I saw the answers to a question I posted recently highlighting the disadvantages of having too little oil in an engine, but that got me wondering what if the complete opposite were to occur!

I know that having an excess amount of oil in the engine is a terrible idea, but I don't actually know why. It seems to me like the excess oil would eliminate at least some of the disadvantages to not having enough oil (like for example the parts would not rub and create friction because there would be plenty of lubrication). Due to this, I am guessing that having too much oil would have very different disadvantages to not having enough oil.

If we imagine that for whatever reason, the engine was full of oil (and I mean literally full to the brim, you can't fit any more in it unless you pour it all over the engine bay):

  • What are the disadvantages to having way too much oil in the engine?
  • How do the disadvantages differ from those that occur from not having enough oil?
  • Would the engine last longer than the same engine would with way too little oil?

Not a Duplicate

The linked post does not answer my questions fully, here is why:

  • The accepted answer only says to take out the oil but doesn't really explain the damage that can be caused
  • Paulster2 said "you can cause damage to the crank and rods themselves" but I don't necessarily understand why so I thought I'd ask a question rather than just adding a comment
  • He also said that some of the moving parts hit the oil 20 times a second, but I thought they did that anyway!
  • Second answer down, which starts to make some good points, could do with some clearer explanation as I'm not sure that I understand it fully
  • The rest of the answers do not provide as much explicit detail as I am looking for in the answer to this question (they are quite short for the most part)
  • This question also has a framework of what happens internally to an engine and what the side effects are as well as component of what the fundamental differences are regarding effects when compared to lack of oil
  • Also, I asked this question in order to build a theoretical understanding of the problem. The other page suggests how to fix an overfilled engine (practical application)
  • 1
    Possible duplicate of How far can I drive with 1 gallon too much oil?
    – Zaid
    Commented Jan 13, 2016 at 20:35
  • I think this question is different. It has a framework of what happens internally to an engine and what the side effects are as well as component of what the fundamental differences are regarding effects when compared to lack of oil. Commented Jan 13, 2016 at 20:51
  • 1
    Yes, also this question I asked in order to build a theoretical understanding of the problem. The other page suggests how to fix an overfilled engine (practical application). Commented Jan 13, 2016 at 20:53
  • @MaxGoodridge Are you still looking for the answer to this? Commented Apr 29, 2016 at 2:05

4 Answers 4


With way too much engine oil in the engine, the problem is that the crankshaft can hit the oil in the bottom of the crankcase when the engine is running. Since the crankshaft is spinning fast, even at idle, each time it slaps the surface of the oil, it will create some bubbles in the oil as the air just behind the spinning crankshaft lobe gets dragged under the surface of the oil. It doesn't take long to get a whole lot of bubbles and the engine oil becomes all frothy and full of bubbles.

Since all oil pumps are designed to pump liquid rather than (compressible) gas, even though the pump is still operating, it's literally sucking air instead of pumping vital engine oil. The net effect is that the oil doesn't get delivered to the places it's supposed to go, so a gross excess of engine oil can kill your engine by the exact same cause as not having enough engine oil.

With that said, if you put in, say, a quart too much, it's probably not going to be a problem (depending on the type and capacity of the engine). If you put twice as much in (I've heard of people doing this because they forgot that they already replaced the oil and do it again!) it's almost certainly going to kill the engine unless corrected before the engine runs for very long.

  • That first paragraph makes sense. Second paragraph though, why does the pump decide to suck in air as I'd expect it to be full of oil! Are you implying that the oil pump also has an air intake? Commented Jan 13, 2016 at 20:56
  • 4
    If the oil is all foamy, then that foamy oil is what gets pulled into the oil pump. Foam doesn't pump very well since it's full of compressible air bubbles and the pump is designed to pump oil, which is an incompressible liquid.
    – Edward
    Commented Jan 13, 2016 at 20:59
  • I see, thank you. Does that also result in a drop in oil pressure because the circulatory speed of the oil is reduced? Commented Jan 13, 2016 at 21:01
  • 2
    Technically, it's not the speed of the oil, but the pressure, but yes, a working oil pressure gauge should indicate a lower pressure as a result of foamy oil.
    – Edward
    Commented Jan 13, 2016 at 21:07
  • 1
    OP asked about an engine that's filled to the brim; wouldn't this completely submerge the crankshaft, such that there would be no slapping of the oil surface to create bubbles? Commented Jul 18, 2016 at 14:52

If an engine is that full, it will pump oil through systems it shouldn't, such as the PCV system into the intake manifold.

Piston rings, even in a fresh engine, always have at least a minimal amount of blow-by from combustion. Normally these combustion products pressurize the crankcase slightly and then vent through the PCV (Positive Crankcase Ventilation) system into the intake.

While the PCV Valve is intended to stop direct oil flow, we know from the use of PCV Catch Cans that some motor oil gets through in the best of circumstances. If the valve cover is full of oil as the OP describes, even more will get through.

The PCV system terminates in the Intake Manifold, so the oil will be added to combustion mix, fouling spark plugs and increasing unburnt carbon emissions.


The real answer is- it depends.

An extra couple of quarts? Probably no real effect. If you put in enough that the crank is sloshing the oil around, you probably will get a foam of oil all around the underside of the pistons, but besides robbing a bunch of horsepower due to windage and maybe causing some oil burning issues (at a certain point your oil rings will just get overpowered because there's so much oil on the walls of each cylinder, plus your PCV system unless you have a catch can, is going to start sucking oil into the engine), it isn't going to kill your engine or anything unless you start getting detonation from all the ingested oil. By the way, I don't believe it will turn EVERYTHING into foam and cause oil starvation- there is still going to be at least a gallon of completely liquid oil under the foam that your oil pump can suck up and use for lubrication.

Eventually you'll put in enough oil that it might hydrolock the pistons from the underside or at least prevent them from moving faster than a very slow maximum speed as it pushes the oil up into passages between the head and the valve cover. Your car might not run, but I don't think it would explode or bend a rod or anything unless you have an incredibly powerful starter motor.

The truth is that too much oil isn't really a big deal unless it's a really silly amount. Many turbo cars come from the factory with oil sprayers under each piston and those things dump a whole bunch of oil constantly all over the undersides of each piston. It causes some oil burning compared to not running them, but it's not a huge difference, and not even a long term reliability issue.

Another thing is that it will vary from car to car since every car's oiling system is slightly different. Some cars have basically sealed off the sump from the crank area with baffles and trays while other cars basically just dangle the crank over the top of the oil. Yet other cars run dry sump systems which store the oil externally and also separate the oil and air. This is a much more complicated (and rare, in production vehicles) approach.


This is strictly theoretical (aka I could be completely wrong), but if an engine is literally filled to the brim as you described, very little damage would occur.

Depending on the type of engine, once the starter is engaged the volume of the crankcase will remain fairly constant, as the volume lost by one piston moving down will be negated by another piston moving up. Once the engine reaches a certain speed, however, the oil will be too viscous to move within the crankcase (remember, the crankcase is completely filled with oil).

Since piston rings are designed to prevent the contents of the combustion chamber from making it into the crankcase (rather than vice versa), oil will begin to leak into the cylinders. It would not take long for the amount of oil in the cylinder to prevent the air/fuel mixture from firing, causing the engine to sputter and die.

In certain cases you may have ruined the piston rings, fouled the spark plugs, or busted your oil seals (rear main seal, oil pan gasket, cam cover seal, etc) but I imagine if you drained the oil to the proper level and removed the pools of oil in the combustion chamber, the engine would eventually fire, burn off whatever oil was remaining in the chamber, and continuing running without any catastrophic issues

After that incident though, you'd probably burn a quart of oil every two hundred miles until you forgot to top it off one day and, well, you know the rest.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .