leaking fuel pressure regulator or fuel injector would also cause a rich idle, and might also make the car hard to start or restart hot.

None of these problems would necessarily generate a fault code on the older on-board diagnostic system on your nearly 20-year-old vehicle.

Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Readers may write to him at: Star Tribune, 425 Portland Ave. S., Minneapolis, Minn. 55488 or via e-mail at Please explain the problem in detail and include a daytime phone number. It isn't always possible to send a personal reply.

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