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What happens if I overfill my engine oil?

Tom Vondrasek | 27th May 2023 | 5 minutes to read

Topping up or changing your oil is an important part of car ownership and, as with anything engine-related, it comes with its own set of lessons to learn.

Overfilling an engine with oil is very easy to do and it can cause some major problems if left unresolved, so it’s not something you want to leave alone and hope it goes away on its own. While the affects can be severe if not dealt with properly, correcting overfilled engine oil is a fairly simple process that just needs a few tools and a bit of know-how.

Below are some of the many things you can do to avoid overfilling your engine oil and what to do if already have:

  1. How To Use the Dipstick To Check Oil Level
  2. What Happens if You Overfill Engine Oil?
  3. Is It Bad To Overfill Engine Oil?
  4. How To Determine if the Engine Oil Has Been Overfilled?
  5. How To Remove Excess Engine Oil?

How To Use the Dipstick To Check Oil Level

One of the main reasons that people tend to overfill their oil is that most people use the oil dipstick to check their engine oil level, which can lead to some issues.

The first thing to remember when checking the dipstick is to always have a rag or paper towel with you so that you can wipe it. Second, make sure that the vehicle’s engine is turned off and has been off for a few minutes. This allows engine oil to drain back into the sump.

Woman checking engine oil with dipstick

If you run the engine while measuring the oil with the dipstick, the oil reading will be lower than it really is. The reason is the engine is pumping oil out of the sump is to keep itself lubricated, lowering the oil level in the sump. It may be hard to read, as chances are there will be oil splashed on the dipstick.

The process for using your dipstick is:

  • Pull out the dipstick
  • Wipe it with a rag
  • Put it back in
  • Pull it out and check the oil level compared to the dipstick marks  

An important thing to note is that the dipstick is designed to read the oil level accurately if the vehicle is on a flat level surface. If the car is parked on a hill, the surface is not flat and level. Oil, being a liquid, will level itself off in the sump. This means that if the dipstick is located in the front part of the motor and facing up a hill, it will read a lower level than if it is facing downhill, where it will read a higher level. Therefore, the first rule when checking oil on a dipstick is to make sure your car is parked on level ground.

The other issue is that because you fill oil from the top of the engine and the sump where the dipstick is located to measure the oil is at the bottom. The part of the dipstick you pull out is on top of the engine to make it easy to get to. Letting gravity do the work, it takes oil a while to work its way from the top of the engine to the bottom and fill the sump. You need to pour oil in, wait a minute or two or three, or even four before pulling out the dipstick and checking oil levels.

If you’re not sure when the best time to change your oil is, see: How Often Should I Change My Oil?

What Happens if You Overfill Engine Oil?

This will depend on how much you have overfilled the engine. If your vehicle takes 7 litres and you put in 10 litres, that can become a real issue. Using the same 7 litre example, if you put in 7.1 litres, that is obviously nowhere near as bad. A lot does depend on the engine and its size and shape as to the impact it can have.

Let the dipstick be the gauge. If it is a few millimetres over or just above the maximum level line, then you can live with it. Anything more and you need to drain some out to be on the safe side. Less is better than more.  

Symptoms can include:

  • Your vehicle may be hard to start or even crank over — like having a flat battery.
  • There may be a puddle of oil that appears under the car.
  • If the car does start, it may run badly and create such a smokescreen from the exhaust you can’t see behind you.
  • It may run alright, but you notice it start to leak oil in different spots.
  • It may have a haze of faint blue smoke from the exhaust.
  • The end of the exhaust may be dripping or spraying a black liquid.

Is It Bad To Overfill Engine Oil?

Yes, it is, especially if by a large amount. Oil does not compress well, so even trying to start an engine with way too much oil can cause damage. It may damage the starter motor or if it does fire the engine itself.

Too much oil pressurises the engine and forces oil up past the piston rings, causing the engine to burn it and creating blue smoke and a black liquid to come out of the exhaust. Your engine will run rough as it is not used to having oil in the combustion chamber.

Excess oil will also force its way out of gaskets and gum up pollution control systems like the catalytic convertor.

How To Determine if the Engine Oil Has Been Overfilled

The easiest and quickest way is to check the dip stick, provided the vehicle is on level ground. This will tell you straight away. If you have completed the oil change you should be checking the dipstick in any case. It won’t tell you by how much but at least it will let you know. If this is the case, remove some oil straight away until it gets to the ‘Max’ line on the dipstick or just below.  Do not even try and start the engine.

The main thing you’ll want to do to make sure you put the correct amount of oil in the engine is to measure it before pouring it into the engine. When buying the oil, you should know how much oil your engine holds and then you usually purchase slightly more — for top ups. Luckily, oil container sizes make this easy. 

Once all the old oil is drained, you simply pour in the correct quantity as stated in the manufacturer’s manual. This way you can’t go wrong. The dip stick will read slightly high as the oil filter will be empty. Once the engine is run it should come back to the correct level.

How To Remove Excess Engine Oil

While this may seem like a big problem, it’s actually a fairly simple process:

  • Drain and clean the oil container you have just used to catch the old oil.
  • Undo the sump plug and drain some oil, not all of it. This is messy as the oil wants to keep pouring out. Ideally, let out more than you need to as you only want to do this part once
  • Retighten the sump plug.
  • You can use the captured oil to pour oil back into the engine if you need to top it up.
  • Check the dipstick and top up if required.
  • Do not exceed the maximum line on the dipstick. Remember a bit less oil is better than too much.
  • Excess oil in the clean container can be used for top ups.

If you’re looking to pick up some engine oil for a top up or full oil change, head into your local Repco outlet or shop our full range online.

Check out our range of engine oils

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