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MG MGB GT V8 Factory Originals Technical - different type of cooling issue

In the 95 deg weather my 1977B with 215 cools ok around town (but not perfect). On the highway at 65-70 mph the temp climbs. It doesn't get to the red on my stock guage, but it shouldn't get hotter at highway speeds. After slowing to 45 mph the water temp returns to normal. I assume there is an airflow disturbance at higher speeds. I have a 16 inch 2900CFM puller fan mounted. I move the radiator forward about 2 inched and had to cut the that panel in front of the radiator to do it. The radiator is the stock width but about 2-3 inches longer. I didn't have this type of heating issue on the highway before I modified the radiator setup(well at least I can't remember it happening). I'd appreiate your thoughts.

My best to you guys.


At highway speeds the water pump creates additional suction, enough to cause the lower radiator hose to collapse if itís old or oil soaked. Iíve had this happen on other cars and it can be tested by squeezing the hose to see if itís easy to collapse.
George Champion

A couple of things I would be checking...

Firstly ; the condition of the hoses, thermostat as George already mentioned.
Secondly ; the pressure cap to make sure it is where it should be and there are no pockets of air still trapped somewhere in the system
Thirdly ; the pump may be cavitating which fits with the temp rise as the speed increases point.

Cheers , Pete.
Peter Thomas

Thanks George and Pete,
Hoses seem to be in good condition by the squeeze method. To test at highway speeds couldn't I just run the car at 2500 rpm in neutral and see if the hose is effected. I'll by a new cap and check system for air. How does one find out if the pump is cavitating? do eveything else and then replace the pump as a last possibility?


Yes running the engine with the car not moving will also tell you about the hose but watch out for rotating pulleys and belts etc.

A cavitating pump is possible but as you suggested it is best left to the last item to check since you need to pull the pump out to check.
The tell tale signs are pitting on the surface of the pump impellor and on the inside of the housing as well depending on the material. aluminium shows signs more clearly than steel or cast iron.
It looks like the impellor has been corroding away.

Just rereading your original post it occurred to me there may be a blockage in the new radiator. Is this possible?

Peter Thomas

I have a similiar set up. Bottom tray cut forward to level with the upper slam panel.
When I first fitted my electric 16inch puller fan, I had it running backwards!
The tubulence kicked up by this big fan did not make the mistake obvious. Little bits of tissue held near the top of the fan shroud appeared to stream in the appropriate direction. I didn't get to the stage of driving it, but it took a day or two before the penny dropped.
Now it never even gets close to overheating, unless the 25A fuse blows, which was about once a year. I'm currently using a 30Amp glass fuse and that is holding.

I've also come across the problem of the water circulating too fast, especially when people with heating problems have removed the thermostat completely. Too fast and the water doesn't get chance to give up all it's heat before it is re-circulated.
Allan Reeling

This thread was discussed between 11/07/2007 and 17/07/2007

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