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MG MGB GT V8 Factory Originals Technical - V8 Overheating

I am having an ongoing problem with my engine running hot. About 2 years ago I did a V8 conversion (Glen Towery) 3.9 litre (from a Triumph Stag). Over the next two years Ive been working on details; ie electrical, fuel pump, brakes. I was thinking of putting AC in, but the one ongoing problem is the engine cooling system (the water pump was changed last year)and I don't think AC would be a good idea because of the engine cooling problem. I'd like to try to get the cooling under control and then revisit AC.

There is a double core radiator and an oil cooler and an electric fan. The heating gauge always read hot after a couple of minutes; just before the red line on the temperature gauge. Under ideal driving conditions, 60 to 70 degrees outside temperature, speeds over 40mph, it stays there.

Once the summer hits or Im in stop and go traffic, the temperature gauge goes into the red and the engine begins to overheat.

Some additional observations:
-The electric fan turns on when the temperature gauge is between midway and red so this seems to be ok.
-If I leave the car idling in my driveway with the hood open, the temperature gauge stays right before the red, but the engine does not overheat (with the hood closed it will overheat).
-With the hood open there seems to be a lot of airflow over the engine, but not much coming down under the car.

Id appreciate any thought on what can be done.

Richard Cohen

The rad is good, the waterpump is moving the water properly, you don't overheat with the bonnet up - have you still got the sound insulating pads on the underside ? is your temp gauge accurate ? Have you the appropriate rad cap ? is the fan pushing when it should be pulling ? Are the fan blades very close to the rad core ?

With the fan running properly, you are looking for the needle to show less than halfway when moving and well under the H when stopped. The stock V8 (probably less power than yours but with headers/exhaust manifolds inside the engine bay) will operate normally at ambient 105F - BUT the twin fans are crucial. Yes - it's twin for redundancy - one small fan will get the stock car home. A large single is fine so long as it works.

Roger W

You say the engine came from a Stag. Is it a Rover or a Triumph engine? I imagine, as it is a 3.9, that it is the Rover. The Triumph V8 is prone to a whole raft of cooling problems that don't trouble the Rover unit. Do your exhaust headers go out through the fenders (as on the RV8) or are they hugging the side of the block as on the factory MGB GTV8?

My car also has a 3.9 Rover engine with RV8 style exhausts. I have a decent radiator and the usual twin unshrouded fans fitted to factory cars. My temp gauge is a capillary one calibrated in degrees F. While driving, in all weathers even on rare hot summer days, it sits comfortably at around 170F. As soon as I stop it starts to climb. The fans come on at about 190 on the gauge, but the temp continues to rise to close on 210 before the fans slowly drag it back down to below 190 and they switch off. I have never had the slightest sign of overheating.

You say the car overheats - what symptoms do you see? Have you tried to find out if the gauge is anywhere near accurate? Many are not. With the bonnet up the heat will come out the top. It's a big opening and hot air rises. On my car with the bonnet closed and the fans running you can feel the warmth pouring out under the sills behind the front wheels.
Mike Howlett

As Mike pointed out you can't always trust the temp gauge to be accurate, you just learn what is the norm for your engine and thus spot the abnormal. Before panicking check the fans are running in the right direction. Check the pressure cap is, in fact, sealing and thus providing you with a system pressure of 15psi. Check the thermostat isn't stuck closed or partially closed.
Running too lean a mixture and/or too advanced ignition timing will also cause overheating.
Allan Reeling

I'd be looking to get the radiator checked by a pro. When I did my conversion in '91 with a 3.5 Rover engine, I used the original RB 4 cyl. radiator turned back to front with the inlet outlet connectors re-positioned.I had big overheating problems so I took it back to the guy who modified the rad. for me & he concluded that the coolant was passing through the rad too quickly to get cooled.He recored the rad with a core that he said would keep a small truck cooled.I never had an overheating problem again & I don't live in a cool climate.I sold the car in 2011 & the current owner now runs a 4.6 Rover engine & uses the car mainly in competitions. HTHs Barrie E
B Egerton

Richard -please confirm the actual engine type - is it an oem Triumph V8 ( they were 3L though) or the Rover from a conversion?

I ask because if it is an original Triumph V8, they were terrible at keeping water in. The main trouble was the position of the water pump leading to overheating, especially when stationary.
Hal Adams

I forgot to mention that if a Rover, there was a problem a few years ago with water pump. The impeller was slipping on the shaft, therefore not paddling the water round. Worth a check if the more obvious stuff proved fruitless, but would be nice to know what engine we are dealing with!
Allan Reeling

I have a factory V8 with a aluminum radiator and 2 Jaguar electric fans with more blades, they came from coolcat express corp. the fans of the jag pull so you have to wire them backwards and the back of the motor needs to be covered because it faces the grill in the MGB I used pipe test caps from Home Depot and 2 T nuts the fit the MGB fan brackets. the fan and motor is part # coolcat10
then come with a mounting bracket that you don't need.
they work great.......Bob

Bob Fisher

This thread was discussed between 13/05/2013 and 23/07/2013

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