Coolant low in radiator

[autosri]

I'm really stuck with this now and desperate for help

After a drive the coolant moves into the overflow tank and raises the level by about 10mm, maybe a touch more and doesn't go back into the rad when cooling, its has never over heated at all

So far it's had a new ally rad, thermostat, pump and 3 different caps, the original 18year old oem unit then motor factors one then nismo1.3bar cap, I've changed the hose to the overflow tank and about a year ago it had an oem headgasket and bolts due to a snapped exhaust manifold stud, but it's been fine for 9 months

I've pressure tested the cooling system to 20psi and no leaks, I've done a compression test and are all 150psi give or take 2psi either way, I've done a leak down test and that passed with no bubbles in coolant, I've had a coolant sniffer test done and that passed

The cooling system has no pressure at start up and slowly builds pressure over 5-10 mins of idling/blipping throttle

I'm so rediculously stuck it's unbelievable and every garage I ring arnt interested in looking at it

It's getting to the point where I'm gunna pull the head off just because

Tganks

[dannyboi200sx]

Checked your water pump?

Sent from my E6633 using Tapatalk

[autosri]

Had a new one in mate but the old one looked like new anyway

[paulsx]

Mine did this after fitting an alloy rad was fine with the oem one, I also went through 3 radiator caps. It never ran hot or overheated but passed fluid into the tank and didn't draw it back. It must be boiling somewhere in the system to vent it

[autosri]

I might swap the oem rad back in just to try it

[autosri]

Just swapped the rad over and gunna go out for a spin later tonigh and see how it goes

[Jon]

You seem to have already covered all the possible problems, so I'll give a random idea.

The hose between the radiator cap and the expansion tank.
As standard, this hose reaches the bottom of the expansion tank. It connects to the top of the expansion tank, then there's a plastic moulded tube as part of the epansion tank which reaches the bottom of the expansion tank.

Does yours?
If not, the cooling down phase will be sucking back air.

The above seemed a daft thing to write, but there are many non-standard things going on in SXOC engine bays.
Is it possible that the hose from the radiator cap is not reaching coolant in the expansion tank in your case ?

[autosri]

I think I have found the issue it's the head gasket gone I tried the oem rad and it did the same so chucked a bottle of steel seal in it and it fixed it so gunna drain it all out and get the head skimmed and a new gasket in there

[paulsx]

I wouldn't blame the hg just yet it could be a small leak elsewhere or the oil cooler failing. You cna get a cheap die kit on ebay that should tell you if there are combustion gases in the cooling system before you do anything

[green_rs13]

^^this^^ do a 'sniff test'...

[jbsauce]

You'd have bubbles in your coolant too surely

[autosri]

I've had a sniff test done and it was fine but if I cruise gently it doesn't do it but only when I nail it it blows coolant

[autosri]

Turns out it was a very slight leak in the head gasket nothing would pick it up even a sniff test but fitted an cosworth mls gasket with cosworth head studs and it’s been perfect since no visible signs of casket failure really but there was a scratch on one of the fire ring seals on the head so had that skimmed and all is good

[autosri]

Oh and I think there must have been a small amount of bubbles as it was so much easier to bleed the coolant afterwards