(Updated on August 25, 2021)
Inside an internal combustion engine, metal components move very rapidly, with tight clearances under intense temperatures and pressures. Without a source of lubrication, these components would quickly wear each other out from metal on metal contact. Engine oil is specifically designed to prevent this catastrophic wear.
Engine oil has a pretty tough job. It needs to be thin enough to allow the engine to start, yet thick enough to provide adequate protection at the engine’s operating temperature.
When the vehicle is parked, the oil sits in the oil pan. Upon ignition, the oil pump switches on and sucks the oil up to start it circulating in order to lubricate all those moving parts. As the oil moves it passes through an oil filter, which collects any contaminants such as metal shavings that get picked up along the way.
It is critical to have the right amount of oil. It is fairly common knowledge that too little oil can cause engine failure, but many people don’t know that too much oil can also cause major engine damage.
Consequences of Overfilling the Engine Oil
It’s always important to check the oil after it gets changed or after topping it off since the consequences of overfilling can be severe. Here is a list of problems you may experience from adding too much engine oil.
1) Engine Damage from Insufficient Lubrication
It seems wrong, but overfilling the engine oil actually leads to reduced lubrication between the moving parts. This happens because the crankshaft sits above the oil pan in most cars and rotates quickly as the car moves.
If the engine oil level is high enough to touch the crankshaft, the crankshaft can whip the oil into a foam, similar to how a whisk aerates egg whites when you’re cooking something.
Frothy oil doesn’t lubricate well or move nicely through the system as it should. The oil flow can slow or stop completely which leads to overheating of the oil. If the engine isn’t lubricated properly parts will wear prematurely, and the engine may even seize up and stop running at all.
2) Failing Seals and Gaskets
When the crankshaft whips the oil, it introduces air into the oil which makes the volume and thus the pressure in the system increase.
Gaskets and seals are designed to hold a certain pressure of oil back from areas it’s not supposed to go, so over pressurizing it means that those are likely to give way. Some of these seals are difficult to access and replace, such as the front and rear main seals of the crankshaft.
3) Spark Plug Fouling
Excess oil can leak out and contact the spark plugs. Once oil coats the plugs, they may no longer receive the high voltage pulse that initiates the spark or the spark might not be able to jump the gap to initiate combustion.
Fouled spark plugs can lead to engine misfires, running rough, or the vehicle may not even start. They are easy to replace should this problem occur.
4) Fouled or Clogged Catalytic Converter
The catalytic converter reduces the toxicity of exhaust gases. After the vehicle has been running, the catalytic converter is very hot. If any oil gets into the combustion chamber or farther in the exhaust system, it’ll likely burn up when it gets to the catalytic converter if it hasn’t already by then.
Oil burning in the catalytic converter will hinder the catalytic converter’s ability to clean exhaust gases and may shorten its life. Catalytic converters are expensive parts to replace.
In extreme cases, excessive oil could clog the catalytic converter. A clogged catalytic converter is a potentially dangerous situation as it could excessively hot and start a fire.
Symptoms of Excess Engine Oil
If you are experiencing any of these problems, stop driving as soon as it’s safe to do so, park on a level surface, and check the engine oil level.
It’s best to check after driving for 5 to 10 minutes so that the car is warm and the oil has been circulating. The engine bay will have a dipstick you can pull out, wipe clean, reinsert, and pull out to check the level.
If the oil level is between the “add” and “full” lines, then you can rule out overfilled oil as the cause of your vehicle’s problems.
1) Recently Added or Changed the Oil
If you are completely certain that the oil level hasn’t been touched in the last few days that the car has been driven, then any other issues you experience probably aren’t from excess engine oil.
This is a problem that tends to show other symptoms within a few miles of driving.
2) Blue Smoke
If you see a slightly blue tinted smoke coming out of the exhaust or engine (or if other drivers see it and wave you down), it is likely burning engine oil. This can also be caused by a leak dripping oil into the hot metal but could be because of excess oil.
3) Burning Smell
The smell of burning oil is pretty unique and acrid. If you smell it, there could be an oil leak or it could be seeping out onto hot engine parts.
4) Leaking Oil
As mentioned above, high oil pressure from too much oil can lead to failures in the weak spots, where oil can then drip out. You may see a puddle or little drips under your engine after the car has been sitting for a little while.
5) High Oil Pressure Light
Not all vehicles have an oil pressure indicator, but those that do may throw a “high pressure” warning if the engine oil has been overfilled.
6) Running Rough, Stalling, and Misfires
These performance issues can be caused by spark plugs fouled when the excess oil contacts them.
7) Car Won’t Start
If the necessary spark for combustion can’t jump the gap in the spark plug because the plug is coated with engine oil, then the vehicle can’t start.
8) Strange Behavior from Oil Gauge
A dashboard oil gauge that swings back and forth between “add” and “full” may indicate overfilled engine oil.
The gauge is designed to work within normal operating conditions, so going outside of those can lead to a “confused” gauge.
9) Overheating Engine
Insufficient lubrication between moving parts increases the friction between those parts, which generates heat.
If this is happening all over the engine when the foamy oil can’t do its job, the overall engine temperature might increase to damaging or dangerous levels.
10) Noisy Driving
The sound of metal on metal can be very unpleasant. Very fast-moving parts contacting one another may generate a screech while slower-moving parts might sound more like a grind or groan.
11) Difficulty Accelerating
This problem can be caused by greasy spark plugs or simply be a result of the car having to work harder to overcome the additional resistance of parts scraping, instead of sliding, past one another.
How Much Oil Should There Be?
Of course the final dipstick reading should show a level between “add” and full, but it’s helpful to know how much oil this actually shows.
In most vehicles, the total amount of engine oil is between 4 and 8 liters. It depends on the engine volume though, so check the owner’s manual for your type of car.
Between the add and full line on the dipstick is usually about 0.5 to 1 liter. So if you need to add more oil, do so incrementally – about an eighth to a quarter liter at a time – to avoid overfilling.
It should be noted that a slight overfill probably won’t cause any problems. The slight excess oil should burn off after a few hundred miles. For instance, if the oil level sits a couple millimeters over the full line on the dipstick, you’re probably fine.
Removing Excess Oil
The good news is that it’s very easy (albeit a bit messy) to remove engine oil if you or someone else accidentally filled it too much. When you are checking the level during or after your oil removal, make sure you wipe off the dipstick before reinserting it to get a proper reading.
If you have a siphon or fluid extractor, you can use the dipstick tube or fill tube to suck out some oil in a fairly neat and painless way.
Another way is to simply place a drain pan below the oil plug and loosen it very slowly with an oil wrench so that it just trickles out. Check the level frequently.
If the oil hasn’t been changed in awhile and it looks dirty, you might just want to change the oil and oil filter while you’re out here.
Even though overfilling the engine oil can have some serious consequences, it’s usually pretty easy to tell that something is wrong and to then diagnose and fix the problem. Anyone who is able to change their own oil at home can fix this for free if nothing has been damaged yet.