Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 26

by Bob Hagin

Q. For some time, I have been on the lookout for an explanation of "dieseling." When the engine is hot, the engine will continue to run even after the key has been turned to the off position. What causes this to happen and what can be done to prevent it from occurring? My friend has a 1985 GMC Safari van with a six-cylinder engine that does this dieseling. A.S. Fair Oaks, CA

A. As you no doubt know, a diesel engine is called a compression- ignition powerplant by virtue of the fact that it has no spark plugs except for "glow-plugs" to get it started. A diesel engine draws air into the cylinders where it's compressed and heated because of the high pressure. At its peak pressure, diesel fuel (a close relative of kerosene) is injected into the engine's combustion chamber and the heat of compression makes it explode. The compression ratio on a diesel engine is somewhere in the neighborhood of 20-to-one while a modern "hot" gasoline engine is around nine-to-one. When a gasoline engine "diesels" (operates without the spark plugs getting voltage) it's because something in the combustion chambers (a spark plug tip, small piece of carbon, sharp metallic edge, etc.) is overheated and glowing red hot. Some of the potential reasons for this phenomenon is that the idle speed is too high, the fuel/air mixture is too lean (maybe caused by a vacuum leak), the spark plugs are the wrong heat range (they literally overheat), or the combustion chambers have a carbon build-up and the carbon is glowing red-hot. All of these can be rectified by a mechanic but for a start, use a higher octane gasoline to see if it helps. Additives in the gas tank can sometimes cure the problem too.

Q. I have a '94 Dodge Caravan with 66,000 miles. We bought it new and have followed Dodge's recommended maintenance schedules. Last March we noticed a regular but slight jerking motion when we slowed down or used the brakes. We took it to the dealership on numerous occasions as the problem worsened to have it repaired, thinking it was the transmission, which was still under warranty. The dealer's shop kept telling us that nothing was wrong but a month ago, the mechanic's test finally indicated that we needed a new transmission. Now we have a totally new transmission including the corresponding computer control board. Now we noticed that the regular but slight jerking motion is back. We are concerned that the transmission will go bad again and our warranty will expire at 70,000 miles. I spoke to the service manager and he said that the '94 Caravans have a 604 transmission/engine and this problem is inherent in this unit. He says that we'll have to learn to live with or ignore this problem because there is nothing they can do to fix it. If the van was yours, would you trade it in? A.W. Norfolk, VA

A. Yes - but not on another Caravan! Contact a local (to you) independent automatic transmission mechanic and see if the independent aftermarket industry has a cure. Sometimes they're more interested in your problem than the factory. Also check on the Internet for a class-action suit based on the failures of these transmissions.

Q. A friend says holding the brake and leaving the automatic transmission in the Drive position at stoplight is OK, saying that it is designed for that. I think it's OK to shift to neutral which I say is better because it produces less wear and tear overall on the components. R.J. Edmonds, WA

A. When your car is stopped at a stoplight with the engine running and the transmission in Drive, the pressure pump in the box is pumping and the torque converter is below the "coupling" speed. Nothing mechanical is happening, so you don't have anything wearing. There's more wear on the transmission when you put it in and out of gear. Leave it in Drive.