MECHANIC CAN GAUGE THE SOURCE OF OIL PRESSURE PROBLEM

Dear Car Talk: I have a 2006 GMC Envoy XL with 227,000 miles. After sitting in the garage overnight, when I start it, the oil pressure gauge will drop, and the oil warning lights come on. If I leave it in park and keep the engine revved up, then the oil pressure will go up. Two blocks from the house is a stoplight, and I have to put it in neutral and rev the engine, or the oil pressure drops and warning lights come on. After about 10 minutes of driving, the oil pressure still drops some at stoplights, but not low enough to cause warning lights to come on. Am I damaging the engine by continuing to drive? Why does it have oil pressure after it’s warmed up but not after sitting overnight? -- Stephen

Beats me, Stephen. Normally, oil pressure is higher when you first start your car, because the oil is cold and more viscous. So why the pressure would start low and then improve as you drive is a mystery.

Yeah, I know, that’s why you wrote to me.

If the reading on your gauge is accurate, then yes, you are damaging the engine by driving it with low oil pressure. So the first thing you need to do is find out if your oil pressure really is low, or if you’re getting an incorrect reading. You do that by taking the gauge out of the equation.

Your mechanic can do that for you. You’ll leave the truck with him overnight. In the morning, he’ll put his own oil gauge on it, and start it up. If his gauge tells him that your oil pressure is exactly what it’s supposed to be, then he’ll know that either your truck’s oil gauge or your oil pressure sending unit is no good.

It’s more likely to be the sending unit. That’s a little sensor that plugs into the side of the engine, reads the pressure and sends that information to the gauge and the idiot lights on the dashboard. A bad sending unit would be the best-case scenario, Stephen; a sending unit is cheap, and very easy to replace.

So that would be good news for both of us: Good for you, because it means you haven’t been harming your truck all this time, and you’ll have a nice, easy, cheap fix. Good for me, because then you won’t write back again and make me hurt myself by thinking harder. I’ll keep my fingers crossed for us.

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