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MG MGB GT V8 Factory Originals Technical - Radiator not performing up to snuff after replacing hoses

I replaced my radiator hoses yesterday, put in new Prestone and water, and now the motor seems to be running much hotter. Is it possible I got a partial air lock in the top rad hose or something? I really tried to hold that hose down, and squeeze it a lot, while filling so that it wouldn't be the high point. I drove the car about 25 miles today and it's definitely running quite a bit hotter than it used to. I mean, coolant is definitely circulating ... top hose gets hot and all that ... but something ain't right and I can't tell what. Any ideas?

(It's a stock factory V8-type system, and the coolant is 50/50 dist water + regular Prestone, with half a bottle of Water Wetter thrown in for good measure.)

I just recently got my Rover 3.5 conversion on the road. One thing I notice right away (and several times later as I have been fine tuning the cooling system etc) is that once the rad has been drained, I fill it up and stand there with the rad cap off and a gallon of water in hand, once the car has gotten up to operating temperature I watch for a big bubble of water to flow over the top of the filler cap and then watch the water sink out of site into the core, I then add @ 1 - gallons of water. So with my experience you are right, it is an air lock in the top rad hose. My feeling is that the top hose is partially empty, once the thermostat opens it push's water into the top hose, sometimes it takes a few times of the thermostat operating before the top hose has been purged of air


I may be mistaken, but I was under the impression that the overflow tank would, after the car heats up and so forth, 'absorb' any air bubbles in the system. That seems to foot with my recollection that many times it runs a bit hot for a couple of hours after flushing the radiator. The water pump shouldn't have any problem pushing water up into the uppermost reaches of the system. Agree/disagree?

(One thing that's a pain with these V8 rads is that the plug is so small, you can't just stick a garden hose in there to flush things out. Also no petcock, so you have to remove the lower hose an make a mess. Pet peeves.)

A friend building a street rod with some cooling problems refered me to this site, and there maybe some useful information for MGBV8s. He's switched from a glycol coolant to water (as the linked tech tips suggest) with a corosion inhibitor and saw a significant differece in the cooling of his SBC (iron block and alum heads) FWIW

Phil - a really brilliant site. Very informative. Do they do pumps for V8s? Do you know anyone else who does high performance pumps? and what is the throughput in gallons per minute with a standard V8 pump anyway?

I don't think they have anything other than the small block Ford and Chevrolet pump, but my aquiantance said they spent considerable time with him on the phone, and I presume that they might do the same with an email. Id ask them what they do to a watrpump to get the performace up. Maybe they'll tell you.

Let me know how it works


As Marc said, that is an incredible site. It shows how the MGB expansion tank could be improved to a surge tank.
George Champion

OK, George. Tell me - what is the diffrence between an expansion tank and a surge tank? This is more new stuff to me - or did they explain this on the Stewart site and I missed it? It's gettig complex but very interesting


The original factory MGBGT V8 and the 77-80 MGB had the radiator so far forward and low that it has a remote radiator cap in what I see listed as an expansion tank. Air from the top of the radiator is routed through a hose to the expansion tank to a tube high on the side that goes to the bottom of the tank. Often this tank is referred to as an overflow tank, but an overflow bottle (also called a coolant saver) is not pressurized and goes after the radiator cap.

On the site above they describe a surge tank as having two hose connections to the low pressuer side of a cross flow radiator, a larger hose from the bottom connected to the lower radiator hose and a small hose connected to the upper part.
George Champion

Just a thought Terrence but it could be down to poor sealing of the water hoses onto the radiator particularly if you are using the 'bulb type' electric fan switch in the top hose. Poor sealing = lower pressure = higher running temp.
Bruces point re bubbling out through the expansion tank is very valid. Once you feel all the air has been expelled, switch off but don't replace the cap. As the car cools return at frequent intervals and top-up as necessary. When cold, replace the cap and all should be well without needing to change your current set-up.
David Canning

The factory V8 should indeed pump any air out of the system after several heating and cooling cycles (unless you only ever drive down steep hills). I recently discovered mine was accumulating air in the top of the rad, which fillied up the expansion tank (which is too small anyway) and pushed coolant out of the overflow. I found the plug on top of the rad was not tight enough and so was probably allowing in air when cooling down instead of sucking coolant from the expansion tank, replacing that with air in through the cap. Some time ago I replaced the O-ring on the rad plug and found it was so soft that 'normal' tightening squeezed it out of place, so erred on the side of caution as well as putting a close-fitting sleeve over the plug to act as an O-ring retainer. It seems that now the O-ring has hardened a little it needs a bit more tightening to seal properly.

Paul Hunt

I've never seen an O ring used with a rad plug. Only fibre washers which are very low tech and have never given me any trouble. Are O rings stock equipment?
On my V8 and my Defender - same type of rad - both have fibre washers.

My stock rad had the O-ring, the uprated rad came with two cotton-reinforced-rubber washers. These started to weep after a short time so my supplier sent me an O-ring. Both thse rads had plastic plugs. A friend with a CB V8 has a stock rad with a brass plug and no washers or O-rings of any kind.

Paul Hunt

My expansion tank was fitted on the right hand side of the engine bay towards the front, just behind the radiator bulkhead. It was mounted so that the pressure cap was about 1/3 below the top of the radiator. Also it was fitted with a 7lb cap which I considered was too low. I couldn't get a 15lb cap with a long enough reach to seal the system.
I've now had a new header tank made from aluminium. It takes a 15lb cap and is mounted on the left hand side of the engine bay about 8" away from the firewall and it is now definitely higher than the radiator level. Has it made a difference? I would say yes to the extent that although the engine stills runs too hot - after a few miles at 100mph the needle normally gets very close to the red, I think that it takes longer to get there. Also whereas previously once at the red, the needle was very slow to drop back to "normal" running temperature (mid-way between N and H), the needle now returns to that point and maybe a tad lower far more quickly.
The new header tank hasn't solved my overheating problem but it is certainly helping and I am hoping that an accumulation of all of the measures will finally get it all under control. I still have trouble filling the system so that the top hose is full of water, but now that the expansion tank is at the high point, I am having a solid pipe made with a bleed nipple which I will put into the top hose at its highest point. This should allow me to fill the system completely by keeping the nipple open and filling the system through the header tank.
The higher pressure did produce a small leak from the bottom hose at the radiator end which has now been tightened.

This thread was discussed between 21/06/2001 and 05/07/2001

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