Yet another 1/2 overheating MD11!

[technolaser]

Yes, I know this is a recurring theme, but I've tried everything suggested in the archives and I'm still left with a strange problem so any ideas are welcome.

I have a 1976 MD11C raw water cooled in a Moody 30. Starts instantly every time, although does smoke a bit. Running for extended periods at low revs/load everything is fine with very strong water flow. Both heads remain at around 50 deg C (sorry, I'm a foreigner!), and #1 cyl is also fine and opening either drain **** results in a plentiful stream. As soon as I run over about 1300rpm under load, the #2 cyl overheats as if there's no water around the liner.

I've changed pump and all piping, new exhaust elbow and manifold gaskets. Drilled/filed (with round file in drill!) and acid soaked the manifold. Tested all channels with hose pipe and have free flow. Removed heads and drilled/filed all channels. Drilled/filed and acid soaked the cylinders in place, tested with hose from all five channels in top of cyl and have free flow. New head gaskets. New thermostat.

I even tried supplying the cooling water via a hose pipe - no gain.

Having seen so many knowledgable Volvo members I'm hoping someone has an idea.

HELP - I'm stuffed!

TIA.

[sailingdog]

Have you flushed out the cooling passageways in the block? It sounds like the ones going to the #2 cylinder have a constriction someplace. That would explain why it is fine at idle but over heats as you power up.

[technolaser]

Sailingdog,

Yes, I have flushed the channels in the cylinder that houses the liners. There are no channels in the block itself.

Is there any way exhaust gasses could be getting into the cooling channels?

[cardiacpaul]

it might be a leaking injector pumping too much fuel?

[technolaser]

I'm not a diesel expert so bear with me. Why would a leaking injector keep water from circulating in the channels?

Or do you mean that a leaking injector would make it overheat?

[sailingdog]

The excess fuel by the leaking injector would cause the cylinder to overheat.

BTW, just flushing the system may not remove a constriction, if the constriction is caused by a sediment build-up or broken impeller blade jammed in the channels, usually at a bend. The channel may allow enough water through, so it appears to be clear, but may be restricted enough that at higher throttle settings, not enough water is circulating to dissapate the heat properly.

[JimsCAL]

Sounds like you tried all the usual suspects. I agree with sailing dog that you may have a partial obstruction in the #2 cylinder passages. Have you tried back flushing? Forcing water in the direction opposite the normal flow may dislodge whatever is resticting the flow.

[Gene T]

The Volvo cooling system is bizarre. At least it was on my MD7a. The whole cooling system relies on an even distribution through multiple parallel water passages. One or more get plugged and you have localized overheating. I had an flash over condition, my description, at higher load the engine temp would increase until the water in the block would flash to steam, cold water would then rush in and the temp gage would drop to cold. This cycle would repeat over and over. The problem was the two water passages in the front of the head down into the block were plugged. I could get access by pulling the freeze plugs and routing them out with a wire while flushing with a hose. It was quite a bit of work and there was a lot of rust in there. The good news it when it was done the engine ran at a nice and even temp.

One other problem could be the thermostat housing. If the bypass hole rusts enough that it gets too big there won't be enough pressure behind the thermostat to allow proper flow through the engine at high load. I ended up adding a restriction to my circuit to make up for the rusted out housing. Had to play with it a bit to get the right size. Too small and the engine runs too cold. You really need to draw out the cooling system through the engine to understand how it is supposed to work.

Another problem with these engines varies with how they sit in the boat. If you leave the water feed open the water level in the engine will be at the same level as on the outside of the hull, which could mean critical water passages could be in moist air and not under water which is worst case for rusting shut. This is a good reason to shut your intake valve when leaving the boat for long periods of time.

Flushing with acid will not unplug passages if they are completely plugged.

[technolaser]

Thanks to everyone for the replies. I will give a flushing solution like Rydlyme a try when I can lay my hands on it. Otherwise I'll just go easy on the revs and see how it goes.

Thanks guys.

[haffiman37]

There is unfortunately only one way if the 'easy' ones (back-flushing through drains) fails: Off with BOTH heads, check and clean ALL passages. You'll be surpriced!
Leaking injector will normally lead to more black smoke than usual, but not overheat.