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rabbi 01-08-2015 14:30

VP D1-20 overheating only at idle
I have a strange engine issue. Maybe someone has an idea what the reason is.

One of the two Volvo penta D1-20 in our catamaran is overheating. To be more precise it always happens when anchoring. Anchoring typically means going around at idle looking for a place, maneuvering back and forth in idle or around 1000 or 1200rpm with little load, maybe running hard in reverse to set the anchor. And always in these critical moments the engine overheat alarm sounds.

I can cruise the faulty engine for hours at 2000 or 2500rpm and a few minutes at WOT are no issue but when i'm at the destination looking for a place to anchor the engine overheats. The engine also overheats when anchoring after a long sail, the cold engine overheats after 10-15 minutes. Ambient temperature is very high here currently.

What i checked so far...
Seawater cooling looks good, output about 11 liters or 2.5 gallons per minute in idle. No steam or smoke from the exhaust, the water exiting the exhaust is warm but not hot.
The impeller is good, the heat exchanger is clean. The coolant pump does indeed pump the coolant. The thermostat works according to spec (starting to open at 90 celsius, fully opened in boiling water). Coolant is new and clean.
I thought about testing the engine without a thermostat but there is a bypass pipe that is closed as the vave opens. If i just remove it the bypass can possibly add to overheatkng so no good test.
Coolant has been drained and refilled a couple of times just in case there are air pockets.
The egine is indeed hot, the coolant is under pressure and steam escapes when the cap is opened.

I'm lost... What can cause overeating at little or no load but does not cause overheatig at medium to high loads?

GILow 01-08-2015 15:28

Re: VP D1-20 overheating only at idle
Just a question, not a suggestion... Does the engine have a temperature gauge? You mention an overheat alarm but I wonder if it's assertion is being corroborated be the gauge?


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MBWhite 01-08-2015 15:33

Re: VP D1-20 overheating only at idle
Where I have seen this is when an impelller is worn or damaged with broken vanes, they do not pump well at low rpm's. However you have stated that impeller has been replaced so that should not be an issue.

Logic tells me it is most likely that water/coolant is not circulating at low engine speeds so check for a restriction and double check the impeller again.

What I would suggest is getting a non-contact IR thermometer:
and run both engines at the same time. Use the thermometer to compare temps between the 2 engines at several different spots to see if you can see where the restriction is occurring. I once tracked down and fixed a similar issue that was caused by a cigarette filter that had been sucked into the intake.

Good luck!

GILow 01-08-2015 15:54

VP D1-20 overheating only at idle
Ah... An idea. Not the raw water impeller, you've checked and you have good flow, it's the coolant pump. That would cause the problem you are seeing.

Edit: I see MBWhite has beaten me to it.

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Ecos 01-08-2015 16:00

Re: VP D1-20 overheating only at idle
After it gets hot will it cool down if you increase the load and rpm? Does it cool down in neutral and higher rpm? What is the water output of the other engine at idle?

hamburking 02-08-2015 08:10

Re: VP D1-20 overheating only at idle
Sounds like the coolant pump. Lots of water over the the raw water pump is good. Coolant pump may be harder to get at.

Or maybe a clog (corrosion or crap) in the heat exchanger.

rabbi 02-08-2015 13:18

Re: VP D1-20 overheating only at idle
Thanks for the feedback!

@gilow: no gauge, only the overheat alarm in the display and beep. The coolant pump is tbe next thing on the list.

@mbwhite: i got an IR thermometer today and will check tomorrow as we approach the next anchorage.

@guy: i shut down the enine when the alarm occurs. I'm afraid to do permanent damage to the engine by ignoring the overheat alarm. So i have no clue if it would cool down at load or higher rpms.
The exhaust water output of the other engine is slightly less than the engine witb the problems. The heat exchanger is really clean, so i guess its not a raw water issue but a coolant circulation issue.

So what is left seems to be the coolant pump and the coolant ducts / pipes inside the engine block. Any idea how to check or service these?
What is the rated output and pressure for a coolant pump and how do i check these?

Thanks again

tinkrman69 02-08-2015 20:11

Re: VP D1-20 overheating only at idle
How old are your engines? a circ. pump impeller wearing out is not unheard of, but not common at all. its so rare no one carries one for spare. But then again? low rpm heat high rpm cool? not even belt driven could give those symptoms. If the impeller is worn it will be pretty obvious. I'v only seen two on engines well over 30 years old. but the veins will be short and well rounded edges.

Of course with the inferior way the make things today I guess that will become common.

tinkrman69 02-08-2015 20:13

Re: VP D1-20 overheating only at idle
adding here if the passages in the cooling system were restricted it would run hot at all speeds and more so at higher rpms.

rabbi 02-08-2015 23:11

Re: VP D1-20 overheating only at idle
@mbwhite: thanks for the PM. I got those docs already but it doesn't really give a hint how to check the circulation pump. Only how to replace...
The belt is ok, no sings of slipping.

@tinkrman69: theengines are 2006 or 2007, my best guess is 3000hrs. The boat got new electronics after a lightning strike so hard to know. The 3000 is just based on the 3-4 season the boat has spent in charter (in Malta, so short season).
my gut feeling tells me a worn pump could give low coolant pump pressure at low rpm and sufficient pressure at higher rpms. However that pump runs at 600 euro so i don't want to just give it try it at this price.

Later today i wil be able to check temperatures. These engines seem to run pretty hot with the thermostat starting to open at 90degree and fully open at 105 degree celsius, so above unpressured boiling temperature.

GILow 02-08-2015 23:32

Re: VP D1-20 overheating only at idle

Originally Posted by rabbi (Post 1882242)
However that pump runs at 600 euro so i don't want to just give it try it at this price.

Ouch! That's AU$900! One small benefit of our clunky old truck donk I had not expected. New water pump AU$150. I am going back to the boat to talk nicely to the engine for a while.

Anyway, you may be able to get a reasonable diagnoses of the coolant pump if you can:

With the engine COLD,

a) remove the thermostat, just for the duration of the testing.
b) observe the flow rate of the coolant at varied engine revs with the thermostat out of the way. (I am assuming this engine is of the conventional type where the thermostat closes to restrict the flow of coolant when the engine is cold.)

In a car, this s easy enough, just pull the radiator cap and you can usually see what is going on, but I have in the past resorted to putting a section of transparent hose in the coolant circuit to see what was going on where the radiator cap was too far from the input side of the radiator to see what was happening.

There are usually enough small bubbles or tiny bits of crap for you to see how fast the cooling water is flowing. If the flow goes to crap at idle then I think you can safely conclude the coolant pump is no longer functioning as it should.

Of course this still could be a sign of partially blocked cooling passages, but I guess you cross that bridge if you come to it.


tinkrman69 03-08-2015 12:14

Re: VP D1-20 overheating only at idle
you can pull both pumps since you have two engines and one os runnin' well. compare the pumps. if you want something more scientific make an adaptor out of PVC to go in the discharge line from the pump, mount a gauge in it and test both engines. see if there is a difference at matching rpms. test both low and high rpms on each engine.

rabbi 03-08-2015 13:26

Re: VP D1-20 overheating only at idle
Today I did some motoring and sailing so had time for testing.
The engine did an hour at 2300 without issues. Temperatures quite similar compared to the other engine. The temperatures at entry and exit point of the heat exchanger was 94 celsius and 78 celsius, so the thermostat was open and the heat excbanger working. Head temperature measured beside the injectors was 95 celsius.
Wind picked up so i did an few hours of sailing and started the engine later to run at 1600rpm for motorsailing. After 20min the overheat alarm sounded and I stopped the engine.
Heat exchanger temperature was 78celsius entry and 75 celsius exit, below the thermostat threshold so the thermostat was closed. The head temperature was 110 celsius...
I started the engine 5 minutes later to run at 2300 rpm and temps went back to normal.
I don't see how blocked cooling pipes could possibly allow the temp to drop at high rpms.

I will test the coolant pump throughput tomorrow but i guess the pump is toast.
The engine block is perkins based so i hope to find a perkins pump for less than the volvo penta cost.

tinkrman69 03-08-2015 14:02

Re: VP D1-20 overheating only at idle
I don't know the availability of Perkins in Germany. but if you have to go out of country, try Knighting Gail tractor in Tennessee USA spelling is up for depute ;-)

Acushla 03-08-2015 17:04

Re: VP D1-20 overheating only at idle
Fresh water cooled engines require a raw water flow of around 35 litres /min for the heat exchanger cooling.
Raw water cooled engines require about 11 litres/min as this is direct cooling.
Therefore I think you should look to the raw water pump first for an impeller/casing wear and/or damage.


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