Automania/Repair and Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 39

by Bob Hagin

Q. We have a 1991 Mitsubishi Galant with a five-speed transmission and it has given us good service. Recently we had to have the clutch replaced after the car had gone over 100,000 miles. After this was done, a mechanic told me that I shouldn't be downshifting the car to slow down. He said that I should just come out of high gear, shift the transmission into neutral, and then use the brakes. He said that brakes are lots cheaper than having a clutch job done. Do you think this is the right thing to do? How long should a clutch last on one of today's modern cars? J.C. Concord, CA

A. If you got over 100,000 miles on the original clutch, you didn't do badly. Clutch disk material is very much like the stuff that is used in brake pads and shoes, and it can take a lot of abuse. A clutch disk wears out pretty fast if you "slip" it during upshifts (first to second, second to third, etc.), but it isn't subjected to the same kind of strain in a downshift (fifth to fourth, forth to third, etc.) or "ride" the pedal by resting your foot on it. Brake jobs are indeed cheaper than clutch jobs and your brakes are well up to the job of slowing down your Mitsubishi at any speed without having to rely on rapid engine deceleration to help. Under normal conditions, a clutch should easily last 100,000 or more - unless a teen-ager is doing most of the driving.

Q. After going through the air conditioner system on my 1970 Pontiac Grand Prix, I found that the engine was overheating pretty badly. During the course of trying to fix the problem I replaced all the hoses, back-flushed the system twice using a chemical cooling system cleaner, replaced the thermostat, checked the ignition timing, replaced the radiator, replaced the intake manifold and carburetor gaskets, and replaced the heater core. Finally I removed the thermostat entirely. None of these cured the problem and the car now sits in my garage. Is it possible that the cooling passages are totally blocked? R.V. Owings, MD

A. I've worked on some cars that have coolant passages so packed with debris that I had to remove their engines, dismantle them it down to the last nut and bolt dig the stuff out through their freeze plug holes. Before you engage is such drastic measures, pressurize the system with a cooling system pressure-tester. If the system and the radiator cap hold pressure, check for a blown head gasket or a crack in the cylinder heads or the block. Slight leakage of burning fuel being pushed into the cooling system from a combustion chamber could cause your overheating as you've described it. To check for a for a blown gasket or a block/head crack, I use a chemical dye-check "sniffer" kit that changes the color of the test fluid from green to yellow if combustion gases are getting into the cooling system. It's fool-proof. I assume that you've already checked to see if the fan is pulling enough air through the radiator.

Q. My problem is poor fuel mileage (six to eight MPG) from my '86 GMC half ton pickup. I've had the four-barrel Q-jet carb rebuilt (including sealing the main discharge well plugs) and have taken it back to the shop seven times for analysis. I've rebuilt the distributor, replaced the spark plugs, wires, air and fuel filters (three times), and replaced the fuel pump. If I race the engine, it will blow out black smoke. If I let the truck idle for 10 to 15 minutes, it will billow black smoke for five to ten seconds after I get underway. This all started in January. J.B. Ft.Wayne, IN

A. Black smoke at idle can only come from excess fuel entering the intake manifold. The causes can be a high float level (like a toilet bowl that keeps running), leaking main discharge tubes, blocked anti- siphon ports or high fuel pump pressure. Check all these (especially pump pressure), and then try another carburetor if all else fails.