Why Overfilling Your Car With Oil Is A Bad Idea

Why Overfilling Your Car With Oil Is A Bad Idea

You've doubtless seen the warning in your owner's manual (you have read the owner's manual, cover to cover, right?) about overfilling your engine with oil. I've never had a problem with that before--except that shortly before I shut the Corvette down for the winter (perhaps a couple of weeks before), I checked the dipstick in the garage, which isn't spectacularly well illuminated. It looks like I was a quart low (which is quite unusual), so I added a quart of Mobil 1.

When I brought it out from hibernation, I noticed that the oil gauge was pegged at 80 psi. I didn't think much of it, because I hadn't driven it in a while, and I couldn't remember what it was normally running. Then yesterday, I turned to the digital display for that, it showed 130 psi. So I called the dealer, and the shop foreman indicated that it was probably the oil pressure sending unit (which does fail on Corvettes now and then).

This morning, I did a little reading online. It appears that when the oil pressure sending unit fails, it goes open circuit, and 130 psi is the open circuit reading. Then I noticed that while the pressure hit 130 psi quite rapidly, it didn't start there. So it was probably not a complete failure of the sending unit.

My thought was: "Hmmm. If I take it to the dealer, and they spend several hundred dollars replacing the sending unit--and it was just that the oil had thickened over winter, when I wasn't driving it, I'm going to feel very stupid." So I took it Wal-Mart for an oil change--and they told me that the engine with overfilled with oil.

I think what happened was that Mobil 1, because it is a synthetic oil, and quite clear, is very easy to miss on the dipstick in less than blazing sunlight--and I overfilled the engine. Unfortunately, I believe that the extra pressure damaged the oil pressure sending unit--because after the oil change, the pressure is still 130 psi nearly all the time.

Or maybe the sending unit was about to fail anyway--I have 69,000 miles on the car. That's my rationalization, and I'm sticking to it! Anyway, I guess I'll take it to the dealer on Tuesday. The shop foreman when I talked to him on the phone Friday indicated that it was a couple hundred dollars to replace. The sending unit itself isn't all that expensive, but the factory method of getting to it involving removing the intake manifold--and once they do that, they replace the manifold gaskets as well.

I could probably do this myself. But the last time I tried anything even close to this ambitious on car repair, I saved not a penny; injured myself slightly; and threw away an entire afternoon. The dealership charges a bit more for a mechanic's time than I charge for my time as a software engineer; why look for frustration and suffering when I can pay someone else to be frustrated and suffer instead?