I've been noticing recently, as the weather has gotten colder, that the temperature gauge on my 2004 XJR isn't staying in the middle. In the summer it would rise to the middle line and stay there, now it rises to slightly bellow it. If cold is 0 and the top of the gauge is 1, the normal operating temperature is 1/2. I'm usually somewhere between 3/8 and 1/2. There is heat, but it's not exactly hot. The gauge rises (but never overheats) going up hills, and falls going down them, or with other increases in speed. Stopped in traffic the gauge will rise to 1/2 and stay there, until the speed picks up when it drops again. The coolant is right up to max and I don't smell any coolant, either inside or outside of the car.

I think that the thermostat may be stuck or sticking open, but I thought I'd ask here. Does that sound likely?

 

Has the fuel consumption deteriorated ? A thermostat that fails in the open position will certainly affect the warm-up of the engine. As replacing a thermostat is fairly cheap, it might be best to just put a new one in and see what happens. After all, to check the one that's there means it has to come out, so you might as well put a new one back !

 

Do you have a P0128 code? That usually indicates a failed thermostat. Get access to a scan tool and watch the actual temperature reading during warmup/driving, but it kinda sounds like a bad thermostat.

 

If you replace the thermostat note carefully how it was installed.

From reading this forum the stat comes in bits and it is not obvious

how they fit together and in the housing.

Consider installing replacement housing with new stat installed

which can be the easier method.

 

Those symptoms are exactly what mine was doing and it was a torn seal on the thermostat. Cheap and easy fix.

 

I would second the suggestion that you replace the thermostat housing as well.

Not long after I got my car, I had an engine fault warning light which was due to the engine not warming up quickly enough. After the thermostat was replaced, the problem came back again after only a few weeks. Following A bit of research on this forum, I "suggested" to the workshop that the housing should be replaced as well... problem solved!

I found out very quickly that this forum knows more about the car than the local dealer does, so I now save myself time and money and get the car serviced by an independant workshop.