Tidgy

yeah, could be its pick up knock at high revs, tbh sounds like it my be worth a run on a dyno to see whats going on.

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.

bit late, but tbh i would just replace them with a set of decent coils, correct setup wont be back breaking etc I've got tein flex z on mine and i love em, firmer than stock, but not back breaking etc

Could try importcarparts

Prices are all over the place, but i would guess 4-5k

have to say im not that up on subaru diesels, but i do remember crank issues being mentioned. tbh just keep up on top of maintenance and just drive it and not worry about it unless you need to.

try importcarparts

could try importcarparts, or might be a good time for an upgrade

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 would get it to a specialist to get it checked out, could well be duff cam sensor, which could be bad if left.

Welcome :D get some pics up :D

Welcome :D

what colour is it? almost looks purple in the pic

I expect you need to use a proper delete kit as its prob stuck open.

importcarparts maybe a good place to contact