XJR temperature issues

[terminalreality]

Can anyone help which I am hoping is only a small issue. My XJR is not reaching it's correct temperature when being driven on a run (not at warp factor just normal driving). It does however reach normal temperature whilst driving round town. This problem has only occured this winter. My first suspicions lead me to suspect the thermostat but i would of thought that if the thermostat fails it would overheat instead of running cold. All suggestions of the problem welcome.
Thanks Andy.

[melhook]

It probably is your thermostat, sticking slightly open. you dont say whether you've got a V8 or straight 6 ? Straight 6 Very easy to remove/ test and change. When cold, drain some coolant, use a drain tray under top hose, and remover hose and catch coolant. Then remove two 10 mm headed bolts on thermostat housing, take out thermostat, noting that bleed valve is at the top. Have a look through the thermostat, and see if the plunger is stuck open. Boil the kettle, fill it right up, make a cup of tea, and find a container big enough to hold the therostat, now poor hot water onto stat, see if it opens. Let it cool down, see if it closes. If it doesn't fully close, you'll need a new one. Might be worth buying one, see if you can be cheeky and get it on a sale or return basis !! Fitting is as simple as it was to take apart, some have gaskets, some don't, the ones that don't, you will need some silicon sealant, ask when you buy the stat, telling them what you want it for. If it's a V8, you'll need to do the same procedure, but you'll need to remove the 3 8mm headed bolts at the top centre of the engine. Testing is the same. This has taken me 3 times longer to type, than it would for me to do the job, tea included !! Any probs please ask. Melv

[terminalreality]

thanks Melv I'll give that a go. BTW it's a V8 1998 model

[richardxjr]

For reference, I just changed my 'stat and it is now up to full temp within 4 minutes from cold starting. I think the stats fail 'open' hence it previously taking forever to get up to temp.

[terminalreality]

Cheers Richard, after more delving on the web it does appear that the 'stat does fail in the open position. Thanks for the response and I'll let you now the outcome.