Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 33

by Bob Hagin

Q. I wonder if you can settle an argument for us? My daughter has a 1995 Jeep Eagle Talon but it is not the turbocharged model. The transmission is equipped with an overdrive. My son-in-law insists that she drive it with the overdrive turned off. I say that it's OK to leave it on. The engine tachs at a lower RPM and therefore it has greater fuel efficiency. He says that using the overdrive damages the transmission. L.D. Bakersfield, CA

A. If the folks at Chrysler had any reservations about the use of their overdrive in your daughter's Eagle, they would have put a warning note in the owner's manual just like they warn against parking the car in high grass. When a transmission is in direct drive, it's like there was no transmission at all - just a straight shaft from the engine to the differential gears. An overdrive gear is actually a step-up ratio that lets the engine cruise at a lower RPM. For instance, if the high or direct drive ratio is 4.5 to one (four and a half turns of the engine makes the drive wheels turn one complete revolution) and the overdrive ratio is .85 to one, it would make the "final" drive ratio 3.825. The engine would then turn a little less than four turns to make the drive wheels turn once. This lower engine speed would result in better fuel mileage but less torque to the drive wheels. This isn't a problem on level ground but going up steep grades would require a downshift which an automatic with an overdrive does automatically. In the case of a stick-shift with an overdrive, the driver has to be sharp enough to downshift into a the next lower gear the keep the engine from "lugging."

Q. I own a 1974 GMC pickup with a 350 cubic inch V8 engine. This engine was designed to use unleaded gas. Now I'm told that to use unleaded gas I have to add an additive. I am getting lots of advice on what to use. The mechanic that works on my truck told me to use a small amount of 2-stroke motor oil so I put about five ounces of it in 15 gallons of gas. I drove it about two blocks when the engine began to sputter and smoke came out of the tail pipe in clouds. It had to be towed home and I drained all the gas out of the tank. The spark plugs looked like they had been dipped in heavy oil. I cleaned them and filled the tank with gas but no additive. Now it runs but not very well. What should I add to unleaded gas or should I use nothing at all? A.R. Renton, WA

A. The problem that "veteran" cars face when using unleaded fuel is that the tetraethyl lead in leaded gas acts as a cushion between the valve face and the seat that it bangs against. The seats and the valve faces can simply pound out at high speed. Your engine is too new to face this problem as the seats and valves are pretty modern in their metalurgical composition. Another type of additive boosts the octane rating of the gasoline and this is necessary in some engines in order to preclude ping and combustion chamber detonation. If your engine hasn't been modified for higher performance, this won't be a problem either. The third type of additive is used to clean out deposits and the gums that build up in a fuel system (carburetor, fuel pump, fuel injectors, etc.) and this would helpful in your GMC. There's lots of them on the market so take your choice. But do a complete tuneup on your engine before you use it. It sounds like it can use some help.

Q. Our 1995 Dodge Neon has 62,000 miles on it and it sometimes gains speed when I use the cruise control. Almost immediately it returns to the speed I had set it for. J.R. Reno, NV

A. Chrysler is aware of the problem and has come up with a modification kit that includes a new vacuum servo unit, a bunch of modification wiring and a few other parts to complete the changeover. It's effective, I'm told, but it's not a job that can be done at home.