Clues to cause of engine noise are in the sound

Hey there, time traveller!
This article was published 28/11/2003 (8329 days ago), so information in it may no longer be current.

QUESTION — I have a 1991 Chevrolet Lumina equipped with a 3.1 litre V6 engine and automatic transmission. When I first start the car in the morning, I can hear a dull knocking sound for a few minutes. The noise is louder and lasts longer when it is colder outside. After the car warms up, the noise goes away. I think the noise is coming from the engine, but I am not sure what is causing it. How serious is this problem?

ANSWER — Engine noises can be difficult to diagnose without hearing them, but you have given a couple clues that help. The first clue is the type of sound. A dull or deep knocking sound generally comes from bearings or pistons. The valve mechanism usually makes a much lighter ticking sound so those parts are probably not causing the noise.

The second clue is the sound disappears as the engine warms up. This would lead me to suspect the pistons. The aluminum alloy of the pistons contracts and expands twice as much as the cast iron cylinder block. When the engine is cold, the aluminum pistons have more clearance between them and the block and a knocking sound may be heard as the piston slaps against the cylinder wall. As the piston warms up, the clearance decreases and the noise goes away.

During the last few years, piston slap or knock on start up has become more common. This may be caused by piston skirt design. The skirt is smaller and makes less contact with the cylinder wall to reduce sliding friction and increase fuel economy. A quiet knock that disappears in a minute or two after engine start up usually is not a problem. If the knock is loud or lasts for more than a couple minutes, then one or more pistons may need replacement.

Identifying the piston or pistons that are making noise can be done by removing one spark plug wire from the spark plug and connecting it to ground. Start the engine and listen for a change in the noise. Without the cylinder firing, there is less load on the piston and it doesn’t slap as loudly. If the noise doesn’t change, try the next cylinder. You have only a few seconds to try this test because the noise disappears as the engine warms up.

You should certainly have a piston slap noise repaired. If left, it will get worse and the piston skirt may break. When this happens, it can cost you a complete engine rebuild or even a new engine!

Finally, now that I have told you the worst case scenario, there may be a simpler cause of the knock. A broken motor mount, loose torque converter mounting bolts or an exhaust pipe touching the body can produce the same sound. The sound could change as the engine warms up because of a change in engine idle speed. Spend time locating the noise before disassembling the engine!

QUESTION — The rear window defroster on my 1988 Ford Tempo doesn’t work very well. The window takes a long time to clear and it doesn’t clear evenly. I think there may be some of the lines on the rear window scratched. Is this causing my problem, and if so, how do I fix them?

ANSWER — A scratched or broken rear window defroster grid line cannot conduct electricity through it and will not heat up. Some of the lines must be OK or your rear window would not defrost at all. Small breaks in the grid can usually be repaired. Large breaks may require rear window replacement.

Purchase a defroster repair kit at your local auto parts supplier. The kit contains a metallic paint that conducts electricity. I have used two types of kits. One uses a brush on paint, and the other uses a special paint filled pen. Both work In preparation, clean the inside of the rear window carefully and allow it to dry completely. Mask both sides of the grid line and apply a thin even coat of paint over the broken grid. Allow it to dry, remove the masking tape, and you are set to go.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca