Heater on Engine temp Goes Up

[adcad]

I've had a good search all over the net and would just like to check what you guys think before changing parts ect!

My car is running sweet AF, always over serviced cambelt up to date ect.
Amongst other things in the past 6 months had new header tank new radiator, new top and bottom hose.
The engine temp stays perfectly just below the halfway mark regardless of driving conditions, I use the m25 most days so it does sit in looooong traffic jams no problem at all! And since the new radiator loses no coolant what so ever and sits perfectly on the MAX line.

BUT

Since its started getting colder out and for the first time in ages I put my heater on and nothing but cold air came out despite being up to temp.
I had already hurt my back that morning so thought that's **** and **** my life, as you do!
Anyway turned heater off and didn't try again till 40 miles into the journey put heater on still no warm air but noticed when I put the heater on the engine temp went right up and fast quickly started mashing buttons on the heater dial changing heat settings from window to feet back to window and then all of a sudden heat started coming out the blowers and the engine temp went back to normal.

This seems to be intermittent issue and doesn't happen all the time.

I've read all sorts from air lock, heater matrix, thermostat to head gasket failure...

Any brains on here that offer any advice? Thanks!

[Red Baron]

First thoughts are an air lock, was the heater valve open when the system was drained and re-filled, was it filled from the header tank rather than reverse filled from the turbo coolant pipe?

Shane

[adcad]

I’m not sure about heater valve, must of missed that part in the guide I followed but yes I filled it from the header tank.


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[SpecB]

Hopefully and most likely an airlock however it could be an issue with the heater matrix or the bypass valve.

Has the coolant level dropped in either the expansion or overflow tank if so that would point to air coming out of the heater matrix which is now trapped in the 'wrong' place could cause overheating. You could try running the car for a bit with the heater on max and the expansion tank cap off to see if you get any bubbles - also try manually 'burping' the top hose with the cap off as well (open when cold!). If there's no joy with that and it keeps happening the next step would be a coolant drop and I'd also consider flushing the heater matrix with fresh water to see what kind of state it's in.

[adcad]

Well thanks for help guys, you were right, it was an airlock. A huge one because as as red barron said, I didn't have heater valve open when I refilled rad , lesson learned! Took about another litre of coolant and a good 15 min on the ramp with engine running to get all air out. Nice easy fix, like it!