Is Engine Flush Good or Bad?Posted by Amsoil, 29th August 2017

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

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

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

Here is what we will cover :

What is engine flush?

 Engine flush is a chemical engine cleaner 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. Engine flush mixes with the engine oil and circulates through the engine, helping dissolve sludge and clean deposits. Then, you drain the oil (along with much of the gunk)), change the oil filter, add fresh oil and get back to driving.

How do 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 accumulate, 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 do not allow the engine oil to fully warm up and evaporate moisture, the ingestion of air-borne dirt, fuel dilution and high heat breaking down the oil. As it settles, engine sludge can clog narrow oil passages or the screen on the oil pickup tube, restricting engine 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.

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

In these cases, a potent, detergent-based flush can help prepare the engine for new engine oil, loosening sticky valves or rings and helping remove harmful engine sludge. While not a required step when switching to AMSOIL synthetic motor 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 Transmission 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-clean-up. Note the sludge around the valve springs and push rod openings.


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

Testimonial time

For the record, I have  used AMSOIL Engine and Transmission Flush on three different pre-owned vehicles in my time, and it has worked great. One of them, a 1999 Honda CR-V, with 206,000 miles 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 my fellow blogger, Ed Newman, for $500 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 on life. AMSOIL Engine and Transmission Flush, as the name indicates, also works great for cleaning automatic transmissions. 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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