Right, I can leave the vehicle ticking over for hours with no problems, I could probably drive the lenght and breadth of the country with no problems but if I put the boot down it instantly over heats.

Basically I drove home from work last night which is a good 15 miles and over various terrains being out in the sticks. So I was up and down hills with no problems, long stretches of straight county roads topping 60 mph, still no problems, but coming up a very steep hill in the village the car infront of me turned so I slowed to an almost stop before flooring it to get some speed up for the hill and it's then when it overheated.

The gauge went up quickly and the light came on but as soon as I passed the crest of the hill and started the descent the temp came down almost as fast as it went up. I then had another steep hill to climb before pulling on to my drive where the temp had risen again and the contents of the expansion tank were emptying all over my drive.

I have been as descriptive as i possibly can and I hope there is someone out there who can shed some light on this for me, please?

Thanks in advance.

What motor are we talking about?????????

If it's petrol, could well be the radiator (Common problem)

I would say viscous coupling. When you are tooting along you have sufficient airflow through the rad to cool it down, but when you are moving slowly the airflow is reduced - hence overheat.On idle the engine is not working so the little air that is drawn in by the faulty fan will be sufficient to cool. As also mentioned the raditor can clog over time, and are underated for the size of the engine, so have to be in tip top condition to work as they should.A good flush should help or if at all suspect replace it. Much cheaper than a pair of heads or new block.

I've recently been through a similar saga with my '99 DSE. Driven locally it wouldn't overheat at all, and it was just running much too well to be a head gasket gone, but put it on a dual carriageway at 70-ish and after around 20 minutes the temp gauge would ping over to the stop and it would go into limp-home 5 mph mode. Really couldn't believe it was problems with the head and the rad seemed to be pretty warm all over and showing no blockages, but after changing thermostat, pressure cap and temp sender, I bit the bullet and had the head off to find no sign of gasket problems. Had to have the head skimmed and new gaskets, then off up the dual carriageway and back to the overheating after 20 minutes of higher revs, just like before.
Then I heard about problems with P38 radiators. Apparently there is some sort of baffle in the rad that sends the water right down and around the whole radiator to keep things cool, but if this baffle fails for any reason, the water goes in through one port at the top of the rad, straight across the top and out of the other port, also on the top, without passing through the whole cooling matrix. The rad still warms up reasonably well, but doesn't do much cooling and after 20 minutes or so the temperature creeps up and it overheats.
It sort of made sense to me, and by this time I'd spent a small fortune on the engine with no success, so I put a new radiator in and have had no problems since. The funny thing was that I'd thought the rad was blocking and was disappointed when I put water in one end to flush it through, only to find that it was flowing freely out of the other end, so I wasn't too hopeful at that point. Perhaps it's something to do with both hoses being on the top, instead of one at the top and the other at the bottom like in most rads. But it did the trick and now pulls as well as ever.
Anyway, I hope this may help others trace their heating problems.

Thanks for all the responses. It's a 1995 4.0 V8 petrol which has a top hose and a bottom hose so the previous post by Sir Ben although very helpfull doesn't seem feasable on mine.
However I took the top hose off tonight and there was no water in it. I also put the hose pipe into the top hose opening in the radiator and put what seamed like a large amount of water into the radiator.
I then left the car for about an hour ticking over to heat up fully, I then put the car on a fast idle, I sat with the revs at 4000 rpm for about 10 mins and the temp gauge didn't move past half way so I upped the aunty to 5000 rpm and the same happened again, nothing.
I'm going to take the thing to the garage in the morning to fill it up with fuel so I'll let you know what it does while under strain to see if that is any different!

Hi,
I have a very similar problem with my 1997 DSE. It overheats only when going uphill (long and steep ones) but I have noticed that if I select 2nd gear it will actually change up into third which means the engine is being heavily loaded. Also if I select 1st it will change up to second.
Any ideas anybody?

Dont overlook the expansion bottle cap. These can be troublesome. If they are not holding pressure you will get boiling water leading to pockets of gas and an overheat reading.