The water is boiling up in the header tank of my 318i and pushing back through the pressure cap,the system seems fine,no blockages,and the head gasket is fine,im at a bit of a loss,anyone got any ideas??

Head or intake gasket leaking into cooling system.start it with cap off cold and see if it keeps bubbling. If yes compression is leaking into cooling system.

Tis is the K18 engine same as K16. It is the twin camshaft petrol engine where the engine is a fully aluminium and very sensitive to temperature.surely the problem you have is the compression gasket. Now you must be very carefull on trying to fix it. You need a good machinery job to check very well the cylinder head and also the cylinder block because the piston liners are replaceable and depends to the overheating removed from the proper position fitted. If everything is correct and by skimming the cylinder head problem will get solved, must buy the new three leaf metal gasket kit and all new cylinder head bolts.

Replace radiator water by adding coolant

The car may have a compromised head gasket, forcing air pressure into the cooling system, displacing water/coolant mixture and forcing it out into the reservoir.

This can be expensive to fix if it's the case, but it could be that it has a faulty radiator cap also.


does it have proper antifreeze/anti-boil mixture, or just plain water?
if it's plain water, it may indicate that this is an ongoing problem that the current owner hasn't fixed.

Check the cap and see if it seems loose, the spring inside should be firm, and the inner sealing ring (rubber) should not be cracked or damaged. if the system is leaking, this can cause similar issues where the coolant boils off into the reservoir.

if the cap seems fine, it will likely be a blown head gasket between a cylinder and a water passage, the problem will continue until the gasket is replaced.

pressure test radiator by colour check system ,or check for petrol vapour in header tank or radiator, check oil in engine to see if it is contaminated with water oil ,could be leaking rocker cover gasket causeing plugs to missfire if gas, petrol smell from water its head gasket ,water in oil headgasket, oil in water headgaskget if any of these requires head skim and head pressure test by a engineer

check radiator is not blocked, check air can pass through radiator (fins not blocked with debris) check that there is no airlock in system. is header tank being pressurised when engine running ( remove cap and start engine, look for bubbles in header tank). If bubbles then you need to check head gasket or even crack in head or cylinder walls. Presume you fitted the thermostat in the right way round. if you have boiled the engine since fitting the thermostat you will probably need to remove it (maybe see if it boils without it fitted) and possibly replace it again. (drop in hot water and see if it opens, if it doesn't then replace it)

a few things to check before we condemn it as a leaking head gasket(pressurising water out).
First check the radiator cap seal,no cracks and that it takes some pressure to push the spring loaded part down.
Does both the top and lower radiator hose get hot as temp gets to normal(where thermostat should be open).if not,the thermostat may be a faulty one or has slipped out of seat as the cover was bolted back down onto it.(temperature will differ either side of it).I have had some of these come in with part of the waterpump impellor rotted away.same symptom of one hot and one cold hose,and water doesnt flow inside radiator with cap off(ensure this is not removed if engine is hot)and temp where the thermo should be open.
Possibly the radiator needs sending out for the top or side tank to be removed and each tube cleaned out.this is a lot more effective than trying to reverse flush with water only.
But if temp is normal these areas should be fine.
Do the hoses swell with pressure and become hard to squeeze in a short time?this could reveal a leaking head gasket and should be tested with a teekay type tester for the presence of cylinder gases in cooling system.
Do the fan(s) come on if sitting idling at normal temp for 5-10mins?water can be pushed out if they dont come on or their cut in/out temp is incorrect.hope this helps.
let me know some times re when it pushes water out,do you hear a boiling sound from inside engine or radiator when this happens,any other info that may help diagnose.

Does the vehicle overheat on the road? If it does and pushes big burps of coolant into the reserve tank then you could have a head gasket leak. Usually what happens with a head gasket leak is that the engine will often start and run normally but will soon start to push coolant out in a kind of foamy burp as the combustion gasses that accumulate in the engine block force their way out. You can have a small head gasket leak that allows combustion gasses to escape into the cooling system without getting coolant back into the engine and often the first symptoms of this kind of a leak is a motor that suddenly overheats, the heater quits blowing hot air and after a short while everything returns to normal for a day or so. As the leak worsens however it may keep pushing coolant out of the reserve tank with a cool or cold engine as the pressure builds to quickly for the reserve tank to handle it fully. Having said that if you have a perfectly good cooling system that is hot and take off the radiator cap it will spray blistering coolant coolant all over so never remove the cap from a warm or hot cooling system! Most additives won't stop a head gasket leak as there is just too much pressure for them to "stick" in the leak and they just get forced out of source of the leak. It's rare but you could have a water pump that's not circulating enough coolant around but usually you have different symptoms than this.

sound to me like you have a blown head gasket, try letting it heat up with header tank open and if coolant overflows when you give it a rev up this would tend to confirm it,check also for water in oil or oil in water, water in exhaust all point to head gasket or head crack problems.

no im a mechanic it sounds like that you have an air lock in the cooling system .i am familiar with ford freda and bongo and the design where the engine is is a bit far from the radiator ,so the problem with air locks magnifies.really the solution is just drain the water and refill it slowly while you are pressing and releasing,just repeat the action on the top radiator hose to get rid of any air trapping within the system,it's as simple as that really.you want it to have just water and no air as this will cause a blockage.