A most curious problem!

[davidivorwynphillips]

On first startup my engine gets too hot (90 deg) and seems to stay at this temperature. If, however, I release the cap on the heat exchanger for a few seconds, the system seems them to happily revert to its normal operating temperature (75-80deg) and stays there even with prolonged usage. My engine is a Mercedes OM 636 and I recently replaced the thermostat. I've checked the opening temperature and drilled a 3mm hole in the thermostat valve plate to make sure there are no air locks. The system has been cleaned out and drained. I'm a bit stumped and so is my Mercedes contact at Lancaster. Any ideas welcome!

 

[RichardS]

It sounds like an airlock somewhere but if it has happened several times then that sounds very unlikely.

If it's a conventional thermostat which simply opens to let coolant through when the engine gets hot then I would remove it completely and see what happens. If it is a twin action thermostat with a bypass line you might also clamp off the bypass line if it has a flexible section to ensure that all the coolant goes through the heat exhanger.

Richard

 

[coopec]

It has to be an air-lock surely?

This guy has a problem with his OM36 too but it is not the same as yours. Maybe the guy he names in his post may be able to help? (The post is 9 years old though)

image: http://www.ybw.com/forums/images/icons/icon1.png
Default Re: Mercedes OM 636 hesitation

I have the same boat and engine and the only times that I have experienced similar problems have been when the fuel filter has been blocked. The man who probably knows more about the OM636 than most is Manfred Princz at Westfield on 01524 791968 I have found him most helpful. He rebuilds them and stocks (or can get) all the spares. Try giving him a call.

Good luck,

Henry


Read more at http://www.ybw.com/forums/showthread.php?97486-Mercedes-OM-636-hesitation#ryOv8JOxhG4LJyHM.99

 

[sarabande]

where is the temp sensor ?

In the engine block, or in the coolant circuit ?

 

[Halo]

The release of pressure seems to solve the problem. Could this be that running without the cap is allowing gas to vent ? If so the likely source of gas is the cylinder head gasket. Just first thoughts
Halo

 

[davidivorwynphillips]

Thanks. I've been in touch with Manfred who is stumped. The temp sensor is in the coolant circuit - on an elbow leading to the thermostat. If as Halo suggests it's a head gasket problem why does it run ok for hours at the lower temperature with the cap in place? I'll try clamping the return circuit as Richard S suggests.

 

[PaulRainbow]

When did this first start happening ? Was it immediately after you changed the 'stat ? Would certainly be worth considering temporarily removing the 'stat. Very much doubt the head gasket.

 

[coopec]

Troubleshooting an overheating engine
Jun 2015 p38-39 (2.00 pages)
View Related Articles
Category: Engines / Practical, maintenance & repair
Author: Hugh Morrison
Description: Hugh works his way through a comprehensive checklist to trace the source of an overheating problem in a Mercedes OM636 engine.
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[davidivorwynphillips]

Thanks for these suggestions I'll take a look at the PBO article and try's bypass the thermostat

 

[johnphilip]

Different engine but exactly the same curious anomaly.
The Bukh 20 on our previous boat had been adapted to indirect cooling and would initially run up to about 90 -95. Ease the pressure cap on the header tank, an audible "glug" and the temp dropped 15 degrees or so and remained there for the rest of the trip. New Day you have to repeat the process. Never found an explanation or any further problem.

 

[NorthUp]

Have you got a calorifier in the coolant circuit?

 

[davidivorwynphillips]

Yes I have!

 

[NorthUp]

Yes I have!

Can you isolate it and see if the problem disappears?
Or, can you find a bleed nipple at a high point of the hoses or the tank? Worth a try anyway.

 

[davidivorwynphillips]

Thanks, I'll have a go

 

[Isaklafleur]

Thanks, I'll have a go

Did you find out what was the issue? I have the same problem...

Thank you
Isak

 

[davidivorwynphillips]

Did you find out what was the issue? I have the same problem...

Thank you
Isak

Never got to the bottom of it! However drilled some small holes in the cap on the heat exchanger to depressurise it and have had no problems over 2 1/2 seasons.

 

[Balbas]

I'd ask what's happening when the engine cools, because that's what's causing the issue. As the engine cools it sounds as though its sucking air into the coolant system. And the place I'd start personally is the pressure release cap. Stick a new one on, check that you've got a good mating/sealing surface on the lid of the heat exchanger / coolant bottle and see how you get on.

If you've got a calorifier, then I'm afraid I don't know anything about them and wouldn't know where to start.

 

[Mistroma]

The release of pressure seems to solve the problem. Could this be that running without the cap is allowing gas to vent ? If so the likely source of gas is the cylinder head gasket. Just first thoughts
Halo

You would get gas leaking into the coolant if the head gasket was leaking between cylinder and water jacket. When the engine is stopped the pressure on the coolant side would force water back into one or more cylinders. OP didn't mention having to fill up the coolant regularly nor problems associated with water in cylinders.

I don't think that the information given by OP would indicate a high probability of a head gasket problem.

 

[davidivorwynphillips]

no evidence of loss of coolant. Also tried replacing the pressure release cap. So still a puzzle!

 

[awol]

May I suggest measuring the temperature somewhere else? One of the cheap infrared devices would check whether the block temperature changes with cap loosening.