1989 Mercury Cougar:
Low Oil Pressure at Idle

Question:

I drive an '89 Mercury Cougar and over the past several months the oil pressure reading on the oil pressure gauge has gotten very low at idle, almost "0". I brought it to my mechanic and he said it needed a new oil pump. My questions are how does he know I need an oil pump without even opening the hood and how come the oil pressure is fine while I'm driving?

Answer:

Your mechanic shouldn't have jumped to conclusions. The first thing to do is an oil pressure test to confirm that the reading you're getting on the instrument panel is correct. If the pressure shows good, then the problem is in the oil pressure gauge. It could be the gauge in the dash itself, the sending unit on the engine or any of the wiring between the two. The most common failure is the sending unit.

If the pressure gauge confirms no (or little) oil pressure at idle, and you already checked the oil level, condition and tried a new filter to eliminate that as a possible cause, step back and listen carefully for engine noise.

Here are some possible pressure loss causes:

1 - The pump is a possibility, but must be confirmed.

2 - Excessive oil clearance between the crankshaft and bearings (typically accompanied by an engine knock).

3 - Camshaft Bearings (no noise, but can be worn and causing a loss of pressure).

4 - Sludge, the oil is so thick you can cut it with a knife.

Instead of a worn out pump not being able to produce enough pressure, worn out bearings have excessive oil clearance and therefore cannot hold back the flow of oil enough to build up the pressure.

Oil pressure will always increase with engine speed, but if there's a real problem it's actually lower than it should be across the entire range of RPM.

Diagnosis of an internal oil pressure problem often requires partial disassembly of the engine. If the engine has high mileage, don't spend a lot of money trying to diagnose the problem. If the car isn't worth the investment, you would be surprised how far some engines will go with low oil pressure at idle, and perhaps using an oil additive.

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