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Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 18

by Bob Hagin

Q. I have a 1994 Toyota light pickup truck that has 78,000 miles on it. It has started developing spark knocking unless I use high-test gasoline. I've had the spark plugs changed and the ignition timing checked. I used to be able to burn regular gasoline in my truck with no problems. What is the truth about various grades of gasolines? Does burning high test gas make any difference in fuel mileage?
T.J. Plymouth, NC

A. There are several things that can make a modern automotive gasoline engine develop spark knock. One is a carbon build-up in the combustion chambers which develops hot-spots and also raises the compression ratio a bit. If this happens, it can be removed by a shop without removing the cylinder head. The use of gasoline that has an octane rating that's too low could cause spark knock but unless your usual gas supplier has changed brands, this usually doesn't happen. Another cause is ignition spark timing that advances too fast and/or too far as the engine runs up. You can check this by using a timing light on the marker and accelerating the engine speed to check the amount of advance. Another common cause is a malfunction in the exhaust gas recirculation (EGR) system. If it doesn't dump the prescribed amount of exhaust gas into the combustion chambers, they run lean and develop a knock. Another cause could be spark plugs that are of the wrong heat range but you've no doubt stayed with the factory recommended spark plug heat range. The engine will also knock if the engine itself develops an overheating problem. Check the EGR system first and then a possible combustion chamber carbon build-up.

Q. I have a 1985 Chevrolet Chevette with a four cylinder engine. If I leave the car parked for two or more days, the carburetor must be primed with gasoline in order to get the car to start. Weather has no bearing on this condition. The car runs fine otherwise and if I use it on a daily basis, it doesn't have any starting problems either.
B.Y. Virginia Beach, VA

A. Squirting gasoline down the throat of a carburetor is a messy job and a bit dangerous, too. If a cold engine is started without reinstalling the air filter, a backfire could make quick work of eyebrows or maybe do even more damage. I've had it happen in my youth. Somewhere in the fuel delivery system, you have a drain-off problem and I'd look into the float bowl system of your carburetor. If a gasket shrinks up or sealing ring dries out in a carburetor, the fuel left in the float bowl can slowly drain into the intake manifold and evaporate. You should also check the fuel pump delivery action for delivery pressure as well as its ability to hold pressure without quickly leaking off. Sometimes the fuel in a float bowl will percolate out if there's too much residual heat under the hood when the engine is shut down, but when this happens, the engine will refuse to start when it's left for an hour or two rather than a few days.

Q. I had the engine rebuilt in my '78 Jeep Wagoneer because it used a quart of oil every 600 to 700 miles. After 6000 miles, it still used a quart of oil every 600 miles and the rebuilder supposidly replaced one piston. Now 35,000 miles later, the problem continues but and the rebuilder has gone out of business. What could be the problem.
B.H. Boise, ID

A. Oil can go past faulty piston rings, past the intake valve stems or pulled into the intake system past an incorrect or faulty positive crankcase ventilation (PCV) valve. Try a non-detergent, single viscosity oil for a while. It's possible that the rings simply never sealed up. Check the PCV system first since it's the easiest and cheapest. Next have a mechanic do a cylinder leak-down test to check for cylinder sealing. Leaking valve stems are hard to check except by a tear-down.

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