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crazyhorse77 05-10-2010 00:15

Challenge: Getting an Overheated Engine to Run Smoothly Again
Hey, on our first day on our new (30 year old) yacht we were so excited and in hurry to leave with the tide we forgot to check engine sea cocks.... problem.

The engine temp got very hot and made a screeching sound. The fresh water in heat exchanger mostly boiled out and i believe the imprellor melted and there was a little bit of white smoke in the engine bay.

After the engine cooled I toped up fresh water and checked for signs of oil in fresh water heat exchanger, none there. Then I checked the oil for signs of water/milky colour and none there. The sea cocks for the engine are now all opened. so far so good. Penty of oil too. There is a broken hoses that i believe is for oil vapour as it connects the engine block to air intake.(in side of pipe has oily film in it)

The engines starts and runns poorly like it is under load.

What do i need to do to resolve this???

I was thinking, replace imprellor and fish out pieces of rubber from filtering in heat exchanger. replace all rubber hoses. yet not sure why it is running so poorly.

Ideas and advice most welcome!

Wotname 05-10-2010 00:28

Knowing what make /model engine would be helpful in the cyber trouble shooting process :).

Meanwhile, are you sure it was OK before you overheated it and are you sure the prop or hull isn't fouled up?

crazyhorse77 05-10-2010 00:53

Hey thanks for reply, the engine is an OM636 Mercedes 42HP.

I havnt checked the prop to see if it is fouled, the boat is in england and its muddy cold water so was hoping to avoid this ;)

Wotname 05-10-2010 01:14

I am not at all familiar with that engine so hopefully others who are will be able to help.

I guess you have checked some obvious things like "does it idle OK and rev normally when out of gear" etc.

crazyhorse77 05-10-2010 09:52

I believe this engine is the same as a volvo model yet i will need to find our which one.

The engine idles in neutral ok... yet it seems a little lower then normal. When reving the engine it does not respond as it did before overheating.

Do you know what would commonly happens to a marine diesel engine when it overheats? I assume when all the water is boiled out the impeller melts.

Any thing that would cause the engine to run poorly and hardly rev at all?

Sailmonkey 05-10-2010 10:00

when the engine overheats, things change. Things like cylinder/piston clearances, valve clearances, etc.... If the engine won't speed up and sound like it's under load all the time, I'd suspect there's far more damage than a melted impeller. Sorry to say, but it may be time to pull the head and inspect the cylinders for "wiped" pistons.

SkiprJohn 05-10-2010 12:06

I have an OM636 but can only tell you that overheating as much as you've had might be a serious problem as Sailmonkey said and I would suspect a head warp or head gasket issue. You'll need to check your impellor as soon as you can and replace it. The impellor on the engine up by the thermostat is metal so you won't need to change that one.
Have you dealt with OM636 Diesel Engine Spares
? They might be able to answer your questions and are pretty close to you.
kind regards,

crazyhorse77 06-10-2010 00:11

Arg... sadly it makes sense what you are saying, I was hoping/wishing there would be something else that would cause it to run poorly.

Can a compression test of some sort be done first to see if it is definably problem with the head?

If the head is faulty would the best plan of attach to take the head off (i take it in needs to be dry docked for this) and take it to a diesel mechanic to see if it needs machining and the valves reseated?

any advice to what to be careful of as i have replaced a head on my old car but never a marine diesel?

many thanks for your advice!

Talbot 06-10-2010 02:58

If the engine has been VERY hot, then it is possible for the pistons to shut down the slot for the piston rings and thus compromise the ability of the ring to adjust to the cylinder - the result is a loss of compression. the solution is new pistons!

YourOldNemesis 06-10-2010 04:29

It sounds to me like you're going to need to pull the head to check the pistons. You've almost certainly scored the cylinder bores and/or cracked at least one piston crown.

There's no need to dry-dock for the ensuing work, unless the engine has to come out of the boat. I don't know what your access is like, but if you have good access above and below the engine, you can replace the pistons in-situ. If the bores need re-honing, then you probably will have to dry-dock and take the block out, but I guess if it comes to that, you can push/pull the boat to the haulout with a couple of dinghies.

Ex-Calif 06-10-2010 07:08

Severely overheated engine - general troubleshooting

- Remove impellor and replace
- If impeller failed clean raw water cooling system including heat exchanger
- remove injectors and check for free rotation of the engine
- perform compression test - low compression could be head (valves burned) piston rings cracked or pistons failed
- If you have access you can borescope the cylinders via the injector ports - look for scoring of cylinder walls

If the engine rotates freely - under starter action or by hand - but develops low power low compression is probably indicated. This could be coupled with hdifficult starting.

I would get a professional involved unless you have a good idea what you are doing.

crazyhorse77 26-10-2010 12:45

Engine Update
I enrolled the skills of a mechanic to tell me what is wrong so i can attach it with my spanners.

The verdict so far;
Melted waterlock for exhaust gas and heated sea water, needs replacing
exhaust tubing blistered on the inside so needs replacing
Exhaust elbow needed replacing due to corrosion
belt for water pump very loose, will need to tighten

Work Completed;
Replaced impeller
Check heat exchanger for failed bits of the impeller

The mechanic said get the exhaust sorted first then he can come back for a closer look

I will keep you posted on updates

Wotname 28-10-2010 01:29

Thanks for the update, it is always good to hear how these things turn out in the long run.

SkiprJohn 28-10-2010 11:38

Yes, please keep us posted. I truly hope there is no internal damage. Keep these guys in mind OM636 Diesel Engine Spares

kind regards,

rtbates 28-10-2010 12:47

What, no over temp alarm??? It should have been screaming at you long before any damage.

I've done the same thing, leave seacock closed, on our Cape Dory's Yanmar and the alarm screamed at me very soon after leaving the slip. No damage what so ever. I could even rest my hand on the valve cover without discomfort...

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