Xantia heater fault

I have a fault on my citroen xantia 1.9 TD SX with air con and climate control, when the heater is on full and the blower is on any position but lets say full on, the temp out of the vent is fairly warm, but within about 5
seconds it cools right down no matter what temp setting it is on it always cools right down BUT every now and then it stays a little warmer showing this is not a consistant fault. I have had the car conected to the citroen fault diagnostic machine but there was no fault shown other than the sensors in the vents and the ambient temp sensor showing way too high, so i changed the sensor but the fault is still there. I also removed the device that controlls the flap that allows heat into the vents thinking the vent was not opening fully, I manually altered the flap but this made no change. When I first encountered this problem I thought the heater matrix was blocked but I am not convinced of this as every time I flush out the matrix it is flowing fine and the water is usually quite clear.
Any help would be greatly appreciated Thanks Phil
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I would say that you have a blocked heater matrix. The fact that you can get a flow through is irrelevant as you will get a good flow even if partly blocked. The matrix is like a small radiator in the dash, to work properly the hot water needs to flow across the whole area of the matrix. if three quarters is blocked you will still get a good flow with a hose pipe but when in use it will cool down quickly. I have heard that by dissconecting the heater pipes behind the cylinder head, you can connect a length of pipe and a funnel, pour a caustic soda mix down the funnel and this will sometimes clear it. BE CAREFULL CAUSTIC SODA CAN BE NASTY STUFF.
The only other option is to replace the heater matrix!!! but bear in mind that when they built the Xantia, the put a heater matrix in the middle of the floor and said "lets build a car around that"
Good Luck
Slim
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