How much damage can overfilling do?

[J]

Let’s say an engine is filled so that there’s about a fourth of the crosshatch over the top of the max on the dipstick. If the crosshatch represents a quart, then this would be overfilled by 1/4th of a quart assuming the oil pan doesn’t change its dimensions after the top of the crosshatch (become narrower for example).

Could this cause damage?

Also I read that damage to bearing seals can occur. Can someone offer an in-depth explanation as to why slightly overfilled oil would affect oil pressure and why any of this would affect seals? From my uneducated point of view the excess sits in the pan. Even a quart over would probably not reach the crank. The seals are for the crank front and back right? The pickup is blind to the amount of oil in the pan as long as it’s not starved and the oil is pumped by a pump driven by the timing belt/chain. The only things I see affecting pressure are the pickup, pump and filter, none of which are affected by overfilling. I don’t see how overfilled oil would cause any issues with pressure or seals. The only consequence to overfilling I can see is if it touches the path of the crank counterweights or connecting rod or it’s so overfilled the seals are submerged. Is that what’s being implied by overfill?

I think maybe the amount of overfill is not established when people warn about damage? For example anything up to a quart overfill could be fine, anything only grossly overfilled like four quarts or more could cause damage?

For auto transmissions people recommend Honda’s take three quarts of ATF but the actual specs say fluid change is only 2.5 quarts. That would be half a quart over. I used to go blindly by three quarts since I hated having half a quart left over but stopped when people told me I would kill something doing that.

Topic

[Joe Mama]

Overfilling is a problem when the oil level becomes high enough to reach the crankshaft. The oil can be whipped into an aerated mixture reducing the lubricative properties. Although you can overfill to some degree (because one can assume engineers build in some sort of variable margin of error) there could be an increased chance of incidental oil-to-crankshaft contact over uneven terrain. In my opininion, an overfill that does not result in crankshaft contact will not affect oil pressure as the system is normally designed to pull from a ‘reservior’ at an established rate.

The same principles apply for ATF in automatic transmissions when oil levels can be high enough to contact the transmission pump.

[Mike]

Overfilling will usually not cause “damage”, at least that can’t be undone. Your ideas about overfilling are pretty much correct. Overfilling will not affect oil pressure unless crankshaft churning aerates the oil so much that the oil pump is picking up a lot of air bubbles instead of oil. I have not seen this happen. You can say that it will cause crankshaft seals to leak, but in practice, lip/shaft seals like that will hold oil in when submerged. I’ve had many axle seals that are the same exact design as crank/cam seals hold in trans oil that is above the level of the seal, and have a lot of fluid drain out when I remove the axle. I’ve only seen excess crankcase pressure cause known good lip seals in the engine leak, and they usually stop leaking on their own when the excess pressure is fixed.

I just dealt with a Jeep Cherokee came thru the shop with at least 10 quarts of oil in it, maybe more, only a couple inches from being up to the top of the dipstick. It was a trade-in that we were taking straight to the auction for wholesale, and they said it was smoking out of the tailpipe. What was going on there is the pistons were being dunked into the oil bath at bottom dead center, loading the rings up with oil, and allowing it to be get into the combustion chambers and produce oil smoke out of the exhaust. I corrected the oil level and advised that the car would have to be driven for a few miles before the exhaust would clean up back to normal, which is exactly what happened.

Another thing I will mention, is that I always put the 3 quarts in when I drain+fill my Honda transmission. It doesn’t hurt anything in that case. The oil pump is essentially inside the valve body, shielding it from being directly affected by the fluid level around it. The Jatco CVT transmission being used by Nissan, Mitsubishi, Chrysler, and others has, however, been known to make noises and have minor drivability problem when overfilled. Not every engine or trans design tolerates incorrect fluid levels the same way, and should be thought of on a case by case basis.

[Author]

Replies