I done this to my old 96 Saturn and I thought I can share this info here. It really does work and it will do no harm to your motor or gas. MMO is realy good stuff in my book. But here ya go:

MMO Piston soak
This DIY proceedure is cheap and may help a JEEP with stuck oil control piston rings to use less oil by freeing them up. You'll need:

1. 12oz or 20oz bottle of "Marvel Mystery Oil" available at Walmart (among other places)
2. A spark plug socket, and old towel and AT LEAST 8 hours where you won't need to drive the car.

Does not ALWAYS work but here goes.
1. Remove spark plugs from "cold" engine
2. Pour 1~2 oz Marvel Mystery Oil into each spark plug hole
3. Allow to remain in overnight (at least 8 hours)
4. In the morning, cover the plug holes loosely with a towel and turn over the engine for 3~ 5 seconds with the plugs OUT to purge any remaining liquid MMO.
5. Replace plugs and drive normally after pouring remaining MMO(if you have any left) into gas tank full of gas

NOTE: The car maybe somewhat difficult to start initially after treatment and will SMOKE like a chimney for the first few minutes or so after it does start. This is normal and will cause no damage. Adding an additional 12oz bottle of MMO to the CRANKCASE (even if you have to slightly overfill it) about 400 miles prior to the next 2 oil changes can be beneficial as well.

 

Oh yea,, my gas mileage in my old saturn went from 23mpg to 32mph. I swear this on my mommas grave.

And this is putting the rest of the MMO in my gas tank after all the other stuff been done.

 

How does this clean the valves when you pour it into the plug hole? You mean just the underside of the valves?

 

I did something similar to this to an old Mazda, only I used ATF instead of MMO. Smoked white smoke like a b*tch for ~3-4 miles of driving... never did a compression check afterwards, but my fuel economy did shoot up around 5mpg on average.
My concern is what this does to the O2 sensors and/or catalytic converters... (that was a LOT of smoke)

Somewhere on USENET I read that the ultimate concoction is a mixture of 1/3rd MMO, 1/3rd Kerosene, and 1/3rd ATF. Whatever, ATF did the job well enough

 

add some to your oil.

This stuff is GOD!!

 

yeah that might do it. Or adding it to your vacuum line while running the car. There are 5-6 different ways of cleaning the intake/valves/piston areas of gunk and carbon.

 

My Saturn use to burn about 1-1.5 quarts of oil a week. That is 300 miles a week. After the MMO it really improved. I added about .5 quarts every week then. I no longer have the Saturn , which now my cousin owns and running great.

 

I added MMO to the tank and my MPG went down Never used it again

 

When I had my old motor, it was burning about 1L of oil every 1000km. Nissan tried ATF, but I am not sure exactly where they put it. Anyhow, it did not work and they had to replace the motor.