Intermittant Heater Problem

My 1999 TD5 110 heater is playing up,  At about 40mph and above I have heat no problem, but as soon as I slow down or stop at a junction it blows cold, as soon as we get going again at a reasonable speed it is fine,  I am also losing coolant but no evidence of a leak (it has been into a garage to be checked) if you stop after a trip you can hear hissing fron the header tank cap.  My feeling is the cap has had it and not holding pressure any ideas?

No heat is caused because your coolant level is too low which is also hard on the engine, If you think it is the cap replace it ASAP and bleed the air out of your engine.
If the cap doesn't fix it have a pressure test run on your coolant system to find the leak, be a head gasket or outer leak.
Good luck,
Mike

Sounds like head gasket.  You definitely need a pressure test done.  I had the same thing happen on our 4.0RR a couple years ago with identical symptoms.  No obvious coolant leak, and it was only found by pressure testing.
 
 

I'd change the pressure bottle and cap before getting into a huge task and bill of stripping down the top of the engine. Unless you've really run the coolant low and overheated the engine the worst case of the head gasket failing is not usually the problem, but it can happen but there are other things to check tht will confirm that - glowing leak check fluid. If it turns up in the tail pipe you have a gasket problem.. The bottles and caps do fail and refreeshing them and a pressure check is a good first start. When did you last change hoses, they fail too and often there can be pin head holes that only leak under presure and the coolant evaporates which is why you don see a puddle or obvious leak too.