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Dufour 24 Swooner
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Discussion Starter · #1 ·
I have certainly seen several threads on the volvo MD 6/7 engines overheating, and I am hoping some of those folks who have dealt with this before might be able to recognize my symptoms to help me narrow down what needs to be fixed.

So when I start up my engine (takes 30-45seconds) it seems to run fine, takes 10 min to get warm, another 5-10 min of running fine in the green, if I keep it at low revs. However, If I put the throttle to even 2/3rds power, it quickly (within 3 min) goes all the way in/through red. I have watched this action closely, and have turned the motor off quickly everytime, but have also felt the water coming out the exhaust and it is cold, or slightly less than cold, but never hot.

I have changed the oil (30w) and flushed the engine with salt away. No change. I'm thinking the thermostat stuck closed, and not sending water through to cool the engine. Does anyone have experience with this? How hard is it to do? These will be the first bolts I ever remove from an engine and any advice, no matter how basic, is much appreciated.

Thanks!
 

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science :

I have the 3-cylinder version of this motor.
If the temperature indicator rises in to the red AND STAYS IN THE RED, I really suspect you have blocked the wee water entry hole to the cylinder head. You will have to remove the exhaust manifold to get to it, bit a piece of stout wire will clear the blockage easily. You might even see a piece of debris blocking it.
Another possibility is that the thermostat is stuck shut. If so, then even though the coolant can get in to the cylinder head, it cannot get out. The thermostat allows you water pump to skim the hot water off the top of the motor. If it sticks shut, it can't skim the fluid off and it gets hot.

I would suggest firstly that you take the thermostat out. It is easy to get out. Just take off the wee cover at the front of the exhaust manifold, take out the thermostat, and put it in to a wee pan of boiling water. If it is working, you will see it open in the boiling water. If it is not opening, then punch the centre of it out (or cut the centre mechanism off with a hacksaw), and put the remainder (the thick gasket) back in to the motor again and re-tighten the wee cover.
Start the motor and run it under load. If the thermostat has been the problem, the temperature indicator will not go in to the red again (indeed it will barely rise , if at all).
If the motor overheats again, then take exhaust manifold off, and check for obstruction where the coolant water enters the head. It will almost certainly be blocked (or near-blocked) by a biscuit like scale. A stout screwdriver will clear it.
You can put the manifold on again with the old gasket as a temporary basis.
Start the motor and check the cooling again..
If it is now cooling OK, then your problem was scaling (or thermostat), or possibly (not much of a possibility), both.
Now, be careful. The exhaust manifold gasket may be leaking a wee bit (you cannot be expected to put a new one on there every time you investigate a problem, but put a new one on there when you have sorted the problem). The leak is not a problem when the engine is running. When the engine stops, water may leak in to the combustion chamber, so be careful. When the motor stops, and you wish to re-start it, turn the motor by hand, and slowly, with the exhaust valve lifter open to purge the water a little. Then drop the lifter, pull the engine stop cord, and with the cord pulled, turn over the motor with the lifter down.
Replace the exhaust gasket at your next chance and the thermostat. MAKE SURE YOU PUT THE EXHAUST GASKET ON THE RIGHT WAY ROUND!!!! I have made that mistake once... seawater dripping out of the air filter on shut-down!!!!
You can run without a thermostat for a while. You will just over-cool, and you can do that for a wee while.
Be careful with the manifold. It is very brittle and EXTREMELY expensive. Loosen it evenly (and slowly), Tighten it evenly, and slowly.

With a new thermostat in there and with the motor cooling as designed, you will see the wee temperature needle rise up to about 11 O'clock, then drop down again when the thermostat opens. This is normal. It must not go in to the red and stay there though.

If you send me a PM I will give you my phone number in Scotland and I will talk you through it, if you wish.
Let us know how you get on.

Rockter.
 

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Science,

Rocketer seems to know what is worth to know on the MD of this generation. So, I am just adding some:

You do not mention if you have fresh or raw water cooling. For MD 7 fresh water cooling was an addition, some different third party suppliers.
With fresh water follows some extra issues:
- there is likely an extra fresh water pump, most often electrical. Does it work?
- have you changed the fresh water liquid? should be a mix of anti-freeze (glycol) and destilled water - the glycol not only anti-freeze but also improves heat conductivity and anti-rust.
- how is the flow through the fresh water part? with time these have a tendecy to clogg, as it is an after-mounted thing, it is easy to dismount and get it rinsed internal.

The raw water goes through the standard gearbox as well. As the engine is old, there will be a build-up of rust flakes which may decrease flow through the raw water system. In particular when the flakes drop from the (inner) walls - both in engine and gearbox. You can try to flush the raw water system with a hose, in both direction (I have done that on a fresh-water MD 7, works. Be careful - more flakes can get moving ... ).


Thermostat is the best bet, agree with that. Notice, if you are going to buy a new one, that it is different thermostats for raw and fresh water cooled.

Good Luck

J
 

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Agree on thermostat. Sounds like it is stuck in bypass.

MD7 is most likely a salt water cooled version.

When you remove the thermostat cover do it gently. There is a copper tube that comes from the pump. You don't want to kink or twist the end of this copper tube. Make sure you pull the thermostat housing off as straight as you can. It has a pressure fit into the thermostat housing ( and pump). It has an oddball cylindrical o ring. Should you muff up the o ring you can find proper diameter o rings for kitchen sinks, but will likely need two or more.

Removeing the thermostat will let you see a bit into the exhaust water passages. Clean what you can while you have the opportunity.
 

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Dufour 24 Swooner
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Discussion Starter · #7 ·
The engine is indeed raw water cooled.

I'll try removing the thermostat first. Does anyone have a picture of what I should expect to find? I have the manuals but they do not show me a picture of the thermostat and I'm not sure what I should check.

Also, any advice as to "how" to be careful when removing would be helpful. I have access to the thermostat, but it's right near a bulkhead so I only have about 6 inches of clearance. Will that be enough to remove it?

Also, can someone help me understand how the heat exchange works? Is that within the exhaust manifold?

Thanks for all you help!
 

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science :
The heat exchange is by direct contact with raw pumped water and the internal metal surfaces. There is no heat exchanger. The thermostat monitors the coolant temperature, and when it gets too hot, opens to allow the water pump to pump off the hot water at the top of the motor. Once pumped off, the thermostat sees the cooling effect of the new cooler water entering and it closes again. This why you see the temperature needle wavering up and down.
The thermostat will be easy to remove with 6" clearance. It is only about 2" long, perhaps less.
Rockter.
 

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When you remove a thermostat, odds are you will need a new gasket and that may not be in stock, so order it in advance. Consider replacing the thermo$tat while you're pulling the old one anyway, and even with a brand new one, CHECK IT in a pot of water brought to a boil, to make sure it is working properly.

You can also check the temperature sender the same way. Look for specs on yours, many of them are variable resistors that range from around 240 ohms "cold" to 40 ohms "overheated". A repair manual should list the right specs for yours.

If you have a non-contact IR thermometer, even a $20 one, that's also a good way to reality check the temperature at different spots on the engine.
 

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Dufour 24 Swooner
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Discussion Starter · #14 ·
Hellosailor - I actually bought an IR thermometer for just this purpose, but dropped it in the bilge before it had a chance to help me. At least it was cheap!

All, I plan to attempt the thermostat removal this weekend. If it is busted, can I repair it with a good cleaning, or do I need a new one? If new, any suggestions for where to get one on the cheap?
 

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If the engine is raw cooled and the thermo is stuck closed, how does any water get into the exhaust
 

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There is a bypass route through part of the manifold, the thermostat directs water through a larger piece of the manifold.

I replaced my temp gaug and sender and stat. I now see the engine running cool. It does come up to maybe 130° under load. Not enough to open the stat.

So if the stat is stuck the water is taking a short circuit and not doing its cooling job.

Don't ask why mine runs so cool, I don't know. But I can hold my hand on the manifold so it's cool alright.

I did have the exhaust manifold off and cleaned a lot of crap out of passages.
 

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A thermostat usually is not repairable. There's a cylinder filled with wax in the heart of it, and as the wax expands with heat, it pushes the thermostat open. The spring is there to help close it again. Pretty much all of the time, when they fail the wax has leaked out of the piston and all you can do is toss it away.

Built-in gasket...damn, I just knew there was a justification for those marine prices!(G)

Since a "bypass" setup involves using the thermostat as a diverter, to divert part of the water through the engine instead of letting it flow around it? You can actually replace the thermostat with any convenient "obstruction" if need be. I did that once with a suitable chunk of rock (large gravel) from a parking lot, figuring rock isn't going to dissolve or melt. Picked over a few until I found one that looked like it would block 3/4 of the water and divert that through the engine, and it was "good enough" to work until a real thermostat could be found. Obviously the engine will take longer than usual to warm up and the rock can't respond to changing loads or temperatures, but sometimes "good enough" will do.
The hard part was when my friend (the owner) said "How did you fix the engine?" and I had to tell him "I put a rock in it."
 

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hellosailor :

YEEEEEEAAAAAAAARRRRRRRRGGGGGGGGHHHHHHHH !!!!!
Do not block the thermostat off with a rock or anything else.
If you do, you will fry the motor.
On that engine, the thermostat opens to allow the hot water from the cylinder head(s) to escape.
On that engine, the thermostat opens to COOL the engine, and closes until it heats up again.
When the thermostat closes it diverts all of the cooling water flow straight to the exhaust elbow. If you block the thermostat you will kill the motor. It will not cool at all. All of the cooling water will by-pass the motor straight to the exhaust elbow. The exhaust will cool, but the manifold and the engine will fry.
Take the thermostat out if you wish (and the motor will run cool) but on no account block off the thermostat housing!!!!
.
 

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Dufour 24 Swooner
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Discussion Starter · #19 ·
Haha, probably not going with the rock approach :laugher

I may just leave the thermostat out then. That's a legitimate option? Any reasons not to do that other than the motor doesn't get warm as fast?

If the only consequence is the engine takes longer to heat up, that doesn't sound bad at all! I like to sail on/off my slip, so really want the motor for zero wind days when I'd run it for long periods anyway, so wouldn't be a problem.

And it's already cold at startup for emergencies, so it wouldn't affect that use either...

Anyone just never replace the thermostat?
 

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Rockter-
"Do not block the thermostat off with a rock or anything else.
If you do, you will fry the motor."

1. Did not block the thermostat, but replaced it with a rock.
2. Did not fry the engine, it was just fine.
3. Dunno about that engine, my point was that a diverter in a diversion system does not have to be very sophisticated. If you have a thermostat the functions by blocking the bypass route and diverting water into the engine as the engine heats up?
Yeah, a suitably sized rock will do. Or ball bearing. Whatever you've got that looks like it will form an obstruction the same size as the open (hot) thermostat.
And we were careful to confirm that by watching the temperature gauge afterwards.

If you think a thermostat is any smarter than a rock, put both in a closet and close the door. Wait to see which one figures out how to get out first.(G)
 
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