Rick's Free Auto Repair Advice

What is an engine flush?

Engine flush — What is it? Do I need it?

An engine flush is a chemical wash of the interior of your engine. In theory, the chemical solvent dissolves carbon buildup and sludge deposits and leaves your engine cleaner. An engine flush can be performed two ways:

Add an engine flush chemical to the oil sump and run the engine

Generally, most pour-in engine flush products are added to the engine crankcase in the ratio of 10-oz. to 1-quart: 4-5-quarts of motor oil. You pour the chemical solvent directly into the oil filler opening on a warm engine. Then run the engine for approximately 10-mins. at idle speed to circulate the solvent throughout the system. Then drain the oil and remove the filter. Replace with fresh oil and a new filter.

Engine flush chemicals:


Dynamic engine flush

Rather than pour the chemical solvent into the oil sump, the technician disconnects the oil pressure sensor and connects the flush machine to the oil pressure port. During the flush procedure, the engine flush machine injects heated solvent directly into the oil pressure port, forcing the hot solvent to circulated right into the oil galleries. During the procedure, the drain plug is removed and drain oil/solvent flow restricted to enable the oil pump to continue sucking oil from the sump.

By injecting the chemical solvent directly into the oil galleries at full strength, rather than diluting it in the crankcase oil, a dynamic engine flush is supposedly far more effective at removing deposits and sludge buildup.

Does an engine flush really work as advertised?

If done properly, an engine flush can remove some varnish, carbon, and sludge from your engine. The issue isn’t whether it dissolves and flushes away any deposits; it does, but whether it removes all engine deposits; it doesn’t.

The big question: Do you need an engine flush?

No. The only companies promoting routine engine flushes are the companies that make the flushing solvents and equipment and the shops selling the service. Not one single carmaker recommends an engine flush as a routine cleaning service. In fact, most carmakers have issued service bulletins that warn their dealers NOT to perform or offer engine flushing services for routine use.

If you change your oil according to the carmaker’s schedule, you don’t need to clean the inside of your engine. The oil does that for you.

If you DON’T change your oil on time, the damage has already been done and an engine flush can cause catastrophic damage to your neglected engine.

What are the risks of an engine flush?

An engine flush can destroy your engine

The theory is that the chemical solvent cleans out your engine. The reality is much different. In cases of sludge and carbon buildup, the solvent, due to its limited contact time, dissolves only the surface layer of buildup, while softening the underlying layers. Later, when fresh oil is added, the new detergents in the fresh oil can dislodge chunks of carbon and sludge. If those clumps are captured by the oil filter, they will quickly clog the new filter, at which point it will go into “bypass” mode and stop filtering.

However, if the clumps get caught in a small oil gallery, they can stop the flow of oil to those engine components, causing oil starvation, rapid wear, and component destruction. This is more likely to happen on a neglected engine.

Again, do you need an engine flush?

Let’s say you are quite good about following the carmaker’s oil change schedule. Will an engine flush help your engine in any way? NO. You won’t see any improvement in performance or fuel mileage. None.

When a shop recommends an engine flush service, they’re actually recommending a “wallet flush” service where the money in your wallet transfers to the shop.

Why do shops recommend engine flushes?

Simple. They’re very profitable for the shop. A dynamic engine flush generally runs about $150 and costs the shop about $20. So when a shop recommends an engine flush service, they’re actually recommending a “wallet flush” service where the money in your wallet transfers to the shop.

Say no to an engine flush “recommendation”

If the shop says they recommend an engine flush, just say no. Or, ask the shop to show you where an engine flush is listed as a recommended service in your owner’s maintenance guide. They can’t because it isn’t listed.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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