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Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 20

by Bob Hagin

Q. My wife has a 1987 Buick Somerset Limited with a 2.5 liter engine. When I start the engine it purrs great but with a slight miss. When I give it the gas, it backfires through the intake system, I think. and then it dies. It will start right back up and purr as before. With the transmission in Park, I can romp on the engine and it runs great. I took it to a local shop where they put it on a computer and said that it showed our Buick needed a MAP sensor. I bought one, installed it and it still did the same thing. The mechanic said that it wasn't the valves or anything like that and that I should take it to another shop. I have already replaced the spark plugs which I gapped correctly, installed new eight m.m. wires and new fuel filters. My wife now wants a new car.
K.S. Memphis, TN

A. Given enough time, a skilled mechanic can find the cause of your problem because the repair business involves around 60 percent observation and 40 percent perspiration. That "slight" misfire at idle should have given your mechanic a clue to tracing down your problem. An antiquated oscilloscope would be handy in pinpointing the particular miscreant cylinder at an idle speed. From there an even more antiquated compression gauge or cylinder leakdown tester could be used to see if the faulty cylinder is leaking compression and/or where. Backfiring into the intake manifold could indicate a leaking intake valve. Those 2.5 liter four-cylinder General Motors engines are "shakers" so it's possible that the intake manifold mounting fasteners shook loose and allowed its gasket to develop a leak. This could run a cylinder "lean" and possibly fry a valve.

Q. In September of last year, I purchased a new Chevrolet C1500 two-wheel-drive pickup. At 3500 miles, a roar developed in the rear end when I would reach 40 to 55 MPH. I reported this at my 3000-mile check at which time a ring and pinion gear was ordered. Late in December the new gear set was installed and it turned out to be worse than the first. They ordered another set and it was installed late in January. The noise now starts at 35 MPH and continues to 70. The selling dealer said that they could not repair it. I called the Chevrolet Customer Assistance and they too have advised that it can not be repaired. Do you believe this can be repaired and if so how?
G.W. Virginia Beach, VA.

A. Any problem on a vehicle can be "fixed" if the shop wants to go far enough to correct the fault. The problem with your truck could be a warped or mis-machined differential housing. The most fool-proof cure would be to replace the entire rear end, but traditionally auto makers prefer to pay a franchised dealer to do a gear replacement on the original unit. Apparently the factory paid to have the job done twice, washed its hands of the problem and left you to fight it out with your selling dealer. Setting up a new set of differential gears is a mean, tedious job and one that I never looked forward to. I've passed your letter on to Chevrolet public relations to see if that office can intervene on your behalf. Let me know if it works.

Q. What would cause the oil light warning system to indicate low oil pressure when there is plenty of oil? I had the oil changed on my 1990 Mercury Cougar and after 700 miles, the low oil indicator light went on. The Mercury mechanic told me that he's seen it before and replaced my oil pump, oil and oil filter. That was 500 miles ago and I have not had the problem since then.
B.F. Merced, CA
A. An oil pump is a major item and usually doesn't wear out for many thousands of miles. It may have been defective or suffered a broken drive shaft. Oil volume and oil pressure are different aspects of the lubrication system. Pressure can be low even if the crankcase is full.

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