Please help me When I'm driving through town at about...

Dear Car Talk

Dear Car Talk | Jun 01, 1995

Dear Tom and Ray:

Please help me! When I'm driving through town at about 25 mile an hour, my oil pressure reading is about 15. Then, when I stop at a red light, the oil pressure drops to zero. When I get back on the highway, the oil pressure builds back up to the normal range. My mechanic says that my oil pump is failing. Is my mechanic correct?

RAY: If it's just the oil pump, consider yourself lucky, Tom. The first thing you need to do is check the accuracy of the gauge. And you do that by having your mechanic check the oil pressure with his own gauge. If your oil pressure really IS zero at idle, then there are two possibilities.

TOM: One is that your oil pump is bad. You should pray for that. If that's the case, your engine is basically sound, but the pump isn't developing enough pressure. A new pump will solve that problem.

RAY: The other possibility is that the engine can't maintain oil pressure because it's ruined. The engine bearings may be so worn out that excessive oil is leaking out around them. In that case, it's time for an engine rebuild or a new car.

TOM: So here's how you tell. If your mechanic's gauge confirms that you have low (or no) oil pressure, the next thing to do is a "leak down test." That's when you pressurize the engine, and see how fast the pressure "leaks down." If you pass the leak down test, thank your lucky stars and go for the oil pump.

RAY: If you fail the leak down test, then you have three choices. You can hand your mechanic $1,500 and ask him to rebuild your engine. You can start checking the classifieds for another car. Or, you can study some more, take Kaplan's Leak Down Preparation course, and take the test again in the spring.

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