I overfilled my car's engine with engine oil, can that harm my engine?
No, not likely, but it really depends on the engine design and how ‘over full' it is. The primary issue is called "windage". It's when the engine oil in the sump (oil pan) rises far enough to allow the rapidly spinning crankshaft to dip into the oil as it spins. It's not a major problem, i.e., one that will quickly destroy the engine, but it can cause some concerns.
First, as the crank spins through the oil, a lot of oil is going to get thrown onto the cylinder walls. This could overtax the oil control ring's (bottom ring on piston) ability to control the amount of oil left on the cylinder walls. And this could cause greater oil consumption, as some oil might end up reaching the combustion chamber, where it will be burned. Blue smoke out of the exhaust is a clue that's happening.
Second, windage causes the oil to absorb air, since the crank is, in effect, acting like a really powerful egg beater. And oil that's full of tiny bubbles is not going to have nearly as much shear strength. This is because air bubbles can block the oil from reaching and adhering to the bearing surfaces in the engine, possibly allowing metal to contact metal. Whether this is really a cause for concern depends on the temperature of the oil, it's viscosity, and how much load the engine experiences. A little aeration is always present, and on a mildly driven commuter vehicle the increase caused by overfilling the sump a bit is almost certainly not an issue.
My advice is to drain out the overage, and forget about it. In other words, don't worry that the engine has been damaged. It's highly unlikely.
Overfilling the engine will damage it.
Overfilling engine oil can be nearly as hazardous to the health of your motor as driving down the road with your car bone dry, but you rarely, if ever, hear anyone talk about this unusual situation under the hood. Although oil is vital to your vehicle's operation, having too much lubricant can actually counteract the very system that's intended to transport it throughout the motor and keep things spinning happily and safely. Understanding what to do should you accidentally overpour can go a long way toward keeping your vehicle in the best shape possible.
Too Much of a Good Thing
A thin layer of oil coating all of the internal moving parts of your motor is what allows its components to rotate at extremely high speeds for thousands of miles without worrying about any damage. Rather than adding even more protection, overfilling engine oil actually has the opposite effect due to how that lubricant circulates inside the engine's crankcase.
In most vehicles, the crankshaft that drives the engine's pistons sits just above the oil reservoir located at the bottom of the oil pan. In order to move lubricant throughout the motor, a pump pressurizes it and draws it up from the pan (or sump), so it can be sprayed in key areas. Overfilling engine oil can raise the level of oil in the pan to the point where the crankshaft starts to make significant contact with the reservoir. Since it moves so fast, it can turn the oil from a liquid into a froth that the pump is no longer able to siphon and distribute. At that point, the engine can starve from lubricant to the degree that serious damage is often the end result.
Warning Signs and What to Do
Too much oil? That's such a change from the typical do I really have to change my oil question!
The typical overfill tends to be minor and there is significant safety space in most mass produced vehicle motors that an extra quart does not create any ill effects, not to say that I advocate doing this.
Once you exceed this point the oil rises above a baffle installed just bellow the crankshaft. This baffle keeps the crankshaft counterweights from slapping pools of oil. While this seems irrelevant at first, remember the concept of surface tension and how much a belly flop into the pool hurts. Your crankshaft is traveling significantly faster when it hits. While it is unlikely to break due to simple impact, what it does do is cause the section of the crankshaft to slow while the rest is still spinning. While this microscopic flexion happens continously each piston moves through intake, compression, power,and exhaust strokes through all of the cylinders at sequential intervals, there are counterweights on most crankshaft and there is a damper on the crankshaft to help absorb these vibrations, but slapping the oil easily exceeds it's capacity to do so. Now, think of how hitting a certain musical note, or frequency of sound, can shatter a crystal wine glass; the same will happen to that big iron crankshaft in time. The bearings will also likely get beaten to oblivion at the same time.
Yes, aeration is a thing, but there are additives in the oil to prevent it and your oil pick up for the pump is down in the liquid below the bubbles if they manage to form significantly. This is more of an issue with oil that hasn't been changed at a regular interval as the additives break down, and isn't an issue for an overfill during service.
Now, exceeding that level, things get interesting. An engine is a big air pump, and fuel is merely a fine mist in that big volume of air going through. That is where your displacement numbers come from. So that 5.0 liter mustang has an engine that's sum of displaced air in one end to end movement of all of the pistons is 5 liters. The back of the piston also moves air in the crank case too. If the oil is high enough to back fill cylinders to the pistons, it now pumps that oil. If it is really full the oil can enter the combustion chamber through the rings or get pushed up through the crank case ventilation system and enter through the intake. Since fluid does not compress, the engine could potentialy hydrolock and bend a connecting rod on start up.
With a diesel engine there is also a risk of excess oil being pushed through the crank case ventilation and causing a run away engine. Diesels will actually run on oil to some extent, and the more oil or fuel that enters, the faster that motor runs, whether or not you are touching the accelerator, until the valve springs can't close the valves fast enough or some part of the rotating and pumping bottom end of the motor liberates itself through the block and or oil pan. I've been in a shop with a run away diesel one time, the noise and smoke was overwhelming. It felt like it shook the floor and the building. Luckily someone stuck a block of wood over the exposed intake air tube and stalled the motor.
Theoretically with a balanced and timed crankshaft the volume in the crankcase does not change as one piston moves up there is a paired one moving down, even in odd cyclinder ed engines, the balance of movement remains the same. But the engine may not be level or some cylinders get more fluid than gas under the piston, and viola! You get a massive piston oil pump that blows multiple seals and gaskets, and most likely the oil cap, upon cranking the engine over.
I had a friend at work in high school who tool his old Ford truck to a lube place. He had a 460 cubic inch motor, or about 7.9 liters, simply Texas kind of big and powerful. The only thing that it couldn't pass on the road was a gas station.The cashier told him after filling the oil that they would have to charge him extra because of how much oil it held. So, knowing his truck he asked to get the manual out of the glove box to confirm its relatively moderate oil volume, and they said they would drive it out as non employees weren't allowed in the shop. The engine made a slow grunt as the starter strained to turn, then there were a couple pops of combustion from the tail pipe and it lurched to a stop, with the sound of torrential rain hitting the concrete floor as oil poured from the engine compartment. It was then they realized the trainee who they gave the old pick up to, probably because they thought now one could screw up such a simple beast of a truck, had filled the engine with oil, literally filled to the oil cap. The motor had to be stripped down, cleaned, and have every seal and gasket replaced. Luckily it didn't bend a rod or cause more significant damage. I don't think anyone could repeat that one if they tried.
So there you go, over fill depends on the volume, and the result can be nothing at all all the way up to needing a new motor.
That is the short answer, read on for the waffly version.
First of all, if you mean a couple of millimetres or so above the "max" level on the dipstick then you should be ok - but if you are way over, then you need to drain some off.
First of all, if the sump is too full, then the crankshaft will be "slapping" the surface of the oil - and in doing so will start to force air into it. This in turn can result in cavitation (a very destructive effect) that can occur in places like the oil pump.
Furthermore, oil is not really very compressible at all - air is - as a result, the oil pump will not pump anywhere near as effectively - so less oil and what there is is partly air!
In yet addition to that, "frothy oil" is not as good as lubrication AND with things like turbo chargers that have "oil bearings", this can result in exceptionally rapid damage to the turbocharger.
Other problems can be "oil getting where it shouldn't" for example in the positive crankcase ventilation (PCV) system.
The PCV system terminates in the inlet manifold - so can result in oil being leaked into the combustion chamber - causing fouled plugs on a petrol engine.
A Diesel engine doesn't have a PCV but DOES still have a breather system and here if there is a considerable excess of oil, a potential for quite a frightening event can occur.
What happens is that when the revs pick up (either motorway or just low gears), oil can be drawn into the inlet manifold, unlike a petrol engine where it can foul the plugs and/or make the car smoke "blue", with a diesel engine it just sees this as lots of additional fuel.
So the revs increase - regardless of the accelerator / throttle position - and as the car starts to accelerate people (understandably) can panic. Turning off the ignition does nothing as the engine is getting plenty of fuel from the intake manifold (and has no spark) - the fact the injectors are idle mean nothing.
So in order to avoid a collision, people shift to neutral where the engine carries on running until something makes it stop - oil dropped below a critical level, a manual placed in fifth gear and the clutch "dropped" with the brakes on full to try and stall it - or other exciting thing you hadn't planned on doing that day.
The Vauxhall Astra Tdi is VERY prone to this - and more than a few engines have been destroyed (conrods usually with the Astra)
So if you have a turbo or a diesel (let alone a diesel turbo), then if you have added more than an extra few mm to the dipstick, then getting some out (just undo the sump plug) NEEDS to be done to protect the engine (and you, if you find the idea of driving at speed with a car that starts developing more power than it should even with no throttle somewhat disturbing.)
The above is a worst case - but it can (and does) happen especially if HEAVILY over-filled.
I felt obligated to answer this because I saw many answers I did not agree with. Being ASE certified in the category of engine repair and engine performance, I can tell you why overfilling your car with oil is very bad and can damage the bearings in the engine. The rule of thumb is not to fill more than 1/2 quart over the specified capacity.
As we know with Pascal's law, a liquid cannot be compressed, but it can be pressurized. Your oil pump pressurizes the oil to be sent to various places inside the engine; this pressurized oil is squirted through small holes in the bearings where the crankshaft and camshafts ride on oil pressure and do not make direct contact with your bearings. This greatly reduces friction and the load is supported solely by the pressure of the moving fluid. This is why each car has a recommended oil weight and viscosity by the vehicle's manufacturer based on bearing clearance.
If your oil is filled up enough to where the crankshaft can dip into the oil it will cause aeration in the oil. This turns the oil into a foam with pockets of air and as we all know a gas CAN be compressed. This aerated oil goes into your oil pump and the oil pump compresses the air effectively reducing oil pressure in the engine. This can lead to main bearing damage, rod bearing damage, cylinder wall scoring, camshaft journal damage to name a few and can lead to catastrophic engine failure, in a worst case scenario.
Most vehicles have an oil dipstick, on the dipstick you will notice a minimum and a maximum fill range. The distance between these marks is usually one quart of oil. You can tell how much your vehicle is overfilled based on this distance.
Edit: I wanted to thank you all for the views and upvotes but also I wanted to add some more information.
On the 2nd generation Prius, if the engine oil is overfilled by more than a half quart, the PCV system will suck the excess oil through the throttle body and stall the engine. We have seen some come back on a flatbed after quick lube changed the oil. Once you drain the oil a little the engine will be able to run again.
Modern engines are of forced lubricated.This oil is circulated by an oil pump. Engine oil quantity for an engine is optimised after doing some calculations and analysis at its design phase considering the amount of clearance volume of the bearings it is going to fill and lubricate it, at which flowrate and pressure it has to be circulated, since it has to stay, lubricate, clean surfaces and carry away the heat from those rotating and sliding surfaces without getting itself burnt due to high temperatures involved.
This oil is stored below the Crankshaft in a container called oil pan. The oil pan is designed in such a way that it holds maximum required amount of oil without being churned out by the rotating parts of the Crankshaft and a minimum amount of oil required for lubrication ensuring that the suction part of the oil pump doesn't suck air i.e the strainer part of the suction pipe always immersed in oil. Entrapment of air in lubrication system with oil will adversely affect the parts of the lubrication system like oil cooler burst, filter element failure, as well as the bearing surfaces.
So a minimum amount of oil level is always ensured in the oil pan at all gradient operations of the automotive engines without exposing the suction part of the oil pump. But for stationary engines like gensets the oil surface plane is always parallel to the ground.
So if we pour maximum amount of oil than the specified level to our engine the heat load increases on the cooling system. Since oil pan acts as a sink to absorb heat energy from oil. More oil means for the same fixed surface area of the oil pan it has to handle more amount of oil to dissipate heat. More the amount of hours the engine runs more the fuel is burnt and more the heat is transferred to the oil which has to cooled. This affects the engine performance.
Next the engine rotating part Crankshaft which is immediately above the oil surface might be continuously immersed in oil based on the amount of excess oil we pour than the specified limit by the engine manufacturer. In most designs the gap between the bottom most surface of the rotating part of the Crankshaft and the oil surface is maintained between 1.25 to 1.5 inches. When Crankshaft is exposed to oil, it acts as resistance to it during its rotation adding to the parasitic load to the engine. This results in power loss, to overcome this we will be throttling more fuel to attain that power. So this results in fuel consumption.
Also engine oil filter in the lubrication system has to handle more amount oil. So it will result in earlier change of the oil filter than the specified service interval.
Since Crankshaft creates slashing and sloshing of the oil, the pressure builds up in the Crankcase and this affects the seals. Which might result in seepage of oil through weak seals. Also oil slashing on hot surfaces causes oil mist formation. Generally oil mist is formed in an engine but more amount might form in this case.
This oil mist mixes with the engine blow-by gas (gas leaked from the combustion area of the engine cylinders via the piston rings to the oil pan side of the Crankcase). There is a percentage limit for the amount oil mist in this blow-by gas.
This oil mist will pass through the Crankcase ventilation system. If the engine is of open ventilation i.e the ventilation system tube is exposed to atmosphere, more oil will pollute the atmosphere and make the engine cabin look dirty.
If the engine has closed ventilation system ( i.e crankshaft bottom side of the Crankcase is connected to the engine air breathing side called the intake system through an oil filter in the Crankcase ventilation system called the blow-by filter), the excess oil mist in the engine blow-by will result in earlier change of the blow-by oil filter before the specified service interval. Also this oil filter in the ventilation system will be able to handle some percentage amount of oil mixed in the engine blow-by gas. Beyond which the filtration efficiency of the blow-by gas filter reduces, which means it will allow the excess oil, than the amount of oil mist percentage it was designed to handle, to pass to the intake system. This oil mixing will affect the performance of intake system parts like the turbocharger, intercooler and intake system pipes. If it is a Diesel engine with exhaust gas circulated, will result in the soot accumulation of the downstream parts after the egr mixing in intake system. This oil will reach the combustion are and results in engine black smoke. This also affects the valve seats causing valve stickiness due to soot accumulation. Also at worst cases oil might reach the exhaust system and might cause catalyst poisoning of the exhaust after-treatment system and its parts, resulting in pollution norms violation.
Pouring excess oil means you are spending more money on oil.
Overall considering above scenarios, it is advised to pour oil into the engine to the maximum level specified by the engine or vehicle manufacturer. Always service your vehicle at vehicle manufacturer authorised service stations. This will save your vehicle life, save environment and your money.
Hope I have answered in detail to the extent I can. Sorry for lengthy essay.
Thank you for reading