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
After each and every oil change — regardless of whether you did it yourself or had a trusted mechanic handle it for you — it’s a good idea to check your car’s dipstick or electronic oil level readout to verify that the amount of lubricant present is below the “max” marking or indicator. Another sign of overfilling engine oil is any unusual behavior from your vehicle’s oil pressure gauge. This could indicate that not enough oil is being pumped through the engine, particularly if the gauge readout is moving back and forth quickly.
If you suspect that your engine has been overfilled with oil, shut it down and do not attempt to drive it anywhere. If you’re in a safe place, you can drain lubricant from your motor in one of two ways. The first and less messy method is by using suction to pull it up through the crankcase, either from the fill cap access point or the dipstick tube (with a siphon or a turkey baster). You could also have your vehicle towed to a shop or, if you have a catch pan, your own driveway, and pull the drain plug or oil filter to pour all of the lubricant out at once. Keep in mind that if it’s overfilled, you might have more oil than your catch pan can handle splashing down onto the ground.
The fact that too much or too little engine oil can harm your vehicle’s motor indicates just how important it is to regularly check your vehicle’s dipstick. It’s a little bit of prevention that can pay off significantly over the life of your automobile.
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Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time. I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.