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AUTO ADVICE: Oil in spark plug; grinding noise when A/C on

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QUESTION& ANSWER

Q: I drive a 1999 Mazda Miata MX-5 and I recently found oil in one of the spark plugs. I was wondering what could be the cause of it. Would it have to do something with the rings?

A: My experience has shown that worn rings will cause oil consumption on the highway and wash the carbon off the top of the pistons, but seldom foul the spark plugs with oil.

You should be able to see the tops of the pistons with the spark plugs removed and the crankshaft turned slowly.

If the top edges of the pistons are a clean aluminum colour, then oil is getting past the rings. The top of the piston should be a light tan or black carbon colour.

Valve seals or worn valve guides usually cause spark plug oil fouling. When the guides or seals are worn, oil is sucked down the intake valve stem and into the combustion chamber.

One would think the oil would be burned in the cylinder, but the spark plug fouls and the cylinder misfires. The backside of the valve will also usually have an oily carbon deposit on it.

The valve stem seals on the Mazda MX-5 are usually not a problem, but they are relatively easy to change, so I would check them first.

To change them, compressed air is blown into the cylinder to hold the valves in place while the valve springs are removed and seals changed.

Make a mistake in the procedure and a valve drops into the engine — not a serious problem but it requires removing the cylinder head to correct. I would suggest leaving seal changing to an experienced mechanic.

If the valve guides are worn, then the cylinder head must be removed and overhauled to fix the problem. If your MX-5 doesn't smoke, and the plug fouls only occasionally, then I would wait until the problem gets bad before spending money on repairs.

Using a spark plug that is one heat range hotter will tend to burn off the oil fouling, so that may be a temporary fix. Don't go use too hot a spark plug heat range or it is possible to damage the piston.


Q: I purchased a 2014 Honda Odyssey Touring model. Recently, every time we start the van after it had sat for about 24 hours, we got a huge grinding noise from the front when the air conditioner engaged (approx. 1-2 seconds after the van starts). This noise continued throughout the summer with every start and only after sitting idle for about 24 hours.

Our local dealership replaced the clutch and compressor twice, and then said it was caused by our alternator and replaced it.

This noise is not heard if the air conditioner is turned off when the van starts, however the noise will occur if we turn it on for the first time.

The dealership has had difficulty solving this problem since the grinding noise only happens for only 1 second, and it happens only after the van has been idle for approximately 24 hours.

That means they get 1 second per day to diagnose it. Any thoughts or direction you have would be helpful.

A: You are hearing the slipping sound of the ac compressor clutch when it engages the compressor the first time.

The compressor’s steel clutch plate is attracted to the steel pulley by an electro-magnet. There is always a little slip when the clutch engages, but you are hearing it slip for a little longer.

There could be a few reasons why it slips longer on initial staRtup. Typically, the engine is on a fast idle and there is no road noise, so you hear the slipping more.

The clutch could be worn, but I doubt it because it has already been replaced. If the air gap between the clutch plate and the pulley is too much, the clutch will slip longer.

This air gap can be adjusted by repositioning the clutch plate on the compressor shaft.

Finally, the compressor may be slightly harder to start turning after it has been sitting because refrigerant and lubrication is not evenly distributed in the system.

I would have the air gap checked on the clutch, but don’t worry about the noise too much. I have seen vehicles operate with this noise for many years without any problems.

Jim Kerr is a master automobile mechanic and retired teacher of automotive technology. Send your questions for Jim via email or mail them to: Herald Wheels, 2717 Joseph Howe Drive, P.O. Box 610, Halifax, N.S. B3J 2T2

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