Just replaced the head gasket on my EL XR6... all going well except when I drive up a mountain or flog the car for an extended period (eg. down a nice windy road) the temp gauge starts to climb and the heater stops working (no heat at all).

It normally sits between the letters A and M on the temp gauge - about 90 degrees in diagnostic mode - and gets to about 100 degrees when the engine is working hard up a hill etc. Heater goes cold at about 97 degrees, and starts heating again when the temp drops to around 93-ish.

I have the bypass thermostat with the tail that seals the heater pipe when the engine is up to temp, so is this normal, or do I have an airlock/dodgy thermostat etc. Thermostat is only 6 months old, radiator about a year old, water pump is a new genuine ford unit, and I don't remember it doing this before I replaced the head gasket. Also it's definately not the heater tap playing games because I disconnected the vacuum line so it's open all the time. Coolant is Nulon long life at 33%.

Anyone else noticed this?

Thanks

you could have the vacum lines around the wrong way ...

Yep vacuum lines is my bet too. Flatten it off the mark and see if it the heater works then. My guess is it doing it under load only. Also at idle take the lid off overflow bottle and have a look to see if coolant is circulating. Airlock is possible but not likely. Thermostat could be at fault. Lines to heater core ok? Heater core could have an obstruction that moves and slows flow.

yep! i had the same prob with mine, it was just a case of swaping the two vacum lines that come off the BBM where the fuel reg is ..

Thanks guys, I'll check the vac lines, might have disturbed them while doing the head. It seems to be engine temp/coolant flow related though because when I back off the heater doesn't work untill the coolant temp gets back down to low 90 degrees. I can see the flow in the coolant expansion tank increase when the engine is hot... then it drops down again and the heater starts working.

The thermostat is an aftermarket Dayco one with the tail to close off the heater pipe when open - I'll try a genuine Ford one next.

Cheers.