Alfa Romeo Forum banner

Pressure in coolant (expansion) tank

1032 Views 18 Replies 9 Participants Last post by  MrAfaKam
Hi!
How much pressure in coolant tank for 1.9JTD 16V (Alfa GT) is normal?
When i open coolant tank the pressurise goes away and antifreeze rises substantially, is it okay?
When the tank is closed there is no bubbles when engine is running, but when it is open there are bubbles, should i be worried about a head gasket? There is no oil in coolant tank and no MAYO in oil cap/dipstick

Thanks!
1 - 2 of 2 Posts

· Registered
Joined
·
1,275 Posts
Is there residual pressure in the cooling system after the engine has cooled down? If yes, then this is not normal, and suggests a possible head gasket issue. However, if the asnwer is no, then this does not preclude the possibility of a head gasket problem...

Regards,
John.
 

· Registered
Joined
·
1,275 Posts
Hi,
After a 20min drive, check the coolant hoses, are they hard ? When I mean hard, like almost rock solid.

after the car has cooled down, open the coolant bottle and do you smell any fuel?

coolant Will pressurise as the it gets hot, but after the car sits overnight the pressure will release and in the morning when you open the cap only a little pressure should be present. If coolant gushes all the way to the top and overflows out then you have a problem.

Unfortunately my guess is your head gasket is gone. You won’t necessarily see oil and water mix because the head gasket damage will most likely be between the coolant and compression areas, this is why your coolant is being over pressurised.

good luck.
I suspect the same, the symptoms do suggest a possible HG leak.

When an engine has fully cooled down there should be zero residual cooling system pressure. The cooling system is a closed system, and when operating correctly, commonly the pressure doesn't reach the cap valve vent pressure, so pressure rises to some value below cap vent relief pressure, and then falls as the temperature returns to ambient. So, the pressure should also return to where it began, i.e. zero (i.e. atmospheric).

It does seem to be possible for a cooling system (with a failed HG) to retain some residual pressure even after having fully cooled. Some HG leaks appear to be temperature and / or pressure sensitive, i.e. they don't start to leak until the temperature reaches X° and / or the pressure reaches Y, and then stop leaking when the temperature falls below X° or pressure falls below Y. This is not a proven theory, but it does explain the particular symptoms I've encountered with two failed HGs in two different engines.

If a HG leak were always 'open' at any temperature, then the system pressure would always return to atmospheric pressure after the engine is shut down. Pressure would return to atmospheric more rapidly than it normally would, because pressure will be able to escape out the 'back door' via the gasket leak, and not have to 'wait' while the temperatrue gradually diminshes back to at / near where it started..
-----------

With a normally operating cooling system there should be no loss of coolant, nor loss of air from the gas pocket in the expansion tank. If the pressure does increase enough that some air is vented it will be 'replenished' as the engine cools / pressure decreases, due to the cap vent alowing unrestricted flow in the reverse direction (hence system pressure never becomes negative when it cools). If some residual pressure remains after the engine has fully cooled down, then this suggests additional 'feral' pressure has entered the system from somewhere, and the likely source is cylinder pressure via a gasket leak, but the leak would also have to be either temperature sensitive, or pressure sensitve (i.e. not leak unless the pressure differential were above X).

As you suggest, hyper-pressurised hoses are indicative of a failed HG causing a large leak. So much pressure may be entering the system that the cap relief valve can't vent fast enough and so the system pressure rises higher than the cap vent pressure, and so the hoses become extremely turgid ('hard'). After shut down the cylinder stops pressurising the water jacket and so system pressure will fairly quickly fall back to cap relief pressure as the cap valve catches up.

However the pressure is then likely to continue falling quite quickly because the failed HG is likely to leak both ways, i.e. pressure entering the cooling system (from a cylinder) and then pressure exiting the cooling system (into a cylinder). A smaller HG leak may not leak enough cylinder pressure to significantly challenge the cap valve, so the system may reach cap vent 'blow-off' pressure but not significantly exceed it, and so the hoses wouldn't become so 'enhardened' with hugely excessive pressure. Still a bad place to be...

Regards,
John.
 
1 - 2 of 2 Posts
Top