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I'm not sure where to start with this one so I thought I'd ask the membership.

Car is 4.2 V8 Sovereign lab 2005

First off I have to admit that I have a small leak which I check on a regular basis and top up but doesn't take much.

System has been pressure tested and was ok, and has been sniff tested and apparently head gasket is fine.

Recently, the cooling fans are kicking in when I switch the engine off even after a 20 mile drive. Tonight when queuing in traffic the temperature started creeping up yet as soon as I started moving the engine temp went back down to normal.

No messages and no error codes.

So - where do I start?

All advice and suggestions welcome.

Tony

I had this sort of problem with mine a while back, only my fans were running flat out as soon as the engine warmed up. Despite a stuck thermostat normally causing a too-cold engine condition this was the cause with mine and a new thermostat fixed it. Beware that there are differences in the thermostat dependent on VIN number.

Hi Tony, I would tend to suspect a radiator / cooling system blockage. The temperature gauge creeping up in slow / stationary traffic and the fans coming on after switch off all indicate, to me, that insufficient cooling is happening in no movement situations. This then leads me to think along the lines of there being insufficient coolant in the system.

You do mention that you have a small leak, but that it doesn't take much to top it off after checking. However, do you know exactly how much coolant is in the system. The leak may have been symptomatic of a cooling system blockage and being going on for some time before you began the routine checks and top up, meaning the system may be down on volume.

Undertaking a full cooling system flush and refilling with the correct amount of fluid to the correct specification would seem a reasonable starting point.

Jim M

...
Undertaking a full cooling system flush and refilling with the correct amount of fluid to the correct specification would seem a reasonable starting point.
...

I noticed a section on flushing cooling systems in the workshop manual. Of note, it says that if you flush or back-flush the engine, you need to separately back-flush the heater afterwards. Presumably the heater matrix acts like a filter, and accumulates the flushed debris. In brief, the Jaguar procedure is

1. Remove the radiator, invert it and back-flush.
2. Remove the thermostat, and back-flush the engine.
3. Back-flush the heater.
4. Remember to put it all back together.

This only appears in the power-train section for petrol engines, but I expect it applies to diesels like mine, though the latter's cooling system is a lot more complex, and removing the thermostat is a much bigger piece of work. This perhaps means it's not so easy to back-flush

Thanks for your input so far.

Seems like I need to flush the system and if doing that would you recommend changing the thermostat whilst it's being done?

I think I will leave the job for my mechanic to do.

Tony

Tony,

Also carefully check the state of the radiator cooling fins, preferably from the front of the car.

A radiator that has deterioration in the cooling fins can often function perfectly ok when it has airflow over the matrix when moving, but it can overheat when there is no airflow, because of the lack of fins that normally facilitate heat dissipation.

Paul

I had exact same on my XK8 and it was the thermostat.

Thanks everyone - I'll keep you posted but may be a couple of weeks.

Cheers

Tony

If the thermostat is failing to open then the engine will run hot, as there is not enough flow via the radiator, if it is open all the time the engine will tend to run cold. (please do correct me if I am wrong!)
It is an easy swap to start with anyway before digging deeper.

Now seems to be sorted

System flushed, new thermostat and also housing.

Plastic housing needed replacing as was very brittle and crumbley.

Now holds it's temperature as it should, even when queuing for half hour on a hot day and fans not kicking in excessively

Thanks for the advice everyone.

Cheers
Tony

Good to hear!