Volvo md7a overheating, temperature fluctuating - Page 3 - SailNet Community
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post #21 of 53 Old 06-11-2014
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Re: Volvo md7a overheating, temperature fluctuating

I'm just starting down this exact road on my MD7A. I have other issues, but at least my motor runs now!
Here's my question though (and be gentle please because I am a newbie.
How much flow should I get just by opening up the thru hull valve that runs into the strainer?
My mechanic said to check and see why we're not getting any water flow, so I'm starting at the beginning and working my way up. I removed the hose from the thru hull to the strainer and opened up the valve just to see what would happen. I got a steady trickle coming out...not sure if this is what it's supposed to do or if it's possibly clogged. My boat is sitting on the Delta here in Nor Cal and the water is extremely murky.
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post #22 of 53 Old 06-11-2014
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Re: Volvo md7a overheating, temperature fluctuating

OK, let's see if I can make an ass of myself here. I'm gonna bet I know your problem.

On my MD7A The water goes through the strainer, into the transmission, then through a really stupid pipe to the water pump, then to the water jacket.

The problem is the connection between the transmission and the water pump. It can be either connection or the pipe itself.

The connection to the transmission is weird and requires a compressible copper washer. You need to get a new one, get a few. The connection to the pump is just a push In kinda fitting with a fat rubber o-ring. The pump housing should have two u shaped holders to keep the pipe in place. If you don't have the right fat o-ring you can use 4 or 5 regular o-rings in series.

The pipe itself may be cracked and sucking air. I eventually cut the middle out of the damn thing and used some hose to make the connection, just keeping the bitter ends for the connections.

That FINALLY stopped the over heating. Took me forever and some help from an old codger to figure it all out.

The engine would run fine for days, then all of a sudden overheat in just a couple of minutes. The pipe didn't have the right copper washer so I didn't know I needed it. They had used silicone to make the seal, this was intermittent. It was after that that I found the copper pipe had a hairline crack in it. IIRC it had been repaired previously. Anyway, the copper pipe was a mess to fit. Once I cut it out and replaced the middle with hose life was sweet.

They don't pump much water at all, just kinda a weak splurt, then another, then another.

When you open the thru hull you should get a good steady stream of water, a little fountain, not a trickle. Stick your finger down in and see if you have some growth or sludge in it. May need to stick a pencil or something through.

Last edited by hpeer; 06-11-2014 at 11:05 PM.
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post #23 of 53 Old 09-09-2014
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Re: Volvo md7a overheating, temperature fluctuating

Definitely check transmission ports (in/out/top plug/bottom plug.) Did mine and they were alm0st blocked completely .
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post #24 of 53 Old 09-10-2014
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Re: Volvo md7a overheating, temperature fluctuating

I had a similar problem on the raw water cooled Universal 5411 on my previous boat. Flushing of the cooling system with acid and a new thermostat eventually stopped the fluctuation. Seemed to be caused by restricted flow and the relatively slow response of the thermostat.
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post #25 of 53 Old 09-13-2014
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Re: Volvo md7a overheating, temperature fluctuating

tx :


What you are describing is perfectly normal for this engine, and for my Volvo MD17C.

When it is cold, the thermostat is in its closed position (with just a wee bit of by-pass leak to it).
The vast majority of the coolant water simply goes to the exhaust manifold (always actively cooled or it will roast), then by-passes the motor and goes straight to the exhaust elbow.

As the water heats in the blocks, the thermostat detects the temperature rise, the temperature sensor needle climbs, the thermostat opens, and the coolant water arriving via the exhaust manifold then skims off some of the hot water off the top of the motor and feeds it to the exhaust. The temperature drops in the blocks, and the temperature sensor drops in to the green again.
This is normal.
If you put your hand on the coolant feed to the exhaust elbow you will feel it heating and cooling alternately as the thermostat feeds alternate hot and cold water to the elbow.

What is not normal is if the temperature needle stays in the red zone.

If you take the thermostat out, the motor will behave as if the thermostat is always open, and the needle will barely climb off the cold stop and will stay in the green zone all the time.

With time in salt water service, the wee water entry ports to the blocks (water entry after the water cools the exhaust manifold) can scale up and the motor can overheat. The ports are easily cleared by a screwdriver or very stout wire (like coat-hanger wire). You have to take the exhaust manifold off though and buy some new exhaust gaskets.

You are welcome to send a PM and we can talk further if you wish.

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Last edited by Rockter; 09-13-2014 at 03:52 AM.
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post #26 of 53 Old 02-29-2016
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Re: Volvo md7a overheating, temperature fluctuating

Hey,
How did you end up fixing the issue? If Rockster was right your engine would still be running!
I have an MD&a with exactly the same heat fluctuation.
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post #27 of 53 Old 02-29-2016
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Re: Volvo md7a overheating, temperature fluctuating

I have the exact same engine and exact same symptoms. I have cleared the small ports in the exhaust manifold, and there is also a small bypass port in the thermostat housing that gets clogged. I believe it provides a minimum flow when the thermostat is closed that will smooth out the temperature fluctuations. In spite of cleaning those ports I still get the temperature spikes so I suspect there is still blockages in inaccessible areas. next step will be some kind of caustic flush I think.

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post #28 of 53 Old 03-01-2016
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Re: Volvo md7a overheating, temperature fluctuating

I would make sure all fittings are air tight first. Those push in fitting are dicey. Check the engine to transmission connection as well.

Good luck.

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post #29 of 53 Old 03-01-2016
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Re: Volvo md7a overheating, temperature fluctuating

Quote:
Originally Posted by christian.hess View Post
if you are using an engine that has almost 10 year old oil

STOP
Exactly. That gunk thats in there now stopped being engine oil long long ago. Engine oil not only lubricates the stuff that goes round and round and up and down, less friction developed heat, and it carries heat away from combustion chambers, cylinder's and pistons. The acid's and contaminant's that have built up in that (once upon a time was oil) stuff are eating away at every thing they touch, seal's , bearing's and the cylinders are most likely scuffed, scared up, ready for a bore job. Change oil, then do compression test on all cylinders, if below required spec, time to rebuild it. As little as 10 % difference in compression between cylinder's, I rebuild.

Last edited by uncle stinky bob; 03-01-2016 at 09:39 PM.
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post #30 of 53 Old 03-01-2016
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Re: Volvo md7a overheating, temperature fluctuating

Was just thinking, also if you really want to know what going on as far a internal engine wear, send a sample of that used oil in to a lab, it's cheap, will tell you if the engine is done based on how badly it's contaminated with metal and it can pin point which parts are wearing badly based on what types of metal are present.
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