It could be either:
Faulty Thermostat
Faulty radiator cap
Head Gasket.
it shouldn't boil, even if it's straight water. although 50:50 coolant is what you should be using.
First, change the thermostat and try bleeding it:
With the
engine OFF and cold. Get a coke bottle, cut the bottom off it and stick it upside down in the radiator filler.
Fill the radiator with coolant, then fill the coke bottle with water. always keep the coke bottle filled.
The open the radiator bleed screw on the opposite side, let the water run until there's no bubbles and the flow is smooth, then close the bleed screw.
There's a 5mm allen key bolt on the thermostat housing. be VERY gentle with it. Again same story, until the flow is smooth with no bubbles, then replace the bolt.
Then finally, do the same for the bleed screw on the heater matrix pipe near the strut tower. Don't lose the cap, it's NOT the same as a tyre valve cap
remove the coke bottle and replace the radiator cap.
This is the cheapest thing to try first.
Your heaters not going cold is a good sign, it might not be the head gasket. However, if it's using coolant, it has to be going somewhere... doesn't always show up with a compression test either.
If there's steam coming from the radiator cap and the coolant hoses are rock hard, then unfortunately it probably is the head gasket.
Temperature woes…
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