Coolant leak only when heater on

[npjrogers]

Following a burst upper heater feed pipe (along the side of the tappet cover) and a replacement thermostat housing for a hairline crack we now find we are still loosing coolant.. about an inch in the header tank after say 30 miles. The problem disappears when the heater control is turned to fully cold ie totally off. We have no evidence of leak inside the van. Has anyone had this before.. Any thoughts or ideas very welcome please as we use the van all year round and the colder weather is now here…

[MinorMatt]

My first idea is that it would be rather useful to know what van you have....

[npjrogers]

Sorry.. It is a’96 non banana smiley 2.5 DI. I think the rotary switch is manual rather than electric.
It appears that the leak is on the engine side of the firewall as we have no evidence of a leak in the cab. It is a swb camper conversion..
Thanks for reading my post and taking trouble to reply.
Nick

[npjrogers]

Ok all… I have researched this in some depth now.. firstly there are two systems fitted on mk5 transits depending on whether an AC unit is fitted…. If there is an Air Conditioning unit fitted then the system usually has a cutoff valve which prevents hot coolant from running through the heater matrix when the AC is on.. you don’t want to compromise the flow of cool air by passing it over a warm matrix..that all makes sense.. some valves are cable driven from the rotary Blue ….. to Red switch on the lower dash. It may also be driven by Vacuum or electric solenoid..
I must now confess my mistake..aka oversight..aka stupidity…
After the coolant was topped up when the hose repair was done there must have been some air still in the system.. possibly in the heater matrix.. of course.. as the air eventually got to the coolant reservoir it lowered the coolant level… hence the assumption of a leak.. If this happens to you it is worth checking whether there is in fact a cutoff valve in the coolant circuit?. The best check is to turn the dial to blue (cold) start the engine till running warm and then check the coolant hoses where they go through the firewall bulkhead.. if there is the system has a valve one of the pipes will be cold.. if both are hot you have a simple flap based system (like mine) which deflects (the hot) air away from the cab interior…
Hope this helps.
Nick

[MinorMatt]

Sorry I didn't get back to you sooner - what you write above is correct

The non AC vehicles have a constant flow across the heater matrix, flaps regulate air temperature. AC ones have a cut off to stop hot air going over the matrix