Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 51

by Bob Hagin

Q. I own a 1990 Dodge Caravan with a 3.0 liter V6 engine and 220,000 miles. The valves and cylinder heads were replaced at 150,000 miles and the transmission was replaced at 44,000 miles. When it hit 207,000 miles, I had to have this replacement transmission rebuilt. I have the oil changed every 3000 to 4000 miles, but now the oil pressure drops down to just above the low-level line after the engine is warmed up. My non-Chrysler mechanic says it's from engine wear and recommends that I use 20W-50 motor oil. Is this OK? Will this engine last 300,000 miles? J.S. Elizabeth, NC

A. Since I've never really trusted electric oil pressure gauges, I suggest that you hook up a hydromechanical pressure gauge to your engine just for a test. If the pressure it truly low at idle (like down to five PSI or so) it's possible that the cause is wear in the main, connecting rod or balance shaft bearings or possibly even a worn oil pump. It certainly won't hurt the engine to go to something with a higher viscosity like 20W-50, but change the oil filter at the same time. Having head work done on a high-mileage vehicle sometimes creates more problems because it puts as-new compression and pressures on worn engine bearings. It's hard to say if your engine will last another 80,000 miles, since it depends a lot on who drives it and how. Hard driving by a teen-ager will lead to its demise quicker than if an old duffer like me drives it. If you're close to a major airport, you might want to locate a testing lab that could do an analysis on a sample of your oil after a run of 3000 or 4000 miles. This technique is often used by aviation shops to catch potential problems without having to do exploratory surgery on a powerplant.

Q. I have a 1966 Chevrolet 3/4-ton pickup truck with a 396 cubic-inch engine. We use it mostly to pull our 28-foot trailer. The truck runs very good and still goes over the steepest passes at 55 MPH. The truck has a slight backfire - a kind of "putt-putt" noise - going down hill. If I step on it a little, it stops but it starts again when I take my foot off the gas pedal. This doesn't happen on level ground. Could you please tell me what causes this and is it harmful to the engine? S.S. Enumclaw, WA

A. When you're decelerating down hill (especially pulling a load), the manifold vacuum goes to its highest value (as high as 30 inches of mercury) and it can pull in lots of fuel from the various carburetor systems. It may not get burned until it leaves the exhaust system where it explodes. I don't think it can hurt your engine since it doesn't create a catastrophic "boom," but you might want to check for a slight vacuum leak in one of the vacuum hoses and if your engine has an early-type G.M. air injection system to control emissions, it may be pumping excessive air into the manifold on deceleration. If it worries you, have a mechanic check out your exhaust and manifold vacuum systems.

Q. Our 1991 Honda Accord four-door has 77,000 miles on it. The engine is a four cylinder and the transmission is an automatic. It developed a slight noise in the area of the right rear wheel, so we took it into our shop to have it checked out. The mechanic said that the wheel bearing on the right rear wheel was noisy and should be replaced. We have had the car maintained very carefully and have followed the recommended schedule in the owner's manual. What could have caused these wheel bearings to go bad with such low mileage on the car? S.H. Danville, CA

A. Unfortunately, the rear wheel bearing dust caps on those cars were built wrong and allow moisture and other contaminants get into the bearings, causing them to corrode. The repair is to replace the bearings, axle nut and dust cap on the noisy side, but check the other side as well. If it's OK, just replace the axle nut and its dust cap.