Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 22

by Bob Hagin

Q. I have a 1981 Cadillac Coupe Deville that has a 4-6-8 transmission and 240,000 miles on the odometer. The car had 47,000 miles when I bought it from a Cadillac dealer in Los Angeles which is about 150 miles from where I live. Since then I have had it serviced by a Cadillac dealer's shop that is only 40 miles from here. About two years ago, I started having trouble when I started up a hill. When the transmission went into the six-cylinder mode, the car would run rough and jump all over the road. The diagnostic technician said that a faulty module was causing the problem. He tried to get one from the General Motors factory but he found that the factory doesn't make them anymore. C.C. Daggett, CA

A. Your Cadillac is officially call the V8-6-4 and it was designed to extend the fuel economy of the basic 6.0 liter engine. The late '70s and the '80s was a period of rising prices in gasoline and the specter of fuel shortages was still in everyone's minds. The system operates by disabling half of the eight cylinders when the car is in a cruise mode and bringing in two more cylinders as the car needs more power going uphill or accelerating on the road. The V8 mode comes into action when starting from a stop. The immobilization is actuated electronically through solenoids that operate on various valves. It was only used on Cadillacs in '81 and '82 and was unique - no other G.M. product used it. You've had good luck with your Seville but most people learned to curse the system. I haven't been able to locate a source of aftermarket modules or rebuilt units so your mechanic has only a couple of choices. He can immobilize the system to make it a full-time V8, try to locate a used unit in a dismantler's yards or checking Cad parts on the internet.

Q. I have an '83 Dodge Rampage that has a hesitation like the one you described in a recent column. You mentioned that it's possible to remove carbon build-up from the cylinders without having to remove the heads. I believe I have that problem. My motor diesels when I shut it off for some five to 15 seconds. I have around 10 pounds more compression than the specifications call for. That along with 105,000 miles on the clock makes me suspicious. The truck has excellent gas and oil mileage. I've heard of being able to remove carbon some way without dismantling, but every shop I go to laughs at me and wants to remove the cylinder head and says that they might as well grind the valves, replace the timing belt, etc. The belt was replace at 70,000 miles. G.S. Shoreline, WA

A. Whenever a mechanic does a static compression test with the spark plugs out and spinning the engine over with the starter motor, he or she has to be careful. Compression gauges can easily be off by 10 pounds and the best I ever hoped for was that it indicated even compression between the cylinders. A system for blasting carbon off valve fillets is done with compressed air and granulated walnut husks but it involves removing the intake manifold. It's an expensive machine and not many shops have one. Modern over-the-counter auto chemicals do a pretty good job of removing carbon from the valves and combustion chambers but you have to make sure that you follow the instruction so as to not damage the catalytic converter, oxygen sensor and fuel delivery parts. Even water slowly dribbled through the intake system of a running engine will remove combustion chamber carbon but it has to be done carefully

Q. My 1999 Nissan Altima occasionally gives off the smell of rotten eggs. I've been told that this is indicates a faulty catalytic converter but our selling dealer says that there is nothing wrong with the car. R.J. Norfolk, VA

A. The main causes of that bad smell is gasoline that has too much sulfur, a periodic engine misfire, or a rich mixture. Try another brand of fuel. If the problem continues, something is wrong with your car.