87 Hilux, 3.5 RV8 Cooling query

[Pos]

Hi,

I've been scouring the forum for a couple of years soaking up information and it's been a fantastic resource.

Last year I acquired a MK2 Hilux (YN67) with the Milners Rover V8 engine conversion, which I've been restoring and making roadworthy. The original SD1 3.5 had a worn bottom end, so we transplanted in a replacement P6 3.5 high comp engine, which is running great.

I have a cooling query, which I don't seem to be able to find an answer to and I would like to get the thoughts and ideas of the folks on here.

Essentially the setup uses the existing Toyota radiator and expansion tank, with the factory ~12psi radiator cap. It's the type with the unsealed expansion tank and bleed hose mounted at the side of the radiator.

I also have a Volvo header tank, mounted level with the top of the inlet manifold, which has a cap rated at ~10.8psi.

The engine generally runs cool when driving, but when sitting in traffic, the temperature increases (around 1/2 - 3/4 on the gauge) and the header tank cap begins to vent coolant. I have only ever filled the header tank 1/4 full.

My understanding is that the system should have a single 15psi cap but I would like to know whether or not people think I should ditch the header tank and keep the expansion tank, or ditch the expansion tank and keep the header tank? Can they work in unison?

Many thanks for any replies and Merry Christmas.

-Tom

[ChrisJC]

Have you got any pictures?

What I think will happen with your setup is that over a few warming / cooling cycles, the water will be drawn from the expansion tank into the system and fill up the header tank to the top. Then, because it has a lower pressure cap, it will escape from the header tank.

I have run both header and expansion tanks, both work just fine. I haven't ever tried both!

My logic is generally that if I can get a header tank as the highest part of the system, then I do that. Failing that, then an expansion tank lower down has worked OK.

Chris.

[Ian Anderson]

Welcome to the world of cooling issues.

First I would get a sealing cap but preferably with a pressure release valve on it so you can release any air caught at the high spot.

I would fill on the header tank and then put the sealing cap there on.

When the engine warms the fluid would be pushed out the other pressure cap on expansion tank into the holding tank, and in turn drawn back in when the engine cools.

Ian

[Pos]

Thank you for the replies, they've been really helpful. The point raised about the coolant being drawn out of the expansion tank into the system and then vented from the header tank makes perfect sense and it would seem to be doing so, given that the marked level is slowly reducing in the expansion tank.

I have created an album with photos of the cooling system that you can see here:
https://photos.app.goo.gl/awdMsEmeW2fNwEHE7

My understanding from the advice provided, is that I should fit a sealing (non pressure rated) cap to the header tank and the correct 15psi bayonet cap to the radiator for effective operation. In that scenario, I can top up and bleed the system correctly from the header tank, and the expansion tank to the side of the radiator will then serve its purpose as the coolant expands and contracts, with only the radiator cap allowing the system to vent and then draw back in.

Many thanks,
-Tom

[Ian Anderson]

From your pictures I would put the pressure cap on the radiator and the sealing cap on the header tank

Ian

[Pos]

Excellent, thanks for the help.

[ChrisJC]

Let us know how it goes!

Chris.

[garrycol]

Maybe you just have the wrong radiator cap on the radiator.

My Land Rover has an overflow bottle that is lower than the the top of the radiator and is outside of the pressure area of the cooling system. When pressure in the system gets too high the cap releases pressure and fluid into the overflow bottle which is open to atmosphere. When the engine cools it creates a low pressure in the coolant and the radiator cap opens allowing the coolant in the bottle to be sucked back into the engine. This needs a radiator cap that opens both with high and low pressure.

If you use a normal radiator cap it will open when pressure rises pushing fluid into the overflow bottle but not open when coolant cools - on the next heading cycle more fluid can be pumped out.

Any car that has the overflow bottle outside of the pressure system will have the relevant type of cap.

[scudderfish]

Is there a way to visually tell the difference?

[stevieturbo]

Is there a way to visually tell the difference?

Look at them, test them.

Some caps will seal under pressure ( to a point ) or vac even when the system cools. These would not have an external overflow bottle, water is never intended to be ejected.

Some caps have dual action that seal under pressure ( to a point, ), but then also have a smaller internal spring that allows air...ie fluid to be drawn back in as system cools and vacuum is present. These would be the type where an overflow bottle is used.

[Pos]

Struggled to source a sealing cap to fit my header tank. Luckily, the pressure relief valve in the existing cap has a brass core, so I've removed the valve spring and plastic innards and brazed the opening closed.

Pic: https://photos.app.goo.gl/ezNGm1gN68wAbVMy9

Got a 15psi radiator cap coming tomorrow and then I'll report back with the results.

-Tom

[Pos]

Maybe you just have the wrong radiator cap on the radiator.

My Land Rover has an overflow bottle that is lower than the the top of the radiator and is outside of the pressure area of the cooling system. When pressure in the system gets too high the cap releases pressure and fluid into the overflow bottle which is open to atmosphere. When the engine cools it creates a low pressure in the coolant and the radiator cap opens allowing the coolant in the bottle to be sucked back into the engine. This needs a radiator cap that opens both with high and low pressure.

If you use a normal radiator cap it will open when pressure rises pushing fluid into the overflow bottle but not open when coolant cools - on the next heading cycle more fluid can be pumped out.

Any car that has the overflow bottle outside of the pressure system will have the relevant type of cap.

Hi, thanks for your reply. It has the factory pressure rated radiator cap installed that works with the factory expansion tank. It is the twin seal type and it allows coolant to vent out and back in. It does have a slightly lower pressure than the V8 requires, so I'm rectifying that tomorrow with a 15psi vented cap that should work the same, albeit at a slightly higher relief pressure.

[Pos]

Just to follow up:

- Brazed the pressure relief valve closed on the header tank cap (effectively making it a non-vented sealing cap).

- Installed a 1.1 bar (15.9 psi) double sealed, vented cap on the radiator, which vents into an expansion tank.

Just been for a good run. Heater gets lovely and warm. Engine temperature didn't rise above 1/2 on the gauge, even whilst hammering it / standing stationary. And most importantly, there are no coolant leaks, nor had any coolant been forced out of the header tank.

It also appears that the system hasn't got hot enough to vent into the expansion tank. There was no trace of coolant being pushed out of, or being drawn back into the radiator. All in all an excellent result.

Thanks for the help and advice
-Tom