ATO

Thanks. I did consider that but got nothing out of the bleed tap on the calorifier, so assumed the outlet pipe was empty. I'll do as Tony says and that will show if it's already bled itself through.

Thanks I'll do that when I refill with coolant. I would've thought the bleed tap would have the same effect but it's clearly not the case, strange.

Oil cooler replaced, all good. In the process of flushing the skin tank I realised there has to be a bleed bolt somewhere as it was clearly trapping air and there was other no way to remove it. I found it hidden behind timber framing for the floor, though it was seized solid needed a lot heat to get it moving. So I've test filled with water and it's running sweetly bang on 82c. Left it for an hour and the temp didn't move, so it looks like the original problem was skin tank air rather than calorifier. However, I'm still a bit stumped by the calorifier circuit, I've now got a hot pipe going in but the pipe coming out is still cold. Tried bleeding on the calorifier, got a spit of water, then nothing. Definitely not affecting the engine running temp though.

Still got to clean out the skin tank and get back to the original problem of air in the system. Thanks for your help and suggestions so far. I'm learning.

It's gear oil. Oil cooler has a leak.

Didn't particularly smell of gear oil but then it's mixed with coolant and water. If I get the oil cooler off and blow air through one of the oil pipe unions while blocking the other, presumably a leak will be obvious.

Not yet, will look at that next time. Spent so long trying to flush the skin tank I ran out of time.

I've been over to the boat and done a bit today. Flushed the engine and skin tank separately. I've mentioned before oil in the coolant but I'm amazed by the amount of oil that has come out of the skin tank. I flushed for an hour and it's still coming out in drips and drabs. Have a look at this photo of the skin tank contents drained into the bilge. I can only think that someone has poured engine oil in the heat exchanger instead or the rocker cover at some point, owner denies any knowledge! Didn't have time to fill up and run it. Do these skin tanks have an internal baffle?

Definitely no non return valve on the calorifier. Pump impeller is on the list if there are still issues after a hosepipe flush/back flush - however again no problems prior to me draining down it's unlikely to be that. Good point on possible blockage in the gearbox heat exchanger, if there is a blockage that's a prime spot. I'll replace the hoses too. I'm definitely leaning toward a blockage somewhere now. It'll be a while before I'll find the time to flush the engine unfortunately.

I think the owner has - we'll deffo try that.

Top connection for skin tank comes from heat exchanger, bottom to water pump. I have no concerns over the piping orientation - it ran fine before. skin tank is lower than the heat exchanger. I think I may have to drain down again totally, flush through/back flush. Thanks all for your continued suggestions.

This isn't my boat or my engine, it's been running well for years. The owner (a neighbour) asked me to service the engine as it has not been serviced for some time. What ever the set up that exists now, it's as it has always been without any input from me. I've just approached it, serviced it and attempted to change the coolant. Whatever the issue is, it's not the existing pipe work, as it'd been bled in the past. I'm just attempting to understand why the calorifier circuit won't bleed. The stat is opening as I've monitored it warming up with an infra red temp gun. It opens around a bit late around 85c. There are three photos in one of the posts above however I'm not sure you can see them from the comments. There's one pic of the pipe coming from the cylinder head, it goes to the lower union of the calorifier. There's a pic of the pipe from the top union where it is Tee'd into the waterpump pipe & there is a pic of the MT cock on the calorifer. I'm starting to think there is some sort of partial blockage in the coolant system within the engine.

It may well be, however it has been like that for years without issue. The problem I'm having has been brought about by me changing the coolant. The engine hadn't been serviced for many years, she only asked me to service it as it was stalling when idling. Change of filters sorted that. However when it did run properly it did so at 85c. I wish I hadn't agreed to help now!

I see what you are saying about the neat coolant, however I did manage to bleed the calorifier on one occasion after trying for ages. That time nothing would come out at all like now, however after running the engine for a while I noticed the pipe from the cylinder head has partially warm for about 30cm from the head, so I tried bleeding again and suddenly coolant spurted out and both in/out pipes were hot. I thought the job was done at that point, however the next time the owner ran the engine a few weeks later, she called me concerned as it seemed to be running very hot after 20mins of idling. The coolant level in the heat exchanger tank had dropped and the calorifier loop just had air in it again. Owner says it idled at around 85c before.

The only pipe connected to the boot is going to the skin tank. Sorry if I didn't make it clear enough.