Is an Engine Flush Good or Bad?Posted by Amsoil, 10th August 2017

To flush or not to flush your engine? The answer is vigorously debated in the garage.

Let’s get right to the point. Is it safe to use an engine flush in an engine with high miles?

Spend a few minutes perusing online forums and you’ll find a range of answers to this question, often involving a 1980s car that someone thrashed on for years, parked in a pasture for a decade and now wants to revive with an engine flush.

Here’s what we’ll cover:

What is an engine flush?

An engine flush is an aftermarket chemical additive designed to clean accumulated engine sludge, deposits, and other gunk. You pour it into your engine’s oil-filler port and idle the engine for about 10-15 minutes. The engine flush mixes with the oil and circulates through the engine, helping dissolve sludge and clean deposits. Then, you drain the engine oil (along with much of the gunk, in theory), change the oil filter, add fresh oil and get back to driving.

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How engine sludge and deposits form

During the heady days of its youth, your engine’s performance delivered maximum power and efficiency. Over time, however, harmful deposits and sludge may have accumulated, causing power and performance loss.

The tiny openings in the oil pickup tube screen can easily plug with sludge, starving the engine of oil.

Deposits and sludge can form in the engine for several reasons, such as frequent short trips that don’t allow the oil to fully warm up and evaporate moisture, the ingestion of air-borne dirt, fuel dilution and high heat breaking down the engine oil. As it settles, engine sludge can clog narrow oil passages or the screen on the oil pickup tube, restricting oil flow to vital parts of the engine, especially the upper valve train. Deposits can cause the rings to stick, reducing engine compression and horsepower.

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Is an engine flush necessary?

A good engine flush can help loosen deposits and dissolve sludge, returning your engine to like-new condition. However, in old engines with high miles, the engine sludge may be the only barrier keeping engine oil from seeping through worn or cracked seals. Removing the sludge exposes the seals for what they really are – rubbish. Soon, your engine begins leaking oil, and your mind instantly associates the engine flush product with an oil leak.

In reality, the seals were already bad; the engine flush simply revealed their true condition.

If you suspect your vehicle falls into this camp, leave the engine alone and skip the engine flush. It’s probably not worth trying to revive an engine in such poor condition without first fixing the bad seals or other defects. In effect, you’re choosing your problem: either sludge and deposits robbing performance or, if you clean the engine, the seals showing their true condition.

Part of a good maintenance regime

That’s not to say an engine flush is never a good idea. In fact, it’s often the first step in helping restore a neglected vehicle to top-notch performance. And often when you buy a used vehicle, that’s what you’re getting – a vehicle whose owner preferred shopping for antiques on Saturday afternoon than changing the car’s engine oil or dropping the gearbox pan. Consequently, your “pre-owned” ride, while not rubbish, may boast a sketchy maintenance record.

In these cases, a potent, detergent-based flush can help prepare the engine for new oil, loosening sticky valves or rings and helping remove harmful engine sludge. While not a required step when switching to AMSOIL synthetic engine oil, we do recommend flushing your engine if you want to give your vehicle a fresh start.

Some people also claim that an engine flush will free large chunks and cause an avalanche of debris to clog passages inside your engine. Don’t worry. AMSOIL Engine and Gear Flush cleans at the molecular level, ensuring the deposits are dissolved and properly exit the engine with the oil when it is drained. This helps the new engine oil get off to a “clean start” and allows it to perform to its engineered capacity.

Cylinder head pre-cleanup. Note the sludge around the valve springs and push rod openings.


Cylinder head post-cleanup. Post-cleanup with AMSOIL Engine and Gear Flush, the cylinder head is noticeably cleaner.

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Testimonial time

'For the record, I’ve used AMSOIL Engine and Gear Flush on three different pre-owned vehicles in my time, and it’s worked great. One of them, a 1999 Honda CR-V, has 206,000 miles on it and still runs like a top without burning hardly any oil. Another, an Oldsmobile Intrigue, ran great until a computer problem forced me to trade it off…for the CR-V. The third I sold to a fellow blogger for £380 and a telescope. Last I heard, it still ran well, but rust was getting to it.'

He may have parked it in a pasture behind his house.

In sum, flush your engine if you want to give your vehicle a new lease of life. AMSOIL Engine and Gear Flush, as the name indicates, also works great for cleaning automatic gear boxes. But if you have any reservations about disturbing engine sludge or deposits that may be holding your old, high-mileage engine together, consider skipping it. It’s up to you.

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