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Old 27-01-2015, 20:58   #1
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diesel UNDERheating

i have a volvo md17c that i recently converted to fresh water cooling. i took it out and ran it hard to test it and it was steady at 185 on the gauge for over an hour with the throttle on the stop. but at lower rpm's the gauge would go up to 210 and then drop down to 180. a buddy had the same issue on the same motor and drilled a small hole in the thermostat which solved the oscillating gauge problem. so, i tried the same thing. didn't work. the other day i took the thermostat out and enlarged the hole from 3/32 to 1/8. put it back in and started the motor. let it idle to warm up but when it got to 210 it didn't drop back down, like it used to. i investigated and discovered that i had not replaced one of the coolant hoses. all the coolant was in the bilge and the impeller was shredded. recovered 5 of the six vanes but one went astray. new impeller and coolant and now the engine won't warm up. i thought maybe the missing vane was stuck in the thermostat so i checked that. no luck. tested thermostat. tried a raw water thermostat (lower temp) with the same result: engine BARELY gets above 100. i stick my finger in the heat exchanger and it's luke warm. ran at 1500 rpm for a few minutes with barely any change. what could make a diesel run so cold???
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Old 28-01-2015, 04:40   #2
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Re: diesel UNDERheating

MD7A here. Similar or same issue.

The engine used to run about on temp. But gauge was so bad, and my memory, I can't swear what that temp was.

Put in new thermostat and new contro panel. Now she barely gets above 110, maybe 115.

I ran a IR thermometer across the engine , I find no hot spots.

I did have the engine out of the boat and cleaned out some passages.
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Old 28-01-2015, 05:39   #3
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Re: diesel UNDERheating

If its been converted from raw to freshwater cooling, the engine was designed to run at less than 60C approx. to stop it cooking the salt out of seawater - so what, apart from changing the cooling system, did you do to re-engineer the engine to run at twice its designed running heat?
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Old 28-01-2015, 06:32   #4
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Re: diesel UNDERheating

hpeer- i had issues with the temp. sender and gauge but, once i got it sorted out, everything was pretty normal. the engine ran in the normal temp range according to heat gun and gauge. now, after puking the coolant and shredding the impeller, it runs cold. i just don't understand what could have changed???
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Old 28-01-2015, 06:36   #5
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Re: diesel UNDERheating

charlie- we're talking F and you're thinking C. when you do the conversion, volvo sells you a new thermostat with a higher temp. the seawater thermostat is rated at 63C and the new freshwater one is rated at 74C. so, the increase in temp is not that great.
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Old 28-01-2015, 07:06   #6
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Re: diesel UNDERheating

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Originally Posted by delmag View Post
........... i investigated and discovered that i had not replaced one of the coolant hoses. all the coolant was in the bilge and the impeller was shredded...................
The impeller is in the sea water circuit. Loosing all the coolant would not have affected the impeller.

Your engine was working fine until you started working on it. You have connected or installed something wrong. Go back and check your work step by step. If you can't see your mistake, get someone else to check your work.

Normally, if an engine won't heat up we would expect to find a thermostat stuck open. In your case, with all the work you've done, it could be that you have bypassed the thermostat altogether.

And find the missing impeller vane. It will cause you problems at some point. It may be in the heat exchanger.
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Old 28-01-2015, 07:11   #7
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Re: diesel UNDERheating

Another thing you may want to check, is some heat exchangers have a thermostat that sits inside a seal lip seal way down in the bottom of the hosing. If you replace the thermostat and don't replace the lip seal way down inside, the thermostat by-passes continuously even when it is closed.

Also, salt water thermostats have a lot larger hole than fresh water cooled thermostats... So if you put a salt water thermostat on a fresh-water cooled engine she's going to run cold.

Cheers,

Zach
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Old 28-01-2015, 07:44   #8
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Re: diesel UNDERheating

rwidman- in this case, there is an impeller in each circuit. the freshwater circuclation pump is a belt driven impeller pump. the sea water pump is gear driven. i agree that the engine was working fine before i attempted to "improve" things. i only disconnected three hoses. two from the pump and one from the thermostat housing. it IS possible that i crossed the two hoses from the pump but i labeled in and out on the pump and i made a diagram of the hose routing on the heat exchanger, according to input from the manufacturer. i've checked the house routing at least three times. i have thought about reversing the hoses just to see what happens but i'd rather not experiment at this point. i don't see how it is possible to bypass the thermostat. there is only ONE place that water enters the engine and ONE place it leaves. the thermostat is between them. i agree that the engine is acting like the thermostat is not even there. i did remove a hose looking for the missing vane. i suspect it's in the heat exchanger but have not investigated yet.
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Old 28-01-2015, 07:50   #9
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Re: diesel UNDERheating

zach- my heat exchanger only has a zinc, no thermostat. i know the sea water thermostat has a lower operating temp. but i just tried it to see if it would have any effect, since i knew it was operational. same results as the fw one.
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Old 28-01-2015, 07:54   #10
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Re: diesel UNDERheating

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rwidman- in this case, there is an impeller in each circuit. the freshwater circuclation pump is a belt driven impeller pump. the sea water pump is gear driven. ............
OK, its different than mine and others I have seen. Still, if it worked before you worked on it, its unlikely that the problem is something new, it just about has to be something you did.

It worked after you installed the conversion but only got screwed up after you started trying to get it to run warmer. Take it back to the point where it worked but ran cool. Use the original instructions and diagrams.
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Old 28-01-2015, 08:22   #11
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Re: diesel UNDERheating

rwidman- i completely agree with your logic. my thought process has been the same and i have attempted to return to the status quo ante. clearly i have failed and that is why it so frustrating. i did very little and changed very little so i can't understand why the results are so different. i'm going to call vdo and verify the accuracy of the sending unit, just to rule that possibility out.
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Old 28-01-2015, 11:02   #12
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Re: diesel UNDERheating

i found the missing impeller vane. it was in the elbow on the input side of the thermostat housing. going to put it back together and see if it makes any difference.
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Old 28-01-2015, 11:26   #13
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Re: diesel UNDERheating

Couple of points :

Quality thermostats have a "jiggle pin", a hole that's plugged with a little ball bearing. It's to allow trapped air to bleed through the thermostat, when it's closed. On some systems it's essential. Your drilled hole does the same, but of course is then a permanent leak through the thermostat. The hole should be at the top.

Some thermostats / housings will not seal around the rim, without a rubber gasket. Omit the rubber gasket, and enough fluid leaks around the thermostat so that the system never reaches operating temperature. Like this one :

1998-2003 Jaguar XJ8 Thermostat - Cooling System - Beck Arnley 98-03 XJ8 Thermostat - 602-05015339 - PartsGeek

I suggest confirming your gauge readings with some kind of IR or other hand-held reader.

You can do quite a lot of diagnosis by feel. When cold, the fresh water hose to the heat exchanger should feel cold. At some point, the hose should start to feel hot. This was the thermostat opening point. If this occurs when the gauge is reading very low, there is your problem.
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Old 28-01-2015, 12:06   #14
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Re: diesel UNDERheating

well, it appears that the missing vane was the problem. it seems that it was blocking the bypass circuit and causing FULL flow, all the time. does this make sense? anyhow, with the vane removed, the engine is running just like it was. thanks for all of the feedback and suggestions!
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Old 28-01-2015, 14:17   #15
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Re: diesel UNDERheating

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Originally Posted by delmag View Post
well, it appears that the missing vane was the problem. it seems that it was blocking the bypass circuit and causing FULL flow, all the time. does this make sense? anyhow, with the vane removed, the engine is running just like it was. thanks for all of the feedback and suggestions!
Good that its fixed. Its important to account for missing impeller pieces as you now know.
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