Volvo MD2020 overheating at high rpm.

[VicS]

Your symptoms fit with a loose circulation pump impeller, although I know nothing about a 2020 circulation pump in particular. Often the impeller/turbine is just pressed onto the end of the pump shaft and I've seen a few that have become a loose fit. They tend to keep up at low revs and everything runs fine but then they start to slip at high revs and don't provide enough coolant circulation. Just an idea..

Interesting thought.

It looks like this and is not user serviceable

On the subject of pumps, although I dont think it applies in this case, flexible vane pumps that are driven by a key acting on the hub of the impeller are known for the impeller to slip on the hub sometimes.

 

[lw395]

Your symptoms fit with a loose circulation pump impeller, although I know nothing about a 2020 circulation pump in particular. Often the impeller/turbine is just pressed onto the end of the pump shaft and I've seen a few that have become a loose fit. They tend to keep up at low revs and everything runs fine but then they start to slip at high revs and don't provide enough coolant circulation. Just an idea..

Saw that on a car forum once, the impeller fell off when the pump was removed!

 

[Heckler]

So it sounds as if the heat exchanger is not getting the heat into the sea water, rather than a shortage of seawater flow.
So, are some heat exchanger tubes blocked? Is it full of scale etc?
Is the heat getting to the heat exchanger? Lack of coolant flow due to thermostat, not enough coolant in the HE, some obstruction such as bits of delaminating hose or lumps of gasket sealant?

An IR thermometer to see which bits are getting hot might be useful?
Sounds like the heat is going to the calorifier rather than the HE.

If the HE is not exchanging it could be the raw water flow is not enough. The calorifier dumping is normal when it gets too hot, mine used to do it.
My money is on the cam in the pump being worn, Vic got it!
S