engine coolant temp/pressure/expansion.

[TactilePaul]

Hi, can anyone adivse me as to the dangers of running coolant with little air expansion and instead a tube running from radiator cap blow off valve to an unpressurised bottle?

the cap (Volvo MD2030) blows at 75Kpa , I have no idea whether it reaches that pressure with a healthy air gap as we have not had any way of knowing if its blowing air out.
Obviously I don't want to put the system under uneccesary pressure.

Back story is that the Volvo method of attaching the pressurised expansion bottle to that cap was flawed, the cap seat is only glued on and is not built to withstand the vibration that came from the substantial brass manifold and hoses!, it failed, repeatedly, a dangerously poor bit of botched design!

so, as hoped, the water pump happily blows all the air from the hot water cylinder heating pipes that are higher up, our only issue is that we have no expansion other than the air gap above the copper matrix taking the raw water through, we have it just above those pipes and so far so good, but i have now attached a bottle to the small brass pipe so that we can see if the water makes it to the cap and the 75kpa is reached and maybe start raising the level so we don't have to worry / check it so often.
the cap sucks back as engine cools, I think at very low pressure judging by the spring ( so it is ok to remove and replace a rad cap when engine is hot as pressure equilibriates when cold?) so maybe we could fill right to the top and put some in the bottle and watch i go up and down, but I realise that the system will be running at 75kpa when up to temp. What about a cap that opens at a much lover level?, i realise that this lowers the boiling point!
any advice welcome.
Thanks.

NB mix of correct and lay terminology intentional!!

 

[Ammonite]

I'm not sure I've fully understood your question but if you want to eliminate the expansion vessel you'll need to replace the existing heat exchanger neck and cap with one of these as your existing cap (the one on the heat exchanger) won't be pressurised so will just dump all the coolant in the bilge if you connect an open ended pipe to this.
https://parts4engines.com/products/volvo-penta-md2030-heat-exchanger-repair-kit
Having installed the kit, brim the heat exchanger and run the engine up to temperature. It will expel a cupful or so of coolant and the level should settle a couple of cms below the base of the neck when cold. Don't keep topping it up as it will just dump the excess out each time.

 

[Bilgediver]

What cap goes where is governed by the design of the system as a whole and whether the expansion tank is pressurised or unpressurised.

If you install an unpressurised expansion tank then you're quire a radiator cap that is set to the correct pressure and one that also includes a non return valve that allows water to return to the engine as it cools after use. The engine should always appear full when it's cap is removed.

If you have a pressurised expansion tank then the cap on the engine is just a blank cap sealing the neckvwhere attached and no springs etc. The pressurised cap is on the expansion tank. The cooling water is free to pass both ways between tank and engine.

 

[Ammonite]

I'm assuming this is what you have now? ie. a closed system with the pressure cap on the expansion vessel?
MD2030-C Expansion Tank - marine diesel engines

 

[TactilePaul]

I'm not sure I've fully understood your question but if you want to eliminate the expansion vessel you'll need to replace the existing heat exchanger neck and cap with one of these as your existing cap (the one on the heat exchanger) won't be pressurised so will just dump all the coolant in the bilge if you connect an open ended pipe to this.
https://parts4engines.com/products/volvo-penta-md2030-heat-exchanger-repair-kit
Having installed the kit, brim the heat exchanger and run the engine up to temperature. It will expel a cupful or so of coolant and the level should settle a couple of cms below the base of the neck when cold. Don't keep topping it up as it will just dump the excess out each time.

there were a few questions to be fair!, thanks for showing me that repair kit, we had to eliminate the expansion vessel as the connecting manifold and its vibration were repeatedly destroying what you can see there is just a bit of glue!
The system works fine without expansion as long as we keep the level low, just above raw water pipes, more than a few cm down, not ideal, hence my question...I'm guessing you don't have any extra volume of coolant extrernal to engiine?, IE to a cylinder/calorifier?

 

[Ammonite]

there were a few questions to be fair!, thanks for showing me that repair kit, we had to eliminate the expansion vessel as the connecting manifold and its vibration were repeatedly destroying what you can see there is just a bit of glue!
The system works fine without expansion as long as we keep the level low, just above raw water pipes, more than a few cm down, not ideal, hence my question...I'm guessing you don't have any extra volume of coolant extrernal to engiine?, IE to a cylinder/calorifier?

If you remove the expansion tank you need to replace it with the parts I linked to in #2 or you risk damaging the engine. The coolant circuit needs to be pressurised as it raises the boiling point of the coolant and prevents coolant loss.