For engine gurus: Why overfilling oil is bad for engine?

[notorious]

Overfilling engine with oil is very bad. The explanation I've heard: "When you overfill oil it can get into catalytic convertors and permanently damage them". If this is correct, please explain me this theory. I don't understand how oil can get into exhaust manifold...

[ainarssems]

1. If there is too much oil it splash around too much and piston rings do not clear all of it from cylinder walls completely so it ends up in cylinder and from there in exhaust.

2. Second is through engine ventilation system it gets in intake manifold and then pass through engine to exhaust.

It's also no good because overfilling with oil reduces air volume in crankcase. This causes greater variations between peaks and dips in crankcase pressure as its pulsating from pistons going up and down and can lead to blown seals.

[notorious]

Thanks for this explanation.

Second point makes sense for me.

First point I still struggle to fully understand as I thought performance of rings doesn't depend on amount of oil under them.

How sensitive Audi V8 engines are for oil overfill? 4.2 / 3.7 engine normally needs 7.5-8 liters of oil and if I fill 9 liters, is this an immediate disaster? Any horror stories?

[47p2]

I owned a 944 for a couple of years back in my (much) younger days. Took it into the Porsche dealer to have a service and two weeks later I noticed a pool of oil on the garage floor. I checked the oil level and was horrified to notice it was well over the high mark on the dipstick. I called the dealer and explained to the service manager that I had a rear crankshaft oil leak and asked if overfilling it with oil could have caused the problem, to which he replied that overfilling puts lots of extra pressure on engine seals and most certainly did cause it to blow the seal. He wasn't so happy when I told him that they had just serviced it and overfilled it and therefore I was holding them liable for repairing the leak. They did agree to replace the faulty seal and filled it to the correct level once finished

[notorious]

Thanks 47p2, interesting horror story with a good end

Do you remember how much higher was the oil level on dipstick? Was it like two times higher than the distance between max and min?

[47p2]

From memory it was around 1" above the high mark

[notorious]

1 inch above high mark is a lot

[47p2]

Which is why it caused the seal to rupture.

The modern method of checking the oil level is to have the engine hot and switch it off and leave for 5 minutes, then check the level. I find this completely inaccurate and always check it when cold, I remove the dipstick and let the dip-tube drain clear, usually around 15 minutes to clear all the residue and then insert the dipstick and take a reading, this gives a far more accurate reading without any pick-up of oil from the tube marring the accuracy. I always make sure the level on the dipstick is about 2mm below the top mark when cold, this allows for any expansion without going over the mark

[47p2]

I carried out a little experiment today on the Mercedes.

Engine cold, removed dipstick and wiped then took a reading, note how it shows to be at the top mark


My theory is some of the oil stays on the dipstick tube between the end of the tube and the end of the dipstick, when you remove the dipstick it drags the oil from the tube onto the dipstick giving a false reading. If you remove the dipstick for 10-15 minutes this allows the tube to clear and give an accurate reading

Engine cold, dipstick removed for 15 minutes, then took a reading, this I guess is the most accurate reading.


After a 380 mile journey I removed the dipstick, wiped it and took a reading, it shows well over the top mark


So I left the dipstick out for 10 minutes to allow the tube to clear and took my final reading which shows the level to be bang on at the top mark

[notorious]

Thanks 47p2,

Very interesting observations.

I'm now thinking about filling pre-measured amount of oil vs trying to measure with dipstick when should I stop filling oil.