What happens if you overfill your engine oil?
Auto technicians refer to engine oil as the lifeblood of a vehicle. Changed regularly, it ensures vital parts of your car’s motor are well lubricated.
When oil is poured into the filler port at the top of the engine, it drains down to a roasting pan-like container called the sump at the bottom of the motor.
The bottom tip of the dipstick used for checking the oil level sits in the sump oil.
Markings on the dipstick show the minimum and maximum amount of oil that should be in the sump. Oil should show on the dipstick between these two marks.
But what happens if there’s too much or not enough oil in your engine?
Not enough oil
An oil pickup near the bottom of the sump is connected by a tube to a pump, which draws oil from the sump and circulates it throughout the running engine.
If the oil level is too low, the pump can’t draw oil from the pickup. This is known as oil starvation.
RAA Car Advice Consultant John Reissenweber says oil starvation can lead to a seized engine and catastrophic engine failure.
“If you hear a rattling or tapping noise from the upper engine, it could mean you’re running low on oil,” John says.
It’s important to regularly check the oil level and consult your auto technician if you notice excessive oil usage.
Too much oil
The crankshaft, which drives the pistons, sits above the oil in the sump. If the oil is so high it touches the rapidly rotating crankshaft, this can aerate or froth the oil, preventing it from being picked up effectively and distributed around the engine.
“This can lead to serious damage,” John warns.
The crankshaft can also fling oil against the cylinder walls and pistons with enough force to push the oil into the combustion chamber where the fuel ignites.
“If this happens, you’ll probably see blue smoke coming from the exhaust,” John says.
“It can also force oil into parts of the engine where it’s not meant to be, and this can create oil leaks.”
The crankshaft hitting the oil sometimes creates a knocking noise.
RAA Car Advice tips for checking the oil level
- Check the oil when the engine is at operating temperature.
- Make sure the car is on level ground.
- Turn off the engine, remove the dipstick within 60 seconds, and then wipe the oil off the dipstick with a clean lint-free rag.
- Fully reinsert the dipstick and then remove it again to check the oil is between the maximum and minimum marks, or on the maximum line if the car’s just been serviced.
- If the oil is near or below the minimum mark, it needs a top-up.
Check your owner’s manual to see how often you should change the oil and oil filter.
Do you need car advice?
RAA members can call our free Car Advice service for answers to their car questions.
Contact the car advice team online
Or call 8202 4689.