Rick's Free Auto Repair Advice

Does engine braking cause damage?

Will engine braking harm my vehicle?

Yes! Drivers with manual transmission vehicles often use engine braking to slow down instead of using the vehicle’s brakes. They’ve been doing it for decades so they’re incredulous when you tell them it’s WRONG. Here’s how engine braking is bad for your car.

Engine braking pulls engine oil into the cylinders

Manifold vacuum increases every time you take your foot off the accelerator. In fact, it’s at its peak when the throttle plate is fully closed. That’s when a gas engine encounters maximum pumping loss or resistance caused by lack of air flow.

Every internal combustion engine has blow-by where a small amount of combustion gas escapes past the piston rings and into the crankcase through the ring gap. The blow by gasses flow from the combustion chamber into the crankcase in normal operation. However, if you use engine braking to slow your vehicle, you cut off air intake while increasing engine RPMs. That creates a vacuum condition in the cylinder when the piston is on its intake stroke. That vacuum condition is what causes engine braking. But that vacuum condition also PULLS blow-by gasses AND oil back UP into the cylinder.

Engine braking with a gasoline engine isn’t like a Jake Brake

Diesel engines don’t have a throttle plate so they don’t encounter pumping loss like a gasoline engine. When you take your foot off the pedal in a diesel engine, manifold vacuum doesn’t rise. So blow-by and oil doesn’t get sucked into the cylinder.

A Jake Brake works by taking advantage of the resistance created during the compression stroke and the lack of “spring-back” during the power stroke. A Jake Brake system opens an exhaust value when the piston reaches the top of its compression stroke. The compressed air is then vented to the atmosphere, so it can’t be used to drive the piston down during the power stroke.

Gas engines don’t have that valve opening feature. When you use engine braking on a gas engine, you’re relying on the vacuum created by a closed throttle plate and the resistance created during the compression stroke. However, since you can’t vent the compressed air mixture when the piston reaches TDC on the compression stroke, you get some “spring back” as the compressed air drives the piston down during the power stroke.

In other words, engine braking is nowhere near as effective as a Jake Brake and it comes with some serious drawbacks.

Extra braking increases oil consumption and accelerates catalytic converter damage

With extra oil now in each cylinder, you get some oil burning as soon as you hit the gas pedal. The oil that doesn’t burn gets swept into the exhaust and dumped into the catalytic converter. Not only does engine braking increase oil consumption, but it also decreases the life of your very expensive catalytic converter. In an older vehicle with worn rings and valve guides, you can increase oil consumption so much that you cause your catalytic converter to melt down.

Engine braking causes carbon deposits

This one is pretty easy to understand; draw extra oil into

engine braking causes carbon deposits

Carbon deposits on piston

the cylinder and then burn it and you get extra carbon deposits. Those deposits can cause detonation and piston ring damage

Engine braking in city driving causes the most damage

Repeated high vacuum episodes caused by engine braking in stop and go traffic causes the catalytic converter to overheat and self destruct. Keep in mind that a catalytic converter’s job is to burn off excess hydrocarbons through a catalytic reaction. The catalytic converter doesn’t care if you feed it raw fuel or motor oil, it’s all HC to the cat.

Engine braking wears clutch components

It’s just plain stupid to wear out a $1,200 clutch to save on brake pads. Every clutch application exercises the clutch master cylinder, slave cylinder, pressure plate, clutch disk, throw-out bearing and flywheel.

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat

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