1998  Forester, just shy of 400000  kms, LPG

This is something I've not come across  before,  everythings generally fit  and running well except that suddenly, on my usual weekly round trip of 250kms  almost entirely on highway, the temperature gauge which I swear never moves beyond more or less halfway, once normal temp is reached, winter or summer, and we are 31º, 32º for he last month and in high summer mid afternoon maxx is usually high 30s and 42, 43 is not uncommon. , traffic, high speed, never budges... has started to creep slowly towards the red. after 3000 revs, 110 kph. The first time I saw it about a week ago i thought oh-oh, and dropped back down to 2500 revs with, and about 20 k to get to the motorway services. by then it gone back to 'normal'.   left it for about fifteen mins then checked  water level in the rad and in the plastic tank normal, no leaks ... carried on and for the rest of the trip, ,same thing , over 3000 gauge creeps slowly pretty close to the red, but slow down, and the old trick of opening the  heater and it goes back to normal   no leaks, levels good, no boiling over . oil level good  ....Failing themostat, ?  but that's usually the opposite, , water pump perhaps,?  time soon for draining and antifreeze renewal, as we can get -2 or -3º in february, some nights.. so shall I drain it all and do a rad flush ,  or is it an early warning...head gaskets,....?  Allthoughts and ideas welcome

I would get a specialist to check out the full system, thermostat etc, but with the heater affecting it doesn't sound great for the HG

I have always thought opening the heater basically dumps the cold water in that radiator and associated tubes into the system, cooling it down a bit ...Not sure  of the link between that and the head gaskets ...  thanks for the reply anyway....

4 hours ago, jonboy said:

I have always thought opening the heater basically dumps the cold water in that radiator and associated tubes into the system, cooling it down a bit ...Not sure  of the link between that and the head gaskets ...  thanks for the reply anyway....

The heat in the cabin comes from the heater matrix which is heated by the engine coolant. So its basically adding additional cooling, which gets rid of the extra heat thats overwhelming the cooling system when its just the normal rad.

When the headgasket goes it can cause gas to seep into the coolant from the cylinder, that gas is hot, which is what causes the heat to rise.

so basically failed headgasket, add temp to the coolant system, cabin heater takes that extra heat out of the system.

hopefully that makes sense.

you can try a sniff test that looks for exhaust gases in the coolant, although its not always 100% reliable.