Take a look at this video: https://www.youtube.com/watch?v=_Vo4fm0JDPY

 

You can never get all the carbon out of the rings. Have you ever hand scraped ring grooves? The carbon is super hard and there is a tool specifically made for cleaning. You can try soaking them with MMO.

 

AC Delco product is the best stuff for this wok, by far. But it's not going to get all the carbon out, may help some (did for me) .

 

Thanks. Is this procedure the same for e46 as it is in the video I posted?

 

I'll second the AC Delco stuff, I used it on my old engine and noticed my compression go up in a couple cylinders. Pretty much do what the video says, I used a funnel to make sure it all got in the cylinder, let it sit for a day (wasn't driving it at the time - 12 hours is pretty much okay though), then used my compressor & a rag to blowout any remaining fluid before starting it. It smokes like crazy for a bit & smells pretty bad, but I just let it idle half an hour then took it for a good rip up the street.

Sadly that motor was too far gone to save, as I found out when I tore it down, but there was a noticeable where the AC Delco cleaner did eat away at a good portion of carbon build up on the pistons, but more so in the rings.

I've also used the seafoam stuff where I sucked it in via the vaccum line, and put some in the oil & fuel, which helped as well. I usually do it about 300km before I change my oil so I can take it for a good rip a couple days to let the seafoam do its thing.

 

Looks like WD-40 also does the trick, maybe even better than the other stuff.

 

Interesting. WD stands for Water Displacement, does it not? So almost 0% chance of hydro locking, I would imagine. May even be more gentle than other degreasers? But how much more or less effective could it be?

 

I've used Subaru Upper Engine Cleaner as a piston soak, and sucked in via vacuum line also.
Didn't work great as a piston soak as its an aerosol and the vapour didn't sit like a pure liquid.
Using it sprayed into a vac line works great, certainly notice a difference.

Whatever you use, don't forget to change your oil shortly afterwards once you've been out for a run.

 

ACDelco X66P--P for pour--the product recommended for this soak, is identical to BMW's upper engine/fuel injector cleaner.

You can see that the bottles are even identical:

http://www.eeuroparts.com/Parts/467...uel-Injection-Cleaner-Concentrate-82140428376/?gclid=CPrt38_l184CFY81aQodt-oO5A

https://www.amazon.com/ACDelco-X66P-Injector-Engine-Cleaner/dp/B0013FMW84

The Subaru aerosol, which is also highly recommended by many, is most likely the same product. This would make it identical to the ACDelco X66A--A for aerosol.

The ACDelco variants are often cheaper, which is why they're recommend.

You can't go wrong with any of them.