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MG MGB Technical - cooling questions

I would like a more effective cooling system. To that end I have been making little changes over time

I have a 1965 MGB, it is in good shape. The water pump in 14000 miles new. The radiator is recorded and is a modern 4 row deal with the older style tanks; it has a cap at the top. I put in twin 10 inch fans that work off a switch in the dash.

I have two areas where I was (am) concerned.

At high speed there is a slight increase in the gage reading and the normal 185 195 can climb to 205 and 215. I thought this could be from the electric fans in front, and the normal fan in back, plus the Moss plastic shroud. I took off the metal water pump mounted fan and the shroud and this helps. It seems (for this time of year and for this moment) to climb about 10 degrees and stabilized. I can turn off the electric fan at highway speeds or leave it on no difference.

At low speeds it over heats. Basically at idle or in slow stop and go traffic. If I rev the engine, it pumps more and the heat comes down, but it will still overheat if the stop and go traffic continues for some time. When I had the metal water pump fan and the electric fans, turning on the electric fans immediately brought the temperature down, and delayed problems. Now, with no metal fan it is improved a little.

I want a thermostat, so I went to a radiator shop and they put a port into the bottom tank near the inlet hose. I have been through a lot of sensors and all of them turn the fans on too late. The dash gage shows 185 climbing to 225 before I get the switch to turn on the fans. The switch I am using now completes the circuit at 160 degrees this is the lowest temperature I could find.

I have 2 questions:

- Water pump I feel that if I was moving more water at idle, some of my cooling issues would go away. I know there are a lot of variations in the quality of these pumps. Where could I get the best low speed flow pump?
- Thermal switch I think the lower tank location is wrong. Where could I put a sensor switch that would allow the dash gage to come up to 185-195, and then turn on the fans? I dont want the fans on all the time but I want them on automatically at this dash temp gage point. Where in the tank should I put a port?
Frank Baker

When you put the new radiator in, did you put the foam filler piece back - the one between the shroud and the radiator. Mine had the same symptoms as yours despite modern cored radiator, aftermarket fan, water wetter, etc, etc. Always overheating, it was becoming a standing joke in the club that there were no known pictures of my car without the bonnet up. One of the guys spotted the missing foam, I bought some, about a pound (so that's several hundred dollars and rising...!) and no more overheating.

Hi Frank I agree that the lower position for the sensor is wrong. The later rubber bumper cars have there factory fitted sensor in the top tank adjacent to the inlet pipe so that it senses the temperature as soon as it rises. It works very well. Is your water pump fitted with a pressed tin impeller? The cast iron impeller is very much better at moving water. Jim
j soutar

Frank on your B what carb or carbs are you running? 1965 MGBs had SUs carburators. If your SUs have a lean air/fuel mix your engine will overheat. If you egine timming is to advance or(retard)she will overheat also.Your in Colorado it does not get that hot? I would run a 180 thermostat and change the water pump pulley and get a later 6 blade plastic engine fan or no fan is fine.Keep the eletric fans they hlep then Buy a cheap tempature gauge from AutoZone. And screw it in to the engine head to make sure your old temp gauge is working right.ALL britsh cars overheat at sometime but if the Timming/carbs/cooling system in good shape she will have less of a chance of overheating. Last you might need a cooling system flush.

Frank Baker

Hi Frank,
First of all, what do you mean by "overheating"? If it's not boiling over, it's not overheating. The fact that it's getting over 200F is not necessarily reason for alarm, especially if it's not overheating. A 50/50 mixture of water and antifreeze boils at about 230F, and the boiling point increases 3F for every 1 psi increase, so a 50/50 mix with a 16 psi cap won't overheat until about 278F.

Furthermore, you want to run the engine as hot as is practicable for the best power, efficiency, and longevity. I've written up a little treatise on MGB cooling and put it on my website -- you might find it interesting:

Every year about this time there is a flood of posts about MGs overheating. Some are real problems, but most people are chasing ghosts. If we would all take a bit more relaxed attitude about cooling, we could all worry less and enjoy our cars more! ;-)

Rob Edwards

The factory electric fans weren't designed to come on until 194F, which equates to about mid-way between N and H, or pointing directly down and the last number for the earlier gauges. For the later C-N-H gauges it isn't overheating until it gets right up to the H, and from personal experience even then it is only a warning and not boiling or steaming. It is this boiling/steaming that is the only true indication of actual overheating. 160F for the fan switch is way too low - that is below the *lowest* thermostat ever fitted to an MGB, it should be higher than the thermostat, which was changed for the more efficient 185F. Any lower than the stat and the two will be fighting each other - with the sensor in the top hose or rad it will overcool, so the stat will start to close, which reduces the flow and hence heat through the rad, so the thermo switch switches off again, cooling reduces, engine gets hotter, stat opens more, radiator gets hotter, fan swithes on again and so on. If you had a head-mounted switch like the V8 you would have the situation of the electric fans on all the time even though the stat had reduced the flow through the rad.

After 17 years I finally got round to fitting the foam strips between the radiator and its mounting panel, and last year which was quite a warm one the temp gauge reading was noticeably lower than previously on hot days running with a rally plaque on the badge bar in front of the radiator grill.
Paul Hunt 2

This thread was discussed between 28/04/2007 and 29/04/2007

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