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I put too much motor oil in my car's motor. About a quart - have I ruined my car? I've been driving it. I know I need to get the oil drained. But I'm worried that I've ruined my car which has been running very well until now.

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2 Answers

up vote 4 down vote

About a quart - have I ruined my car?

In short, no. If you read the answers on this similar question, even a gallon of extra oil generally won't ruin a car.

This is what I suggested and still recommend:

If the oil isn't hot, almost any sort of plastic tubing can be used for siphoning. It's easiest to go in via the dip stick.

Remember not to use the "suck start" siphoning method as you don't want a mouthful of oil. If you have a long enough piece of tubing, you can stick the excess down the dipstick tube, put your thumb over the end, pull out the slack (that is now full of oil) and you'll have an immediate siphon.

Also remember to siphon the oil back into the original container - you might as well keep it until you do need it.

It's not that big a deal. With an extra quart, most normal cars driven by normal people will just have slightly lower mileage due to pumping losses. Go ahead and get it back to the correct level, though: no need to waste gas or trouble your peace of mind.

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If the car were driven for many miles (>100--1000), I'd be concerned about fouling the catalytic converter, which can get expensive. I didn't see anyone mention this point on this post or the linked post. – mac Dec 3 '12 at 23:49
@mac, fouling a cat? Why? – Bob Cross♦ Dec 4 '12 at 2:02
my understanding is overfilling leads to increased oil consumption, which means oil smoke out the tailpipe, which can foul the cat. A couple of my vehicles have explicit warnings against overfilling on the oil filler that state something like "overfilling can lead to catalytic converter damage" – mac Dec 4 '12 at 15:01
@mac, if you consistently drove your car at +1 quarts, refilling every time the excess burned away, I would agree that additional exhaust components from the oil wouldn't be good for the cat. For a single overfill, that seems less likely. That said, this is all a game of risk management. If the driver is concerned about even a low probability outcome, the remediation procedure is clear: remove the excess. – Bob Cross♦ Dec 4 '12 at 16:16
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You most likely will not cause damage by running only an extra quart. Many people racing with stock oil pans will run half a quart to a quart of extra oil to prevent the oil pump pickup from drawing in air under racing conditions. Under sustained high RPM, oil can be pumped out of the sump faster than it can drain back. This issue combined with high cornering forces would move the remaining oil in the pan away from the oil pump pickup tube, causing oil starvation and engine damage.

Your engine's optimal maximum oil level is determined by filling up to where there is a safe gap between moving parts and the oil. What you put too much oil in your engine, you risk frothing the oil due to the crankshaft and connecting rods whipping air into it. The air bubbles in the oil will interfere with the oil's shear strength, which is what keeps metal parts from rubbing directly together. Not good.

Another problem is the additional oil splashing around the crankcase from the crankshaft and connecting rods whipping it up. It will coat the sides of the cylinder walls and could result in excessive oil consumption. In the worst case, carbon deposits from the burnt oil could cause the piston rings to stick, making oil consumption even worse and reducing power.

... but again, one quart is really not going to be enough for you to run into these problems. More than one quart is asking for trouble though. See: How far can I drive with 1 gallon too much oil?

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