Poor heater output

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Location: Kent

Poor heater output

Postby ChrisDixon » Tue Dec 23, 2008 7:43 pm

I'm getting poor heater output until the car has been run for a good while and then only when out of town and driving faster. So far the heater pump has been replaced to no avail. The engine warms up quickly so that rules out the thermostat and the hoses are all hot around the pump and the valve. I've thought abouit trying a new valve in case the original is only partly operatiing, but a quick check on the internet suggests only second hand ones are available at around £40. Are there some further tests I can run? If its not the valve the next obvious suspect is a blocked heater matrix, which I assume is a pain to replace. Are these prone to airlocks and is there a procedure for testing flow? Would disconnecting the hose on the return side of the heater circuit give an indication?

All suggestions gratefully received. Regards, Chris.
'95 XJR Manual
'86 XJ-SC 3.6 Manual

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Postby robv12 » Sun Jan 04, 2009 1:50 pm


I'm not up on X300 heating systems, but David Marks is, maybe worth a call??

On 40's the heater flap motors can seize up, this could be your problem, maybe someone with more knowledge can help here

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1990 xj40 heater

Postby joejag308 » Fri May 22, 2009 8:31 am

With no heat to the floor whatever i decided to let someone with the experience have a look.......i had exhausted other options ......The problem turned out to be rather a shock...
One of the swivel pins on the main outlet flap to the cabin had broken and to replace it meant having the heater taken out en block......then a new pin either made or buying a new one ........then after servicing the rest of the heater while it was out of the frame you have to install it back in.
With an XJ40 it is never guaranteed to go back as it came out and possible other items of wear and tear become evident. So a very expensive episode....the alternative was either buy a new heater or keep sticking your screwdriver up the flap in winter to allow the hot air to the footwell then in summer pull it back down to keep your feet cool.....
descisions descisions.......Oh to own a Jaguar

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