Coolant System Residual Pressure
 

Pretty sure deep down I know the answer to this already...


My 1.8 non-turbo overheated, still not been able to track down why really. At the time I just threw a load of water in without bleeding to get home.

Anyway, last weekend I levelled the coolant properly, and carried out what diagnostics I could.
The blue 'sniff' test was negative for exhaust gases.
Took it for a test run - no evidence of oil in water or water in oil, even after 200mi to work this week.

However, there is always a good amount of residual pressure left in the system even after its cooled fully (overnight or 10hrs in works carpark). Top hoses are almost rock hard, can barely squeeze them at this point.
There's also evidence of slight coolant loss, the past 2 days I've put 100ml ish in each afternoon.


Temperatures are nice and steady at around 85C, helped in no small part by a lazy 88C stat...


So anyway, I can easily see how it could be an airlock, but I can't see how that could be causing residual pressure? Am I looking at HGF?

An important law of physics tell us that the pressure of a fixed volume or gas rises as the temperature rises and falls as the temperature falls, such that pressure x volume equals a constant x temperature. So, a cooling system at ambient temperature that's opened will be set to atmospheric pressure when you next drive the car. The cooling system will/should return to that same ambient pressure after cooling down to the previous ambient temperature. Any significant overpressure thereafter means there's more permanent gas in the system than before. Translation, there's exhaust gas pressurising the coolaing system (even a bromothymolblue test notwithstanding).


TC

And for simplicity,

Pressure = problem

Your getting residual pressure from somewhere unfortunately. Try the head gasket

That's the exact conclusion I'd reached, just wanted a second opinion.


Bummer. So much for the MLS being fit and forget, I only changed it in November (not for HGF reasons). Just hope the head is OK...

I personally don't rate the mls, I've far fewer issues with the original style elastomer.
It would likely be fit and forget if it were used within its operating parameters, the overheat certainly won't have done any favours.

Well yesterday I managed to pick up an elastomer gasket for free from the local T4/Mini specialist because "they're rubbish and you'll get 6 months top from it and there's a pile I can't sell". Which is good.


Confirmed diagnosis of HGF today, coolant smells of petrol. With it being the MLS that's currently fitted, what are the chances of it being the gasket that has failed vs the head warping?

Probably depends on if it still had water in it when you stopped.

A small drop of water will hold the temperature down to a safe limit but if you take too long to stop and the engine runs dry the temperature will ROCKET up and distort the head.---:eek:

Red light of doom was on for maybe 5 seconds before I shut it off, reached a temp of 120C on the IPK, but had been sitting at 112 for a good while before that.
Was fortunate that it lasted just long enough to get to the layby I was aiming for.

You should be OK. A temperature of 112C is considered to be within the 'normal' band. The gauge even wouldn't move until 115C-plus. And 120C is only just starting to get hot. I believe the light pops on a bit over that. The real worry point is 130-140C up.


TC

If it’s new old stock check the elastomer bead is completely intact, quite rare but some had issues with quality out the box