7 Overfill Engine Oil Symptoms

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Engine oil is the life source of any engine, but giving the engine more than it needs can lead to catastrophic results and failures, so overfill engine oil symptoms; what are they exactly?

The symptoms of too much oil in an engine are white smoke from the exhaust, rough engine idle, engine misfire, engine stalling, poor engine performance, knocking sounds, and a runaway diesel engine.

With the seven symptoms listed, let’s go through them and see why they can cause costly damage to your vehicle.

one of the overfill engine oil symptoms are excessive smoke from the exhaust

7 Overfill Engine Oil Symptoms

These symptoms will not show on a vehicle where the oil is just a smidge over the full mark but on cars with over a quart (0,95 liters) more oil than the manufacturer intended.

1. White Smoke From Exhaust

The first symptom that will appear when there is too much oil in your car’s engine is white smoke from the exhaust.

Because of the excess oil, the engine will consume some of the oil either through the PCV (Positive Crankcase Ventilation) valve or the oil moving past the piston rings.

In both cases, engine oil ends up in the combustion chamber, and since oil does not burn that well in gas engines, it results in white smoke from the exhaust.

2. Rough Engine Idle

Since the oil does not burn well in gas engines, it will start contaminating the spark plugs.

The oil will coat the spark plug and result in the sparkplug not being able to spark as it should, resulting in the engine running rough at idle.

This is also one of the common issues of cars shaking at idle because of fouled spark plugs.

3. Engine Misfire

With some of the sparkplugs on the engine now coated with oil, the engine can also start misfiring.

A misfire happens when there isn’t a spark for a few combustion cycles, and suddenly the sparkplug can throw a spark again, igniting a very rich mixture of gas and oil in the combustion chamber.

As a result, the engine will also spew out unburned fuel into the exhaust, which can cause the car to smell like gas.

Here is how a V8 engine sounds with a misfire:

Town car tail pipe misfire sound

4. Engine Stalling

The engine stalling can be achieved in two ways either all the sparkplugs are not working anymore due to contamination, or the engine is so full of oil that it is hydro-locked.

With all the sparkplugs contaminated, it is only a matter of time till they all misfire in just the correct order to stall the engine.

If you overfill the engine to the extreme, the engine might turn over and instantly stall because the oil is so much that it stops the pistons from moving.

This could have led to significant engine damage, like the oil rings breaking parts of the piston off to even the piston breaking at the wrist pin that connects it to the conrod.

5. Poor Engine Performance

If the oil is at such a level that it comes in contact with the rotating assembly, the crank, conrods, or even balancing shafts, it will put more resistance on the engine.

The higher the resistance on the rotating assembly, the less power the car will have; it is the same as a person running on a road compared to running in knee-deep water.

Another reason why performance is also influenced is the sparkplugs that are contaminated and can’t spark, resulting in the engine running even worse.

6. Knocking Sounds

Knocking sounds will start appearing after quite a while of driving with too much oil since the longer you run the engine with too much oil, the larger the chances of bearing failure.

The bearing failure can be from the rotating assembly just nicking the oil enough to put unnecessary strain on the conrod bearings.

Or the oil starts to foam up (air gets trapped inside the oil), resulting in starvation since the oil pump struggles to pump foamed-up oil; thus, the bearing surfaces cant be lubricated.

7. Runaway Diesel

All of the above is common in a gas-powered car since it runs on an external spark source; the spark plug and diesel engines don’t have sparkplugs but rely on compression ignition (combustion under pressure).

Since diesel and oil are very close to one another in terms of combustion, if a diesel engine is overfilled with oil and you start the engine, it will begin to rev up and won’t stop until the engine explodes or there isn’t any more oil to combust.

This is called runaway diesel; it is highly dangerous as the engine can explode by over-rotating.

If this happens to you while behind the wheel of the vehicle, this is what you do:

  • In a manual transmission vehicle, put it in the highest gear possible, engage the parking brake, press down on the brakes, and release the clutch so the engine stalls; if this fails, follow the next rule on automatics.
  • In an automatic transmission vehicle, you need to act fast, open the vehicle’s hood, and try to block the vehicle’s air intake, suffocating the engine because no air equals no combustion.

With too much oil in a diesel engine it can result in a runaway diesel, as demonstrated in this video of a Land Rover Freelander:

Insane Runaway Diesel

Final Thoughts On Overfill Engine Oil Symptoms

By overfilling by just a quart or to the brim of the engine, too much oil can result in havoc.

Either the engine will try to consume it and pour out white smoke from the exhaust, the engine can get permanent damage, or it will induce a runaway diesel scenario in a diesel-engined car.

Hopefully, you found this article helpful and will drain some of your engine oil if it is too much or send your vehicle to a mechanic to get the car looked over if you have already experienced some of these symptoms.

Too much oil in a car symptom is also when it shows the oil level over the maximum line
An excellent example of a dipstick showing the engine is overfilled with oil (oil is above the top mark).


Can I Drive My Car With Too Much Oil In It?

No, the chances are too high that the engine can get permanent damage from running with too much oil.
Your best option is to remove the oil by draining some through the sump plug or using an oil extractor pump that sucks it out through the oil dipstick tube.

Is It OK To Overfill Engine Oil A Little?

If it is just over the full line on the dipstick, it won’t harm the engine, but if it is half an inch or more, it starts entering the danger zone.
Engines are built to have a buffer zone for overfilling; this buffer zone is not half a quart (0.47 liters); it might just only be 0.2 quarts (0.19 liters).

How Much Is Too Much Oil On A Dipstick?

The dipstick usually has three lines, the one closest to the end is empty, followed by half, and the one furthest away from the tip is the full mark.
Touching or nudging over the full line is acceptable, but if it is more than half an inch (12.7 millimeters) over full, you are beginning to take chances.