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Old 19-07-2020, 11:05   #1
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suddenly overheating MD17D

Hi,

I own an N343 which is equipped with an VP MD17D engine with VP fresh water cooling system. I had exchange the fresh water pump 3 years ago as the one before did leak.
Last winter the engine got fitted with a new (used but tested) exhaust manifold. The engine did never make any problems, starts immediately, never had heat problems, smokes a little (grayish, definition no stream).

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.

What did we do since then:
- checked and changed impeller (the old one was not worn at all and just fitted during the winter, ... anyway)
- flip the seawater pump lid around as it was a bit worn
- checked the seawater intake
- dismounted the heat exchanger and checked it for blockage (i was newly fitted last year)
- checked the piping of the fresh water side
- drained the coolant to see whether the is any dirt, mud ... - comes out fine and clean
- checked the thermostat

What we can see is, that the freshwater pumps (I do at least a good amount of coolant being pushed into the expansion tank). What I cannot judge is whether the pump develops sufficient pressure to pumps a sufficient flow through the heat exchange and the cylinders.

We can run the engine without load in different rpm with no issues. As soon as we put load on it it starts to overheat. The temperature of the heads and the cylinders is as well considerably higher then the temp at the thermostat housing (20 degrees more). The heat exchanger gets warm, but I somehow feel that it used to be hotter in the past (but have no data to prove).

Anyone any idea what else to do/check?

Thanks for any idea.

Frank
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Old 19-07-2020, 11:20   #2
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Re: suddenly overheating MD17D

Could you trace the flow? It sounds like there is a constriction somewhere, or possibly a weak pump. Constriction is more likely from your description. How much water, and how hot is the water, coming out of the heat exchanger? Backing up, how much is getting to the HE? If that system is flowing well but not much heat is being picked up in the HE (discharge not hot), then start on the coolant circuit the same way. Is coolant flowing through the heat exchanger? Is it losing temperature like it should in the HE?

It really should be analogous to tracing down a 12v circuit that isn't working.
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Old 19-07-2020, 11:29   #3
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Re: suddenly overheating MD17D

One item is missing from your list of things checked - the coolant pump, called the water pump on automotive engines. If it died on you or went weak, you'd have something like you described.

Running under load will heat things up much faster than the same RPMs in neutral.

Flow and pressure. Flow is what's important to cooling. Pressure could be high below the water pump but zero after the HE if the HE is blocked.
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Old 19-07-2020, 11:46   #4
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Re: suddenly overheating MD17D

We had rebuilt the coolant pump 3y ago. New bearings, new seals and a brass pump wheel to replace the old worn plastic wheel. Hence, i was pretty (perhaps too much) certain it should be ok. The pump has perhaps 100-150hrs run time (at most)

Would this pump degrade in a sudden, i.e. using the engine for 2h10min before starting to overheat and then continue to degrade? When we now start the engine it starts to overheat after 15 min. I have a spare one, as well rebuilt only though. They are not available from VP anymore. I'll exchange the pump next time I'll be at the boat (is unfortunately 400 km away).

The exit pipe from the heat exchanger is pretty cool. I can easily touch and hold it and it feels colder than good warm shower water. I have unfortunately no temperature reading from it.

Could the changed exhaust manifold fail in a direction that would cause this behaviour?

Thanks for your reply.
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Old 19-07-2020, 12:06   #5
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Re: suddenly overheating MD17D

Hmmm. Sounds like you have raw water flow, but are not exchanging heat. That could be blockage or the pump, both in the coolant circuit. When you checked the heat exchanger, did you check flow through it on the coolant side? Pull the core and clean it?

I'm still suspicious of the coolant pump. A HE blockage or encrustation would gradually lower heat exchange, while this seems more a failure. Pumps have moving parts and by definition can fail, even with nice new bronze impellers. Failures also happen at two points - when brand new due to a failure in the overhaul/installation, and long thereafter, when it wears out.

I can't speak to wet exhaust manifolds.
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Old 19-07-2020, 12:19   #6
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Re: suddenly overheating MD17D

I cannot take out the package at this heat exchanger. We checked the raw water pipes in it. Very clean (is new, has perhaps 40 hrs of running, so no built up in the pipes), no signs of seaweed or old impeller particles). We blowed out the fresh water side of it. There is no noticeable resistance.

I agree to the coolant pump hypothesis and will exchange the pump next time I will be there. I will try as well to find a way to test the fresh water side of the heat exchanger (other than just blowing through).

Still curious about the exhaust manifold as this was the only change that had been made on the engine since last season. Could of course be (and likely is) only coincidence that the fresh water circuit fails at the same time as we had the manifold exchanged.
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Old 19-07-2020, 13:26   #7
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Re: suddenly overheating MD17D

Whoa. What's the condition of your thermostat?
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Old 19-07-2020, 13:42   #8
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Re: suddenly overheating MD17D

I removed and tested the thermostat in a pot on a stove. It worked. I can see it as well working when using it in idle (measured the temperature on the thermostat housing, temperature is oscillating between 73 and 78C, it is a 74C thermostat). That said, the cylinder head (the first one That is easy to measure) is easily 10C hotter in idle.
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Old 19-07-2020, 14:19   #9
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Re: suddenly overheating MD17D

I have been down the same path with my MD17D over the last few years......
Tried everything you have tried..

I had been down the new cover plate , flip the cover plate route...no improvement. Then a friend said he had dealt with the exact same issue.

Impellor wears the cover plate... it also wears the inner face which is part of the pump body and thus can't be flipped.. and it has been wearing away at this for some 35 years.

Answer... replace entire pump... worked for him.

I managed to buy a genuine Johnson pump in Australia so they are out there.. I look forward to fitting it... one day.. some day..
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Old 19-07-2020, 14:26   #10
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Re: suddenly overheating MD17D

If the only difference between last year running OK and this year is the manifold was changed I would look to the manifold as being suspect. More so with a used manifold.



It's quite possible the exhaust manifold has a internal corrosion issue that does not allow full flow of coolant as in rusted through and not having 100% cooling when the T-stat opens.. which would explain why it works OK not under load. Look at the coolant flow chart in the manual and use a non contact thermometer to look for even cooling.
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Old 19-07-2020, 14:42   #11
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Re: suddenly overheating MD17D

You can check the output of the raw water pump. You can also measure how much it pumps out the exhaust. I assume the new manifold did not include the mixing elbow? If it didinclude the mixing elbow that may be where you problem is. Clogged mixing elbow restricting raw water flow.
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Old 19-07-2020, 16:57   #12
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Re: suddenly overheating MD17D

Have you checked the train?


Is any part of the train getting hot when you are in gear? You can test this by hand.



Maybe there is something wrapped on the prop? Maybe there is some mechanical resistance

in the train?


Another suspect is clogged tubes in the exchanger. At times stuff gets sucked all the way into the exchanger and some of the tubes get locked. It is a relatively easy check if your caps are easily removed (much as I tend to remove the whole exchanger and work on a bench in much more comfort).


I wonder what you find.



b.
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Old 20-07-2020, 13:42   #13
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Re: suddenly overheating MD17D

Thanks for your answers.

Yes, the mixing elbow is as well new this season.
I'll check the raw water pump housing and will replace the fresh water pump.

I still wonder how the manifold would degrade so it first works for 2 hours very well and then suddenly cause overheating. It does not leak, I.e. Iam not losing coolant.

I'll dive the prop. Would something that is wrapped around the prop and cause mechanical resistance create as well other symptoms. I do not seem to loose power or propulsion. Boot operates at the same speeds related to rpms
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Old 20-07-2020, 14:23   #14
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Re: suddenly overheating MD17D

Ah... the MD17D fresh water circ pump......

If it is the one that I am used to the impellers go forever... the problem is the seals... they fail.. water gets to bearings .. a true cascading failure....
Then you source a new one... problem is it is over 30 years old... rubber seals are already perished... and you can see where this is going.
2008 was my 'year of the pump'... and not just that pump... boat was about 23 yo at the time and pretty much every pump on the boat failed over the course of the year...

Anyway back on topic... those FW pumps are as rare as hens' teeth... send yours to a good pump shop and get new seals. That was the road finally I went down.. I had 4 or 5 new ones ( sourced out of Chile , Finland, UK and Australia ) start leaking either straight out of the box or withing 50 hours... all but one replaced under warranty.. in the end the Melbourne Au dealer had a new one rebuilt free of charge for me...
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Old 20-07-2020, 14:57   #15
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Re: suddenly overheating MD17D

Using this picture https://www.marinepartseurope.com/en/volvo-penta-explodedview-7742230-90-15204B.aspx


Is pipe # 71 in the new to you exhaust manifold? Is the thermostat #20 a two stage (as in flat disc on the end) and OE or Wahler or Caloric brand? Is # 29 OE or has it been replaced with an elbow (there's a restrictor in it)? You do have the small vent hole on the Tstat at 1200, yes? And you have an OE design o-ring #21?



The thermostat has to open when hot but it also has to close off the tube # 71. The flow actually redirects/reverses through the exhaust manifold when the thermostat opens. If the tube is too short or the thermostat has no plate it would give you the symptoms you describe.

A Pyrometer (Laser temp/IR thermometer) is ideal to check if the exhaust manifold is getting coolant it needs. It would show if you have the flow going on properly. Ideally it would measure the same temp all across. If the front is getting hotter than the back half (or vice versa) you have a flow issue related to the thermostat or piping or internal corrosion issue. If the exhaust manifold was a salt water cooled engine in a past life that would do it and it would show on the IR gun. The salt cooled manifolds would rust out the cast divider plates inside the manifold.

You can always use an electric coolant pump as opposed to #30. A heater circulator pump off a Mercedes, late model VW or AUDI would work fine. Mounted off the engine and supply 12V with proper wiring and a relay or switch rated for the amps.


You could check the salt water side by measuring how much time it takes at a given RPM to drain a bucket with a measured amount. For example if you take the hose off the raw water pump outlet/inlet hose to the heat exchanger and route it to the bilge or a second bucket, put the inlet side raw water pump hose to a bucket with water and a line drawn at say 1/2 full, run engine at 1500 or so RPM and time how long it takes to drain the bucket. The motor will not overheat in the time you need. The hook the raw water hose back up and repeat with same RPM and same water amount in bucket. Time to empty bucket should be the same as long as the RPM is the same and there is no blockage.

Feel free to PM me for my phone and I would be happy to walk you through it since I've been told my technical writing skills/language comprehension skills apparently suck.
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