Looking for a bit of informed advice if anyone can help?

Out driving yesterday, all was normal until I joined the dual carriageway and accelerated up towards 70 mph. As I approached this speed, the engine temperature warning light came on and the temperature gauge needle shot up to the red zone. I immediately decelerated and went to pull over, to find that when I got back to 55 mph, the warning light went off and the gauge stabilised in the normal position. I pulled over at the next lay by and checked all the usual things (hoses, water expansion tank, oil, radiator) for leaks and capacities to find no issues at all. I continued my journey and all was well for about 15 miles until I tested it out again by accelerating up towards 70, where the same thing happened, followed by the return to normal when I decelerated. This happened twice more on more journey under the same circumstances.

On arrival at my destination (60 miles from home), I called the RAC for advice as I am a member. Their technical geek on the phone advised that it sounded like a water pump issue, that being that the pump would have to work harder at higher revs to pump the coolant around, but could obviously manage at lower speeds. However, they wouldn't be able to fix it at the roadside (obviously), so would recover me home, advising me not to drive it any further than I needed to - or do so at my own risk.

I called the local garage this morning who service the car (not a Jag dealership). Thoughts revolved around the "possibility" of a faulty water pump, although he pretty much dismissed this as it would either work or it wouldn't, in his opinion the speed of the car wouldn't be a factor. Possible thermostat sticking issue, although again he thought that unlikely. His other suggestion was that the radiator could be clogged up with solidified, corroded coolant, reducing the ability for the coolant to be circulated completely around the engine. Typically being summer, his books are full for the next week or so, he can't fit me in until middle of next week.

When I asked about driving it until then, he said it shouldn't be a problem at speeds where the temperature remained "normal".

I would much appreciate it if anyone is able to offer me any advice or share similar issues? Can anyone confirm (or rubbish!) the advice i have been given already? I really don't need a big bill for this at the moment (it's a ten year old diesel, with PLENTY on the clock! It could be "cut your losses" time, much as i enjoy driving it!

 

I don't have a lot of experience with diesel but one thing that is universal among engines is that when fuel delivery is extremely LEAN engine temperature will rise exponentially. My take on it is at high revolutions you are experiencing fuel starvation, especially since reducing load reduces temperature.
The first steps would be fuel filter, fuel pressure regulator and last the fuel high pressure pump.

 

I will wager the RAC man is correct. The impellers on our water pumps corrode; they can move enough coolant at low speed, but start aerating the coolant as the engine speeds up, and air in the coolant does not cool well at all.


This happens with both diesel and petrol engines.