MK2 Heater problems

[chucklebus]

Hi...I need some help with my heater please.
When the engine's warm and I turn on the heater, it blows warm air for about 10 seconds and then it goes cold.
I've flushed the pipes and can get water to flow through the matrix inlet/outlet pipes so I'm assuming the matrix is ok.
Both pipes also get hot when the engine's running, and I don't have a bypass valve fitted.

Does the hot/cold lever on the dash open/close a valve on the coolant pipes? Haynes manual says to have it set to hot when emptying/refilling the system, but why? I thought this lever only operated a flap?

[gazareth]

yeah the lever only operates the flaps so the water circulates no matter where either lever is. the flaps have a thin foam layer on them which tends to break up over time which affects the heat output. if you take out your centre console you can get at the flaps to check them.

[Santiago]

...I've flushed the pipes and can get water to flow through the matrix inlet/outlet pipes so I'm assuming the matrix is ok...

Unfortunately, that isn't a particularly safe assumption.  If you were to cut open a heater matrix, you'd find that there are a dozen or so narrow pipes that are all fed in parallel by the main 'in' pipe.  Just because you have flow from inlet to outlet pipes tells you almost nothing except that not all of them are completely blocked.  Flushing is hampered by the fact that there are all these alternative small pipes for the fluid to choose, so it's going to mainly flush the pipes that are least blocked and do little or nothing to ones that are bad.

Your best chance of improving things is to use a hosepipe sealed onto the inlet or outlet, and another length to route the expelled water out of the engine bay, then give it full bore mains water pressure for a few  minutes.  Swap the hoses over and repeat. 

[gazareth]

yeah that's the best way. flush it out in both directions a couple of times. but my money is on the flaps. it happened to mine and the heating got so poor I had to investigate further. and turned out to be the heater flap foam had all fallen off leaving big holes in the flap and bypassing all the hot air.