Another Overheating Problem

[Sydney]

Hi all,
EJ20 N/A Subaru in a late bay with radiator and scoop mounted between the chassis rails.
I have no thermostat fitted, and during the winter time and until recently the water temperature has remained between 75 & 85 Deg C, depending on the outside temperature. However during the recent hot spell the temperature started increasing to over 100 degC when climbing hills, and cooling to 95 degC on the way down at around 65mph. The odd thing is that when I hit a long queue of traffic on the motorway, I tried to keep the revs low and the temperature stabilised at just below 100degC. I then had to put it into 3rd gear (30mph) and the temperature dropped rapidly by about 5-10 degrees.
The upshot being that in third gear (more revs, higher engine heat? and higher pump speed) the temperature is lower than at the same speed in fourth. It reverted back to high temperatures again when I sped up and put it back into fourth.
There seems to be sufficient airflow through the rad to cool the water but it just isn't circulating well enough when I'm in top gear, even at 65mph.
Has anyone encountered this issue before or have an idea what's causing it?
My initial thoughts are slipping belt or poor pump.
Thanks for your help.

[malum]

Maybe air trapped in the system (can cause rapid fluctation of temperature)

Maybe head gasket?

[Kai]

Matey I know built a ej20 turbo engine into a bay using 28mm copper tube for the pipe work. Can't remember what expansion tank he used.


The only way he was able to get it to fu croon properly was to persist in bleeding the system properly as it constantly would over heat. He ended up raising the back and and persevering with the bleeding process to expell all the air in the system. Runs sweet now after a long spell of over it overheating. Running thermostat now also.
_________________
Think......

Don't sleep and drive!!!

[malum]

Here's me bleeding mine
It's an absolute bugger to get all the air out