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Oil Gauge Troubles

For the first ten minutes after my car is started (until it warms up), the oil gauge drops to 0 while accelerating. When I take my foot off the gas, it goes back up. After the car is warm, the gauge works fine and unless the car cools completely down, it gives me no more trouble until the next morning. We have tried:

1. Changing the sending unit (twice);

2. Changing the oil;

3. Working on the electrical contacts;

4. Cleaning all of the parts around it.

Our mechanic is stumped. Any other ideas?

Has your “mechanic” checked the engine oil pressure with a calibrated oil pressure gauge? Your oil pump may be failing.

Yes, and it works fine in the shop. Oil pressure is great. Unfortunately, he can’t test it on the road during the acceleration phase, but he believes the oil pump is fine. He is also basing that on the fact that once the car is warm, things work well. But maybe I should revisit the oil pump with him. Thanks for the suggestion.

I think that the mechanic needs to figure out how to rig a way to have the pressure gauge on it under the conditions that this happens - i.e. accelerating the engine while cold. One thing about cold oil is that it is thicker than hot oil. Its harder to pump and doesn’t move through all of the places it needs to go as well.

Good point. It might be worth it just to replace the oil pump to see if it works. If it does, great. If it doesn’t, I have a new oil pump. We have been fiddling with this for awhile. He is embarrassed that he hasn’t been able to fix it, and I am sure he was trying to spare me the cost of replacing an oil pump when he wasn’t sure it was the problem.

My bet is on wiring problem somewhere else. Has he traced all paths from the sending unit back to the gauge when warm and when cold? Does the indicator drop to zero when the engine is accelerated but the car is not moving? Sometimes wiring problems can vary by temperature.

He checked all of the contacts and cleaned them, but I don’t know if he did it both warm and cold. My husband agrees with you that it is a wiring problem. I will use your wording “trace all paths from the sending unit to the gauge” and see if he has done that.

No, the indicator does not drop if the car is in park and the gas pedal is pushed. But I should probably confirm that again tomorrow morning. I tried it once and it didn’t drop, but I haven’t tested it since.

Disconnect all wires to the alternator and test drive it.

Please help me understand what that will do. Will that indicate an electrical problem? I obviously know just enough about cars to talk about a problem, but I don’t know enough about the inner workings to understand how this will help. I know my mechanic hasn’t done this, however, and I am glad to suggest it to him once I can intelligently talk to him about it!

A failing rectifier in the alternator can cause the problem.

My mechanic will be very impressed when I use the term failing rectifier! Thanks for your help!