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Old 20-07-2020, 23:11   #16
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Re: suddenly overheating MD17D

Hi Scubaseas

Your hint with the little pipe in the manifold would be an explanation. The manifold was used in salt water but in a fresh water cooling system and was pressure tested. The thermostat is the one that was in before and has the little plate at the end but no hole at 1200.

However, I do have the warm water heater heated by the engine and this adds about 2cm between the manifold and the thermostat housing for the outlet. So if the pipe that was in the manifold was not made for that it is likely that the thermostat is not able to bridge the whole gap. This is easily measured and tested next time I will be at the boat (which is 400km away and still have a working life).

I guess that the coolant is not properly routed through the cylinders when this pipe is open and this would explain as well the temperature diff between the thermostat housing and the cylinder heads.

What I am still puzzled about is why it did perfectly work for the first two hours before it started to degrade and it feels as it would degrade even more every time I used the engine (under load). The last time made it barely out of the marina (about 1.3 nm).

It would mean as well that removing the thermostat in emergency situations (broken and no spare on board) would not help at all as the is nothing that would close that little pipe. True?

Regarding the circulation pump: the two ones that I have were both rebuilt recently (the one installed about 3y ago) with ceramic seals, a new shaft and a brass pump wheel. So it not very likely that this pump failed.

Your tech explanation writing skills are just fine :-)

Thanks

Frank
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Old 21-07-2020, 00:01   #17
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Re: suddenly overheating MD17D

You don't say how old the engine is or how many hours on it.

Any buildup on the raw water side on the tube stack insulates it - slows the heat transfer. Over a few years it is natural for the tube stack to get coated. Another blockage of the tube stack is the zinc that sluffs off of a zinc in the raw water path. Even with good water flow the exchanger is not doing a good job.

I don't have a Volvo engine so my take on the is related to engines I have owned. Take the end caps off of the heat exchanger and run a stiff wire through the tubes. If there is zinc buildup you will find it. On my engine I have discovered that every time I change the engine zinc I have to remove the end cap that it screws into and clean out the cap too.

Every five years or 2000 hours remove the tube stack and clean it. Get a length of pvc pipe that it will fit into, glue a cap on the one end of the pipe, put the tube stack in it and fill it with white vinegar. Let it sit for at least 24 hours. Hose out the tube stack and take note of how much gunk washes out. You will be amazed. The heat exchanger will function much better.
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Old 21-07-2020, 01:53   #18
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Re: suddenly overheating MD17D

Sounds way more to me like a problem on the freshwater side, most likely an airlock somewhere in the block or head, or possibly some non-soluble debris that has shifted position.

If there are any obvious high spots in the freshwater system that can be opened to possibly relieve any trapped air (sensors, hoses, etc.) that could be a simple diagnostic solution.
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Old 21-07-2020, 03:00   #19
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Re: suddenly overheating MD17D

The heat exchanger is new (built on last year) and i had it opened. It is free and i can well see through the pipes with no built up detected.

I removed as well the temp sensor to open up the the fresh water circuit and release air in the cylinder heads and manifold.

The engine is prob 35y old but has been used with fresh water cooling only. A non dissolvable debris that had moved and now blocks something would of course be an explanation.
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Old 21-07-2020, 05:25   #20
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Re: suddenly overheating MD17D

Quote:
Originally Posted by __fdom__ View Post
Hi Scubaseas

Your hint with the little pipe in the manifold would be an explanation. The manifold was used in salt water but in a fresh water cooling system and was pressure tested. The thermostat is the one that was in before and has the little plate at the end but no hole at 1200.

However, I do have the warm water heater heated by the engine and this adds about 2cm between the manifold and the thermostat housing for the outlet. So if the pipe that was in the manifold was not made for that it is likely that the thermostat is not able to bridge the whole gap. This is easily measured and tested next time I will be at the boat (which is 400km away and still have a working life).

I guess that the coolant is not properly routed through the cylinders when this pipe is open and this would explain as well the temperature diff between the thermostat housing and the cylinder heads.

What I am still puzzled about is why it did perfectly work for the first two hours before it started to degrade and it feels as it would degrade even more every time I used the engine (under load). The last time made it barely out of the marina (about 1.3 nm).

It would mean as well that removing the thermostat in emergency situations (broken and no spare on board) would not help at all as the is nothing that would close that little pipe. True?

Regarding the circulation pump: the two ones that I have were both rebuilt recently (the one installed about 3y ago) with ceramic seals, a new shaft and a brass pump wheel. So it not very likely that this pump failed.

Your tech explanation writing skills are just fine :-)

Thanks

Frank
If the water heater adaptor you have is like the one shown in the drawing below, it seems likely that the tstat should be between it and the manifold. The parts manual is unclear about where the actual location is, and since there are no part numbers for the pipe, there's no way to check if there are two different lengths available (unless you can find an old parts manual that has the p/ns listed). The only service manual I could find said nothing at all about the heat exchanged engine, and it was from VP. I would say the chances are better that there is only one length pipe, and either the tstat is in the wrong location between the waterheater adaptor and the bronze endplate, or/and that you've just not gotten all the air out of the system yet.

Either would explain why there are 'inconsistent heating' issues.
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Old 21-07-2020, 05:36   #21
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Re: suddenly overheating MD17D

The thermostat is definitely fitted between the outlet and endplate in my engine now. This could be as well an explanation. It is easily changed when being at the boat again.

Thank you all so much for the discussion. It helped me already a lot to understand and brought up a lot of new ideas.

Best
Frank
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Old 21-07-2020, 06:33   #22
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Re: suddenly overheating MD17D

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Hi,

<snip>
We put the boat in the water end of May and had to motor from ship yard into the marina, which is in total 2.5 hours. For the first 2h10min everything was OK, temperature around 75-78C (at the thermostat housing). We motored at around 1800-1900 rpm. Then the temperature started slowly to increase. We still made it into the marina. When reducing rpm in the marina the temperature started to increase towards the red an just touched the red when we finnished docking.
<snip>
Check the fan belt. Replace and/or tighten.
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Old 21-07-2020, 07:58   #23
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Re: suddenly overheating MD17D

Quote:
The thermostat is definitely fitted between the outlet and endplate in my engine now. This could be as well an explanation.
Indeed. Check height of pipe when you have it off. They only made one pipe. There should also be a small (5mm or so passage) restrictor in the one of the two hose fittings going to the calorifier/heater. If I recall this is in the elbow to the block that holds the three hoses/pipes. You could try clamping off one of the hoses to the calorifier/heater and see if this improves the cooling if relocating the tstat does not resolve the issue.

Quote:
What I am still puzzled about is why it did perfectly work for the first two hours before it started to degrade and it feels as it would degrade even more every time I used the engine (under load). The last time made it barely out of the marina (about 1.3 nm).
You may have blown a head gasket? As in the first over heat may have affected the head gasket. This is fairly low probability but should be checked to rule it out. You can check this by putting a cooling system pressure tester on and see if the pressure continually rises. Make sure you don't let the pressure go over about 14psi when the tester is on. let it warm up, put on tester (be careful opening the cap on the filler as it may be under pressure) and the start motor and see if the pressure slowly builds. Vent and see if it builds again. If it does you have a head gasket that is not fully sealing the compression chamber from the cooling system. The pressure should be constant when it's running. It may go up slightly with revving the engine. You can rent a tester from Autozone, borrow one from your local mechanic maybe or just put a pressure gauge (0 to 20psi or so) on the small line that goes from tstat housing to the expansion/overflow tank either with a tee or cut the hose and block off one end. If this is the case it puts an air bubble on the head in a small area and lets a hot spot be created that leads to overheating. The pressure inside the chamber when running is in the thousands. You can have high pressure gas migrate from the combustion chamber to the cooling jacket and not have any of the typical blown head gasket symptoms like smoke, hydrolocking, water in oil, etc.

Quote:
It would mean as well that removing the thermostat in emergency situations (broken and no spare on board) would not help at all as the is nothing that would close that little pipe. True?
Pretty much yes. If you go over about 1/4 load it'll overheat if the tstat is removed. If your tstat does not look like below simply drill a 1/16" hole as pictured. Calorstat made the one below. Wahler tstats are made of brass. Wahler was also the OE manufacturer for Volvo.


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Old 21-07-2020, 08:44   #24
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Re: suddenly overheating MD17D

Since it took two hours to overheat you do not have a vapor lock. A blockage in the fresh water circuit would have shown up much quicker.
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Old 21-07-2020, 10:25   #25
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Re: suddenly overheating MD17D

The thermostat looks like the one on the picture, just made from brass.

Btw, the little pipes (71 in one picture and 7 in the other picture) have different part numbers:

71: 807120
7: 829921

If they do differ in length it could be that the wrong one is in the manifold in case the manifold did not have a warm water heater outlet attached in its previous life.

I hope the best for the head gaskets :-)
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Old 21-07-2020, 11:02   #26
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Re: suddenly overheating MD17D

If there's not a vent hole in the tstat drill one or at very least check coolant level after the tstat opens. Engine should be initially filled without it running and rechecked after it reaches temperature. If there is no vent on the tstat you'll be under filled about 2 quarts even if you bled at the temp sensor. On Volvo's gas motors in that vintage they used two petcocks for air bleeding.



Hope the head gaskets are OK too but if it's any consolation they are pretty easy to change if needed.


I've seen a lot of MD11s overheat after someone changed the heaters/hoses or exhaust manifold etc. MD17 uses the same system, just one more cylinder. The water heater circuit needs a restrictor somewhere in the system or the engine will overheat at 1/2 to full load. On the MD11s the restrictor was in the elbow either on the tstat cover or under the exhaust manifold. Note sure where it is on the MD17s but it has to be in the system someplace.
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Old 01-08-2020, 04:04   #27
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Re: suddenly overheating MD17D

@Scubaseas thank you for all your advice.

The little pipe in the exhaust manifold is highly highly the culprit. There are different pipes depending whether you have a hot water outlet for the warm water boiler or not. The manifold I got was apparently mounted without hot water outlet in its previous life. I had this pipe in my spares but was unable to remove the old one. I guess it will need to get drilled out.

I remove the complete hot water outlet (and inlet). I drilled as well a little hole (1mm) into thermostat at 1200.

I run the engine for 1.5hrs at 1500 rpm in gear with the propeller spinning. The temperature at the thermostat housing did never reach 80C and was always between 76 and about 78C. I did not dare to run it t higher rpm in gear while being docked and I could not go out. So the real sea trial has to wait another 3 weeks.

However, I was not able to run the engine in gear at 1500 rpm for more that 20 mins before it started to overheat. This and as well the fact that your hints sound totally logical and that it is related to a change that had been made to the engine give me quite some confidence for the sea trial.

So for all the MD17 or MD11 owners who change the manifold please check as well the pipe in front of the thermostat. It should be about 12mm in the manifold with hot water outlet and about 35mm without.

What a great community!!! Thanks for all the help.
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