Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 23

by Bob Hagin

Q. I have a '79 Mercedes 450 SEL with 225,000 miles on it. I bought it in 1990 with 165,000 for a good price. At 175,000 miles I had the two camshafts replaced and other than normal maintenance items, it's been a great car. Six months ago it began to run poorly. The local Mercedes dealer replaced the fuel injectors but it still ran bad. An independent Mercedes repair shop installed some missing vacuum hoses, did a minor tune-up and it ran fine. I now wonder if the original injectors were bad. I was informed at the time that one plug was fouled and that it would need an overhaul. It was suggested that the problem was a seized ring. The compression is good but it now burns a quart of oil every 750 miles. Another suggestion was that there is a valve guide floating in the head. I wonder if trying to free a seized ring chemically by pouring automatic transmission fluid or squirting in a penetrating oil would work. I don't want to go shopping for a cheap solution to an expensive problem, but I want to know all the options to an $8000 overhaul. J.G. Boise, OD

A. In an oil-burner, a compression test can be misleading since a lot of oil in a cylinder can seal up a bad ring and raise the compression number by a dozen pounds of more. A cylinder leak-down test (external air pressure in the each of the cylinders and checking to see if they all hold it evenly) is more accurate. I've never been able to get a stuck piston ring to free up chemically but I guess it's been done. If you've changed the oil and filter frequently (every 3000 miles or so) I don't think a stuck ring is the problem but it may be a broken one or a collapsed piston ring land. A loose valve guide is possible but I'd look to a bad valve stem seal on that cylinder if your Mercedes uses them.

Q. I have a 1992 Ford Probe LX with the 3.0 liter engine. After a period of sitting overnight or after an eight hour work shift, the brakes don't work properly. I can press the pedal all the way to the floor and the front brakes still won't lock up, although they do eventually grab and stop the car. After driving and applying the brakes a few times, they grip better but they're still soft. The Ford dealer has replaced the main brake cylinder twice but the problem remains. Is it a design defect or if they're just not catching the problem? B.W. Battle Creek, MI

A. Pressing the brake pedal all the way to the floor is dangerous. On some cars, it can damage one of the seals in the tandum master cylinder and cause the brakes to be soft or the pedal to be very low. You problem could also be in the brake booster so it should be bench-tested along with the master cylinder. A soft pedal usually indicates air in the system but I assume that the Ford mechanics did a power-bleed on it. There could also be an internal leak-down in the accumulator valve or any of the other components. I haven't heard of it being a design problem so I'm pretty sure that the technicians just haven't found it. Try another shop, maybe one that specializes in brakes.

Q. In May of 1993 we bought an '89 Plymouth Voyager from a dealer. In August of 1994 we had to have the transmission replace at the dealership at a cost of $1600. We were never satisfied with the transmission and returned it to the dealer's shop but were told that the problem didn't exist. A year later we went back and again were told that there was no problem. The transmission immediately failed completely. An independent transmission shop repaired it for another $1300 and it now works fine. We feel the dealer installed a faulty transmission and was negligent in his follow-up tests. We feel he should reimburse us. What should we do? M.C. Pittsburg, CA

A. After two years, I don't think you're going to have much luck. Small claims court is your only hope, I think. The Chrysler minivans had lots of transmission problems then but the factory would never admit it.