Engine Running Cold as Speed rises !!!

[The2Js]

I have just done a 1000 mile roubd trip to Cornwall in my '98 fsh 61k xk8. I travelled on varying types of road and at varying speeds !!!! I cruised on the motorway for approx 300 miles on one leg of the journey and the coolant gauge never rose above the cold mark !!! Conversley, when we slowed down the temp gauge would rise but never up to the median mark, only if we had been running around at 30 mph or so for lengthy periods. Also, we were not able to get the heater to gives any heat ..uncomfortable in some of the cold weather we were travelling in. The heater problem has been with the car for a while now. Has anybody else suffered this woe and can they suggest a fix / is it dangerous Thanx Jonathan

[Nik]

Hi Jonathan ,
Sounds like you have a problem with the waterpump/thermostat , I assume that the coolant level is ok , I'm sure one of the techies will be along shortly to explain all ,

Best of luck ,

Nik

[mike020150]

Hi Jonathan,
I agree with Nik, from what you've written it sounds like the engine thermostat has failed 'open'. In any case, thermostats should be treated as precautionary replacement items and changed every few years, so it's a good place to start looking for a solution. Mike.

[ronnie2507]

Hi jonathon,
just reading your message, i've just posted a one roughly the same although my car temp appears to be normal. The heater however fails to blow hot air even though the display has been set to hot and the engine has been ran for some time.. did u manage to sort your problem out,
many thanks Ron

[BestGear]

.....and the coolant gauge never rose above the cold mark...... !!! Conversley, when we slowed down the temp gauge would rise but never up to the median mark........Also, we were not able to get the heater to gives any heat .....

Quite a few clues there!

Firstly, as has been said, check that the coolant level is correct and there are no leaks - a simple pressure test can reveal a lot.

If it was the thermostat that had failed - sticking open - once the car had run 5-10 miles or so, the car would run as normal. If it was stuck closed, then the car would most likley overheat within 3-5 miles.

If it was the water pump, the car would have overheated in a few miles - you would certainly not have driven 1000 miles!

Did the problem just appear as it is now, or did it develop over a period of time?

I would first check the temperature gauge and sender, as it may be incorrectly reading. If you have access to an infrared thermometer, use it to check the top of the radiator - that will give some evidence if the car is indeed warming up. You can even just check that the radiator top hose with your hand to confirm it is indeed heating up.

Next, I guess would be the possibility of airlocks, but that would only really appear if the car had a leak somewhere (very possible on these cars) or if the coolant had been flushed and replaced and not all air removed when refilled.

The hoses on the XK are not of very good quality - it appears over time that several of them leak. Typical hoses are the two under the inlet manifold, but on my own car, I found that both small hoses at the EGR and throttle body had very small holes in them, allowing coolant loss and air introduction.

If you do have more than one fault, then the failing heater could be down to the heater valve remaining closed - see my post here: http://www.jec-xk8-forum.org.uk/xk8bb2/ ... .php?t=691

With simple diagnostics this should be easy to crack - the clues are certainly there.

David

[BestGear]

I just noticed that the OP was several months ago - so either the problem is fixed or the car has probably been sold!

David