Over the past 2-3 weeks I have noticed that if I drive my mk4 golf steady and drive like an old man my heater gets hot after about 10-15 mins but then if I give it some poke down country lanes etc the heater then goes stone cold until I stop. The heat eventually returns ands gets red hot again. The temp gauge never moves off 90' once up there so it's put doubt in my mind if it's the thermostat at fault. It has also started to lose water over the course of about 6-8 days after the expansion bottle has been filled the warning flashes on the dash for coolant. The head gasket was done about 9k ago and I'm hoping that it isn't going again. I have had a quick look for any signs of coolant leaking and found signs of antifreeze on the chassis leg under the expansion bottle and also a dot of coolant under the flange on the gearbox end of the head. (This is the pre heater type flange with the wires on it.) Not had a chance to pull the timing cover off to check water pump but I'm thinking it maybe at fault due to having to replace the last one only a few thousand mile ago as the one fitted with the head gasket started to leak after only 5000 mile.
my tdi dose this bud not worked out if it's the head gasket yet but that's my thinking that or a cracked head was going to check the water pump and thermostat first when I get time mine seems to hold pressure and if you boot it it push the water level down and comes back up when the cap is released also has the hot and cold if you boot it
Unfortunately that's either head gasket or cracked head. Common on 115's and 130's but less common on the 150's (all share the same head but the 150's use 12.9 head bolts rather than 10.9). Don't go skimming the head willy nilly either as they're sensitive to the compression raise. If you do, i'd advise upping the head gasket thickness accordingly (unless you're on a 3 hole gasket already and then you can't really up the thickness). Basically the cylinders are pressurising the coolant (hence the coolant stains under the expansion bottle) which makes you lose coolant and will also reduce the coolant level until you release the pressure. You may hear it hissing out of the cap after taking it for a blast.
