Was driving from the alignment shop Sunday afternoon when I noticed the temperature guage suddenly start to rise (was initially pretty much at the 12 O'Clock range). As the gauge neared the red line, I turned on the hearter full blast and the gauge returned to its 'normal' postion.
Does this indicate an issue? Has not arised before/since, however, the cooling fan does remain on for a few minutes after operating the vehicle for a while and then parking it.

 

First of all, the temp gauge is not a true indication of engine temp, the engine temp can vary wildly and the needle will stay in the middle (it was intentionally designed like this).

Treat it like a warning light, if the needle rises above half way, it's overheating.

 

You definitely have a cooling system problem. It could be many things, but a sticking thermostat is common.

You should not drive the car until it is fixed. Aluminum engines have poor overheating tolerance and can be easily damaged.

 

Understand....but it DID rise, almost getting to the max limit, until I turned on the heater...then it returned to normal...as this has not occurred before, just want to know what could have caused it. (it rised from the normal to the almost-max position in about 30 seconds before I turned on the heaters).
Could this have been a sticky thermostat? Some other issue? I checked the fluid level upon returning home at it is right where it should be.

EDIT--I see another post JUST before I posted this...what do you suggest? I am certain a garage would LOVE to diagnose, but without the temp being high (at least on the gauge), what would they be looking for?
Is the cooling fan being on for a few mins an issue? Or is that normal after operating this year's Jag?
Should I have a new thermostat put in (or myself if I can FIND it!! :-)), and maybe have the cooling system flushed? Hate to do a whole lot without zeroing in on the real issue.

 

You can monitor the actual temperature with some OBDII readers for diagnosis.

Thermostats are cheap and should be considered a maintenance item when the cooling system is flushed. I would start there. Also look for obvious signed of a radiator blocked with debris.

 

+1 on OBD if you want the true temp but for sure if the needle rises like that then it's a lot hotter than it should be. Not just a bit, a lot. Treat with care and +1 on it's probably the thermostat.

The 3.0 PCM will try to avoid overheat by cutting cylinders but I believe the 4.0 does NOT do that (I think the PCM is too busy to manage it, but that's a guess). So be afraid - sorry to say.

 

Can the obd reader detect this type of code error if the engine light never even came on? Have done googling and other Jag owners who have same issue, fixed either by thermostat or sensors. Guess if odb cannot detect this intermittent issue (only once so far), i will replace the thermostat. I'll query this site to see if there are instrctions for doing so.

 

Some OBDII readers have real time monitoring of key engine parameters, like coolant temperature. This will not be a error code.

The thermostat fails far more often in these cars that the temperature sensor does.

 

If the temp went high enough then the PCM could flag a code. Most codes are Pending ones first, because most have to occur twice (on separate drives). Any decent tool can pick up pending codes but some software decides not to tell you!

However, I'm not sure the PCM will have a code for this. (Clearly it SHOULD have, but I don't know if it actually does. Short of reading painstakingly through the list of codes...)

OBD name is ECT. (But some tools don't use that name!!)

For 2003+ cars (2002.5 in UK), there's ETM so you can watch the temp - but early cars either don't have ETM or we don't seem to know how to trigger it.

(see acronyms, below)