brrr heaters again!!!!

[Fezzey mk5 driver]

hi everyone, if you've read my previous posts on here about my heaters I'm sorry for this latest one but I'm stumped again!!  driving back from work yesterday, motorway speeds 70 mph the heaters lost heat, when I slowed down at the slip road they got warmer again but not fully hot, it did the same tonight so when Ive just got home I took header tank cap off and a lot of hissing came out, is this a sign of air in the system? squeezed the heater pipes near where the HCV used to be (now just piped straight into matrix) stuck head back in car and heaters toasty hot again!!! can anyone shed some light please? cheers everyone

[Tdci-Peter]

6 hours ago, Fezzey mk5 driver said:

I took header tank cap off and a lot of hissing came out, is this a sign of air in the system? squeezed the heater pipes near where the HCV used to be (now just piped straight into matrix) stuck head back in car and heaters toasty hot again!!

Was the engine really hot, with the temp gauge right up, when you released the pressure? Then the gas coming out would mainly be steam, which would condense to water where the tank cap vents to. There could be a de-laminated hose, or something else blocking the system somewhere, causing overheating, and preventing the flow to the heater.

More sinister is if the system was not too hot, and it was gas that was escaping from the tank. The most likely cause for that would be a head gasket leak. Combustion gasses leak into the coolant, cause air locks in the heater circuit, and pressurise it.

Tests for head gasket leaks include: Smell of anti-freeze & white smoke from the exhaust. Loss of coolant. oil in the coolant. Rising oil level. Grey sticky emulsion forming in cooler parts inside the engine, like around the oil filler cap and in the crankcase breather hoses. CO dissolved in the coolant, which most garages can test for.

On a car of your's age, if a small HG leak was seriously suspected, I would chuck in some K-Seal. It worked on my old Vectra, at least for a while. The car failed MoT & was scrapped before the HG leak returned.