Loosing water under pressure

[DonE]

Happy New Year all. I took my Stag for its first run today after recently flushing the rad and refilling with blucol. I had changed the standard expansion tank pressure cap to a 20pound psi one.
After 2 or 3 miles the temp guage started getting towards the red. When I returned to the garage there was steaming hot water spraying out of the expansion tank (old style not new header tank) pressure cap. Water in the expansion tank was bubbling. Although the temp guage was showing hot, the Kenlow fan did not kick in
Anyone know if this could be an air lock or problem with the expansion tank?

[Hettiherald]

Sounds very much like an air lock to me. Especially as you have just flushed the system. Do a search for radiator bleeding or flushing, loads of tips about getting rid of air.
Dave

[DonE]

Thanks for the advice Dave. Also been advised to jack up front left of car when re-filling rad to help clear air.
Rgds
Don

[StagnJag]

Is the new cap ok? If you still have the old one, swap back and see if it is any better. Was it ok before you did the coolant change?

I seem to recall that some caps appear to fit, but are too shallow and don't seal properly. Did you need to push down firmly on the cap to tighten it up? Could you have broken the seal between the bottle and its brass neck?

That's all I can think of.

Rgds

Dave

[Lingen]

Don.
There should not be any need to jack the car up/park it on a slope etc. to fill the cooling system. If you have the correct thermostat with an air bleed incorporated in it, and the small waterways through the front of the inlet manifold are not blocked, then it should fill immediately at the radiator without any gymnastics. Running the engine for a while with the heater on "Hot" should finally purge any small air bubbles from the system. If the pressure cap is sealing correctly, as already advised, and the problem remains, then I wish you all the best!!
Mike.

[Guest]

Could be air lock, I replaced my coolant and went for a spin, temp towards the red, top hose stone cold, gave the thermostat housing a gentle tap, top hose got hot and temp returned to normal so my stat had stuck! Think it was a new stat too

[DonE]

Thanks Mike, I will try that.
Rgds
Don

[DonE]

Hi Dave
It had overheated once before I changed the coolant (on the way home after buying the car!)
Fiddled with the Kenlow thermostat as it seemed to have been set to max and was not kicking in when hot. On advice, I replaced the old cap with a new 20psi one when changing the coolant. It had been running fine and the fan was working correctly until I took it out for a run on New Year's Day. I plan to fit a retro Header tank next to help the cooling and maybe that will fix any potential issues with the seal on the expansion tank.
rgds
Don