Decarbonzing small diesel

C

chrisyse

.
Apr 7, 2011
37
1989 catalina 34 Bridgeport
Has anyone used Mystery oil to decarbonize a small diesel- To remove carbon build up on cylinder rings and valves?
A long time ago an expert on Atomic 4's, recommended that the engine be periodically decarbonized with mystery oil. He recomended pouring about a 1/4 cup of Mystery oil directly into each cylinder, rotate the engine, and let it sit for a few days.
My small diesel engine is 25 years old, and runs at the cool temp of 160F and is depositing a thin layer of soot on the boat's stern.. Since I plan to replace the injectors. I feel it's also an opportunity to directly decarbonize. The only downside is a large cloud of smoke on initial engine startup.
engine- Universal M25XP 23 hp
 
Bill Roosa

Bill Roosa

.
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Steam explosions do a great job at removing carbon buildup in the combustion chamber. A spray bottle of water misting into the intake manifold while the engine is running at 1/2 throttle works great.
 
Gene S

Gene S

.
Nov 29, 2015
181
Delphia 37 Tacoma
A 1/4 cup would surely give you a hydrolock. I would ask a diesel mech first.
 
flynhi4u

flynhi4u

.
Oct 28, 2013
678
Hunter 20 Lake Monroe
Gotta love the mystery juice. New Waco airplanes used to require it to be added to every tank of fuel. Why they did this was a "mystery" even to the FAA but they allowed it to be part of the plane's operation manual.

Sam
 
kloudie1

kloudie1

.
Nov 6, 2006
9,892
Hunter 34 Mandeville Louisiana
Hydrolock is not a problem IF the injectors are out.. You can safely put the MMO in and let it sit a few daze (?) then put a rag over the injector holes and spin the engine 10-15 seconds with the starter to blow out the MMO. Now put the injectors back in.. pull the compression release and turn the crank a few turns to make sure the engine is free.. then turn off compression release and fire it up.. I am not an advocate of doing this.. just passing on info to avoid a hydrolock if you decide to do it... Note that a diesel can start and run on MMO; in other words, even with the fuel off, when you go "compression on ", the engine can fire.. make sure you are not in there if it decides to do that.
 
C

chrisyse

.
Apr 7, 2011
37
1989 catalina 34 Bridgeport
kloudie1,
It appears no one has decarbonized a small diesel. Water spray is a bad idea, the resultant explosions damage the pistons.
To avoid running on MMO, I would flush with a 1/4 cup of diesel/turn over w/o injectors and then let the engine sit for weeks to air out.
I'm not going to experiment, and therefore I'm dropping the idea unless a diesel guy recommends it.
 
kloudie1

kloudie1

.
Nov 6, 2006
9,892
Hunter 34 Mandeville Louisiana
Good idea, Chris... Decarbonizing usually is not necessary on small diesels .. unlike gasoline engines, they run with a lot of excess air.. which helps to avoid carbon building up. The exhaust ell on Yanmars is an exception.. I don't know exactly why the Universals don't do that, but they seem to have a lot less build-up in the exhaust pipe itself..
Letting the engine start on MMO would not be a problem except for the smoke and strange smell (the first diesel engines ran on coal dust).. My caution is about not turning it over with a wrench with a little residual MMO in the cylinders; if it were to fire with a regular wrench on the crank bolt, you could get a lot of damage to you and the boat. This bit of advice comes from a personal experience many years ago .. while freeing a small diesel in a tractor and turning it over by hand with a wrench .. no one was hurt and only my laundry lady knew how that affected me ! ..
 
  • Like
Likes: flynhi4u
flynhi4u

flynhi4u

.
Oct 28, 2013
678
Hunter 20 Lake Monroe
Learning is defined as a change in Behavior due to experience. I bet you never do that again :)

Sam
 
  • Like
Likes: kloudie1
C

chrisyse

.
Apr 7, 2011
37
1989 catalina 34 Bridgeport
KLoudie,
Thanks, I'll limit my work to replacing the injectors and be very careful when manually turning the engine over. I had planned to do it with the injectors removed, but there other times during normal maintenance, like a few times during the winter to minimize acid corrosion.
Thanks for helping,
Chris
 
P

Pat

.
Jun 7, 2004
1,250
Oday 272LE Ninnescah Yacht Club, Wichita, Ks.
I've always put a bit of Marvel in when changing the oil. I would guess just roughly a 1/4 quart max. It has been running fine since 1986.....a pretty small percentage percentage of the two quarts. I really know nothing about diesels, but MMO was recommended for my Volvo so I though why not.....Maybe I lucked out on this one.... Patrick
 
Justin_NSA

Justin_NSA

.
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
The Marvel site recommends it in the fuel for top-end lubrication, but no claims of removing carbon, just lubrication.
ADDED TO GAS OR DIESEL:
Marvel Mystery Oil® lubricates the entire fuel system-fuel pumps, fuel injectors or carburetors and the top-portion of the cylinders. These are areas, that by design, motor oil does not reach. Using Marvel Mystery Oil® in your fuel extends the life of these vital components by providing them with lubrication that fuel alone does not provide.

I've had dramatic results for getting rid of pinging (carbon buildup) on gas engines with SeaFoam. I've never tried it on a diesel. I added it thru the intake per instructions. It's more like an intervention. There are risks. I'm not suggesting you try it.
 
NYSail

NYSail

.
Jan 6, 2006
3,060
Beneteau 423 Mt. Sinai, NY
i use seafoam on old injectors. Made such a huge difference.
 
You must log in or register to reply here.

Top Bottom