Motor flush on oil change requires cold engine?
 

Motor Medic Motor Flush - 1 qt (Not 911. 2007 Jeep 3.7L)

Add to full crankcase when engine is cold. Idle engine five minutes. Shut off and drain.

I read the directions after I have already warmed up the engine for ten minutes.

I look at the contents and it's #2 diesel fuel and kerosene plus distillates.

I'd rather do it with the oil hot. However, I have no idea why it's supposed to be done when cold.

Thoughts?

Hot oil is thinner, so they must want it to be the cold thickness. Maybe to prevent main bearing damage? Just a guess, so don't take this as gospel.

That makes sense to me. Diesel and kero would just thin it further.

Your thoughts are supported by the requirement to idle the engine only.

Mechanic in a can = snake oil. Why are you doing the flush?

I often wonder about liquid added to the gas tank or crankcase? Most or almost all are snake oil which I agree but if you think about it, diesel, tranny fluid, seafoam and such is know to remove or thin out sludge or break down thick grease or oil as a cleaner. It must be of some use to clean clogged up oil passages or sludge inside an old engine. I am thinking about running some Seafoam through my wife's sienna just to clean some gunk out. Engine runs fine without any issues.

BTW, Kontak, it was resetting the computer that did the trick. Turn key and leave for 30 second. Thanks again.

It also cleans the sludge that is sealing the worn piston rings. Now you have blow by and a smoke machine. A buddy of mine had this happen back in the 70's.

That's happened to a friend in high school after his auto shop teacher told him that half quart of tranny fluid will clean the internals of his very tires motor. It smoked like a pot head after that

Customer said her oil looked like mud. It was past the end of its "clean" life but was not horrible.

So I bought the flush X2 and the car parts place gave me a free discontinued five quart jug of 5W-20 and I bought a cheap filter and then another oil change bundle. I was going to flush twice.

If I do it I will do it once with the original oil while cold. Change the oil and completely warm up and drive around. Then change again.

Why?

That's what I chose to do.

Not being critical...just wondered why you chose that path. Hope it solves whatever problem you’re trying to address.

The only motor flush I would ever do is add a quart or two of ATF and drive a few hundred miles before draining it. Having torn down more engines than I can count, the only place I ever see appreciable amounts of sludge is in the oil pan.

If you wanted to really clean the insides of the engine, the best way would be to shut it off hot and let it sit for hours, then drain it and drop the oil pan. Clean the oil pan in your parts washer until it’s spotless inside, rinse it with fresh water and let it air dry before reinstalling it. There is always some real crap in the pan that does not drain unless the engine has been maintained like Air Force One.

I always change oil hot but I do it on time and let it drain for a very long time. The bottom line is that it takes a long time for all of the dirty oil to drain into the sump and if you just shut it off, you’ll never drain it all. The absolute best way is to drain it cold and remove the pan if you are trying to get the dirt out.

That’s strange because I know first hand of the exact opposite happening. A guy buys an old truck with a “bad motor” in northern CA and dumps a bunch of ATF in it for the 500 mile drive home. He figures, “wtf, who cares,” because he’s going to have to rebuild the engine anyway. It smoked bad and burned a ton of oil. By the time he got it home to SoCal, it had completely stopped smoking and stopped using oil. It had a gummed-up oil ring or 3 and the ATF cleaned it and caused it to start sealing again. Good thing he didn’t rebuild it.

My first 'car' was a '58 Jeep with an F head four cylinder in it.
When we took that engine down, the sludge in the lifter gallery was the goop/crud that aferburn 549 describes. In fact, it was soooo bad the parts of the gallery that would have been 'pockets' were filled solid with it. The pistons fell out of the block when I took the rod caps off with the engine upside down on the engine stand. I knew I was in trouble when that happened.

I wanted to see what the original oil looked like coming out of the car after the sludge stuff (distillates) was put in. I did it cold and set a timer for five minutes.

It looked very ready to be changed but nothing scary looking came out.

So how many small pockets of old oil with distillate remain in the engine? I don't know. So I've put in new oil with a new filter and I'm going to run that a (very short) while to dilute the old oil and distillate and let the fresh additives do some scrubbing before draining hot.

It may all be for not but if there is a buildup of sludge I probably would have detected that with looking at the original drained oil.

I did have an early 2000's Passat with a 1.8 Turbo in it. It would throw the oil pressure light and I used the anti sludge stuff to no avail. Still would say it was starving.

I just used a thinner oil. No problem for 10k miles or so then my kid took the car out.

I'm guessing some of the solvents would evaporate off if the engine was hot. Evaporate off before they had disolved any crud.

Amsoil disagrees with using ATF for engine flushes.
https://blog.amsoil.com/transmission-fluid-as-an-engine-flush/

Multi vis oil is thinner when cold, "W" winter at 0 deg. F; thicker when at 212 deg. F. (the second number---such as with 5W-30)

Thank you. I knew it was a range but never knew the mechanics.