940 cooling problem at idle

[prebreye]

Hello.

I own a 940 with the b230fk, max boost pressure around 0.9 bar.
The car is loosing coolant via the cap on the expansion tank sometimes. First, headgasket and the whole cooling circuit has been tested and is fine. New cap too. The car runs fine, no sign of overheating, the fan never switch in second speed,..
Anyway, then I use the car and for example run with a pressure like 0.2 or 0.3 like on motorway and if I slow down to a traffic light or junction and stop as example, often, after few seconds, the cooling is boiling and going out of the tank. I suspect a problem with the cooling of the turbo (like if at idle the water flow in the turbo was stop). If I revs the engine, the problem diseapears. The same if the car runs at idle for a long time. I suspect the something around the turbo. Especially, when the engine is running, the flow is from the top to the bottom (to the pump) while if the engine stop the flow should be the opposite due to the thermosyphon effect. Otherwise, even under high load while driving, everythings seems fine.
Any ideas about which problem it can be ?
Or should I had an extra electrical pump to have a better flow in the turbo at low rpm, idle and also after stop ?
Thank you

[Clan]

Originally Posted by prebreye

Hello.

I own a 940 with the b230fk, max boost pressure around 0.9 bar.
The car is loosing coolant via the cap on the expansion tank sometimes. First, headgasket and the whole cooling circuit has been tested and is fine. New cap too. The car runs fine, no sign of overheating, the fan never switch in second speed,..
Anyway, then I use the car and for example run with a pressure like 0.2 or 0.3 like on motorway and if I slow down to a traffic light or junction and stop as example, often, after few seconds, the cooling is boiling and going out of the tank. I suspect a problem with the cooling of the turbo (like if at idle the water flow in the turbo was stop). If I revs the engine, the problem diseapears. The same if the car runs at idle for a long time. I suspect the something around the turbo. Especially, when the engine is running, the flow is from the top to the bottom (to the pump) while if the engine stop the flow should be the opposite due to the thermosyphon effect. Otherwise, even under high load while driving, everythings seems fine.
Any ideas about which problem it can be ?
Or should I had an extra electrical pump to have a better flow in the turbo at low rpm, idle and also after stop ?
Thank you

The cooling system certainly doesn't need any extra help , you need to find out what is happening . How about the water pump impeller , have you got a cheap non genuine pump on there? what about the 2 water cooling pipes and hoses to the turbo ? blocked or pipes collapsed?
What does the Temperature gauge say when you are having this problem?
is the radiator fan pumping out very hot air at the time you have boiling is hot ?
The flow should always be healthy and in at the bottom of the pump and out at the top hose when ever the engine is hot and running at any speed..

[MiniNinjaRob]

Does sound like a water pump issue, working just enough at working revs but low revs it just can’t cope, may be the pump itself or something else is blocked/collapsing giving pump too much to do?

[prebreye]

The water pump is rather new from a serious shop here in Sweden.
The 2 pipes and hoses looks fine to the turbo (at least from an external point of view)
The temperature gauge stays perfectly in the middle, the fan is never going to the second speed, and seems most of the time not to be on while driving but turning on in first speed if I drive slowly or in traffic jams.
If I open the hood when the water is boiling sometimes the fan most of the time the fan is off but in fact most of the problem appears while driving and slowing down.
But if the fan turn on really hot air is going out. And no pronblem of overheating under hard conditions like driving upphill while towing.
In short, the cooling systems seems to works perfectly

[prebreye]

Originally Posted by MiniNinjaRob

Does sound like a water pump issue, working just enough at working revs but low revs it just can’t cope, may be the pump itself or something else is blocked/collapsing giving pump too much to do?

That's one possibility. Otherwise, the coolant circuit should be fine: a recent radiator, recent hoses, no sign of dust inside..

[Laird Scooby]

What antifreeze did you use when you replaced the radiator, pump etc? It sounds horribly like a blockage somewhere and as others have suggested, at higher engine rpm, the water pump generates just enough flow to overcome the problem but when you slow down, it doesn't.

[prebreye]

I used glycol bought at the volvo store, mixed with 50% water.
I've added a light and a switch to monitor and control the 2nd speed of the engine and even towing heavy upphill at slow speed, the fan doesn't kick in 2nd speed and the gauge stays in the middle. Thus, it seems that the cooling of the engine itself is fine.
But maybe as said before the water pump is too weak, or maybe the cooling pipe of the turbo or inside the turbo are almost blocked.

[Rversteeg]

The gauge is quite heavily dampened around the middle position, so normally you will not see it move once it has reached this position. My E-fan only kicks in when standing still in heavy traffic, but I cannot see any gauge movement.

How high did you top up the coolant system? The expansion volume above the coolant liquid level is quite small if you top it up to max when cold. I usually fill it up to a third or halfway between min and max. Any higher and you will loose coolant through the cap until the cold level has dropped halfway.

[prebreye]

I was adding coolant up to the min level written on the tank.