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Compression Testing On Older Vehicles
Compression Testing On Older Vehicles
One of my first jobs after school was at a gas station. And I remember, we had a '67 Willie's Jeep there that had a Buick V6 in it. And I had as much fun working on that vehicle as I did driving it. We used to use it for snow plowing and service calls. But because it was so simple and primitive, it was an easy vehicle to learn mechanics on because you could see how everything worked and nothing was in the way of getting at one given part.
Any case, we've got some e-mail to answer this week from a viewer with a fairly primitive vehicle, and that's an '82 Honda Accord that he's having some trouble with. It's got a hundred and eighty-five thousand K on it. He says it's in good shape, but it's consuming some oil. So this answer could apply to other people with cars that are starting to consume oil as well.
He says that he's got some blue smoke in the exhaust when he starts it up and a little but when it's idling sometimes. He's done a compression test. He wants to know whether just to change the valve seals in the engine, or to rebuild the engine completely. Peter, replacing the valve seals in the engine could quite possibly address the oil consumption situation. In terms of that blue smoke that you're seeing on startup - that's often the hallmark of an engine that's got worn valve seals. When the engine's shut off, the oil runs down the valves, gets past the valve seals, enters the combustion chamber, very slowly during the engine shutdown period. And then when you re-start it you see a little puff of blue smoke coming out the exhaust until you run it for a minute or two to clear it. So changing the valve seals conceivably could address that problem.
However, you mentioned in your letter that when you had a compression test done - and I'm holding a compression tester here in my hand - when he had the compression test done, the difference between the dry reading and the wet reading was about sixty PSI. Now when we do a compression test, we thread this tester into the spark plug hole on each cylinder, crank the engine over with the fuel system disabled, and read the amount of compression pressure available each time the engine passes over the compression stroke. After several cranks of the engine, we can get a stable reading for each cylinder, record it and then see if it's up to spec. Our viewer mentioned that he had a difference of sixty PSI in his compression test between dry and wet. And that seems to indicate that he's got worn piston rings, and cylinder walls. But one thing I might suggest that, when you're taking readings like that on a high-mileage car, you'll find that in addition to a compression test to follow up with what we call a cylinder leakage test - and I'm holding a cylinder leakage tester in my hand right now - would be helpful to pinpoint exactly where the problem is.
The two things that were proposed here - replacing the valve seals - fairly inexpensive job that can be done with the engine still installed in the frame of the car. Might be a good starting point for our viewer because he's got some of the symptoms of valve stem leakage. However, if he's talking about rebuilding the engine, much more expensive job. Engine has to come out. Very costly and time-consuming job and something that you want to look at the whole car closely before you embark on to make sure that the whole package is worth the money that you're going to invest in that one small area.
'Til next week, I'm Bill Gardiner for Motoring 2008.
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