Volvo thermostat puzzle

[Mirelle]

I replaced the thermostat on my MD2. The old one, a bellows type, is no longer made and the replacement unit (from a Volvo main agent) is a Motorrad item which looks rather different. It certainly works differently, in that the old one opened and closed progressively whereas this one, to judge by a finger placed on the outlet pipe, seems to snap open, then snap shut again, so the water exiting the manifold is either hot or cold but never warm.

Is this right? Engine seems happy enough, but may be running a degree or two warmer than before.

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[bedouin]

I haven't investigated in depth - but my MD2B certainly has a habit of oscillating quickly between hot and cold, which could be a symptom of the same behaviour you are seeing. In my case it does depend a bit on the load/revs with it sometimes oscillating and sometimes reaching a steady temperature.

This probably means that the engine is running too cold most of the time.

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[DIOGENES]

On my MD 11C at medium revs the water exhausts a constant warm temperature but at high revs it alternates hot and cold. Has done for years with no problems.

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[upstream]

Funny you say that, my MD11c does exactly the same. I thought it may be a problem but it seems to run quite happily. Although at higher rpm's it does get rather close to the red band before dropping back. I wonder if its a sign of water jacket blocking? One cylinder does run slightly hotter than the other.
The comment regarding the therostat snapping open and closed is a new one for me.

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[drawp]

Guys. If the temperature gauge does oscillate between hot and cold, it is an indication that the cooling ports through your engine block are partially obstructed, probably by scale and rust. As the thermostat opens and closes, the temperature reacts accordingly because all the flow is going through the thermostat and not through the engine block. I had the same oscillating problem and, acting on the advice of Mike Pilkington of Pilkington Marine in Newton Abbott, I de-scaled the exhaust manifold (using oxalic acid) and flushed/cleared the water channels in the engine block. Since then the engine runs at a constant temp. The harder it works the higher the temp.

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[Johnjo]

Had this with my MD2B when I had it, different thermostat to the original,
engine was okay with the original themostat until it fell apart, ran hot and cold with the new one,large puff of steam when the thermostat opened,
Overcame this by drilling a very small hole in the thermostat surround to enable a steadier flow
of water, cured my problem and the engine ran at the correct temperature,

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[Mirelle]

Ah! Thanks. That seems like the logical answer.

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[Mirelle]

I should have mentioned this

but the reason for changing the thermostat was that the aft cylinder was running much too hot, so I removed the manifold and rear cylinder head, cleaned out the passageways and re-assembled the engine; in doing so I noticed that the old thermostat had failed part open so I replaced it. The alarmingly high temperature of the rear cylinder has been cured, but it is still a little warmer than the forward one.

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[Johnjo]

Yeh had that problem too, same back cylinder as well,
Did all the openings in the head, then stuck a screwdriver down the waterway alongside
the cylinder, the jacket was filled about a third of the way up with very fine sand/silt,
the option was to remove the cylinder or do what I did, which was to blast it out with
a pressure washer, it was soft, bit messy though!!!!

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[Mirelle]

D**n! I did'nt think of that! I'll try running the engine with the drain cock open and see if that flushes anything out, as a first step - if not, head off again....

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[upstream]

Chaps,
Is there any sensable way of flushing the water ways with a degunger whilst the boat is afloat and without dismantling anything to severe, I had my exhaust manifold off last year and it was partially blocked. This all cleaned has reduced the occolation slightly but I fear that the block is also blocked. I had imagined bits of hose and buckets to recyle the degunging fluid. Tell me if I'm way of the mark. I have had it on good authority that brick cleaner from a builders merchants is a good degunger!

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[Johnjo]

Its bad enough possibly having to take the head off again, Its the cost of the gaskets that gets me!
One other thing Im sure you are aware of is the manifold gaskets, If fitted the wrong way round,
they close off the water ports!!


As to cleaning the internal waterways out with a descaling agent,
In my experience it is a waste off time and money, If the engine is scaled up internally
the only way to do it properly is to strip it, sorry!!!!!!!

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[Mirelle]

You are not kidding about the cost of gaskets! Ouch! Very good point about the manifold gaskets, also.

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[tr7v8]

Someone told me that kettle descaler works, but never tried it.
Other wise on road cars I've had success with Holts radflush, ignore the instructions it needs to be in for a while whilst hot and then pull a hose off so it comes out quick. Probably not caustic enuf for a raw water cooled engine though.

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Draco 2500

 

[Johnjo]

On my old volvo, I had to use a drill to clear the ports ,as if they had been welded over!

Mirelle if you have water coming out of the drains at the bottom of the pots,
would have thought that the jacket was clear, mine were clogged, had a poke around and all that was coming out was black silt, would have thought that feeling the pots top and bottom might give you a clue, hotter at the bottom possibly, if same would not worry to much about it.

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[Mirelle]

Whatever you do, take the thermostat out first or you will be along to the dealers for a new one!

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[vyv_cox]

As others have said there seems to be no chemical method of removing sludge that may be a combination of calcium and sodium carbonate, sand, rust, carbon, clays and salt. Flushing with hydrochloric acid will remove a lot of this but not the sand or clays. Raw water cooled engines are unlikely to contain any components that will be attacked by HCl but removing the thermostat might be wise.

An old Yanmar 12 that I owned suffered severe overheating problems that turned out to be caused by blockage of a small coolant passageway in the head. The culprit seemed to be a mixture of salt and carbon and I needed to drill it out. It was something like the consistency of a hardwood and no chemical method could have come anywhere near removing it.

Incidentally, looking for Brick Cleaner with Google seems to indicate that there is a wide variety of chemicals in various makes, ranging from detergents to acids and/or alkalis. I suggest it would be safer to stick to a known chemical rather than experiment with the unknown.

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[pyrojames]

I had this problem with my MD2B a long time ago, and although it can be blocked cooling pasages requiring major work, it can also be much more simple. There is a small hole in the base of the thermostat housing which allows a small amount of water through to the thermostat, when the thermostat is closed. As the hole is only 1/8 inch, it can get very easily blocked, and cause the temperature sender unit to get a hot/cold blast, as the thermostat cycles open/shut. Before you go stripping your engine, check this is clear first!

PyroJames.

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