The only thing I can think of is that coolant may not be circulating at higher engine speeds. You've already changed the water pump and the thermostat, but I would ask is the replacement thermostat a double stage type? These are recognisable by the circular disc at the bottom. This disc opens a bypass port when the thermostat is closed, and closes the bypass port when the thermostat opens. Its purpose is to allow for even cooling and eliminate engine hotspots. If the bypass port were to be open when the thermostat is open coolant would take the easy option and would not flow through the radiator. If this was happening it would explain your problem. I would remove the thermostat, cut a short length of 15mm copper pipe, and then cut that piece lengthways to give you a piece of copper. Hold the thermostat open in a vice and wrap the piece of copper around the spindle so that the thermostat is held fully open. Then refit the thermostat making sure as you do the disc is closing the bypass port. Coolant will now have nowhere to go other than through the radiator. Refill the cooling system and go for a test drive to see if the problem is cured. If it is then you know where to concentrate your efforts.

Thinking along similar lines, a while ago I did try cutting several slots in the surround of the thermostat with a Dremel, and re-installing it. This did keep the temperature more under control for a bit, but it still crept over 90 eventually!

Cutting slots in the thermostat will achieve nothing, just as neither will removing the thermostat altogether. As I said before, it is imperative that the bypass port is closed off when the thermostat is opened, otherwise the coolant will take the easier option of returning to the block via the bypass instead of through the radiator, resulting in overheating because engine heat will not be dissipated.

Thanks railroad - this is something I had not considered as the radiator does get very hot so assumed the flow was OK, but what you say makes sense

Sounds like a blockage in the flow somewhere.

Have you had any problems with corrosion in the coolant at all?

Have you tried flushing out the coolant circuit with a hose?
Do each component seperately - the heater matrix is often a culprit because the coolant entry and exit are at the top and so any deposits settle into the bottom of the matrix.

After a thorough flushing use a double dose of proprietry cleaner in water only.

When you refill, use deionised water to prevent any possible scale deposits forming.

I've only owned it for just under 3 years, although in that time it has had numerous (4 or 5 now, I think) coolant changes! I always use de-ionised water myself, and requested that when the garage did the most recent waterpump/TB change, they used it. Said no-one normally asked for it, but they had some they were happy to use!

The heater does provide a steady flow of hot air, if that was blocked I'd expect that not to be the case?

A clean and flush is the next thing on the list, then - any recommended products?

Cheers Cyd

I once had a similar problem after changing a radiator and thermostat, The new thermostat was faulty. I eventually went to Toyota and bought a genuine thermostat and that cured the problem.

Nissen rads are fine.Does the car have a/con?

I doubt if this vent will make any difference. Overheat under load or at speed tends to suggest lack of heat disipation.Does the new rad have the correct depth "core"?Also may be an idea to get a combustion test done on cooling system,just incase the system is being pressurised /heated by combustion process.Just a few ideas.

Happy Christmas all,

I posted this a while ago, and now the colder weather has arrived, the gauge now indicates only 80 celcius while driving around town even with the heater off. (Please bare in mind this is not a superbly economical modern diesel, and did not do this when I first bought it several years ago)

So the bizarre thing is that it now seems to be under-reading at colder temperatures, but still wants to get hot on the motorway, although as the ambient temp is now so much lower, it does not get up to 100+ like in the summer.

Bascially, the temp gauge seems to want to sit anywhere apart from the usual just under 90 it used to.

Does anyone think this could be a faulty temp sensor, or a bubble of air around the sensor itself causing such exagerrated readings, whether high or low?