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MG MGB Technical - Doesn't cool that well

Haven't really had a lot of problems with the cooling, until recently. It seems my 76 GT copes well with the everyday commuting (10 miles each way), but for longer stretches of fast motorway cruising with high revs and high gears, the engine runs very hot and eventually boils over. With a lot of drama and impressive sound and visual effects (looks like a very small dragon), it throws out the coolant through the pressure relief hose on the radiator. Opening up the heater to max helps, but obviously, this is not a solution in the long run. Even Norway gets hot in the summer, now with global warming and all. Previous owner installed a ratty radiator. I have flushed it once, but coolant still gets very dirty, so I will flush it again tonight and put in some radiator cleaner. Can blocked coolant passages be the problem? Will cleaning liquid take care of that? If the system leaks, where should I start looking? Has the first cleaning I did revealed a weak point in the cooling system? Could it be the thermostat, and how do I know it is malfunctioning? I really need to sort this before summer: I was planning on taking it to Scotland, to visit a cask of malt whisky I boughjt there a couple of years ago. As always, thanks a lot for any ideas. I have always found this forum most helpful.
OYH

OYH,
Sorry to hear of the overheating.
If the coolant is dirty reverse flushing the radiator would be a good idea. This is where water is forced through the radiator in the opposite direction to the way it normally flows. There may be scale, rust and sediment blocking the lower part of the radiator and this should clear it.

I would not use radiator cleaner too much since if the radiator is ratty as you said the cleaner may damage it if you use it too often.

Is there a good radiator repair place you know?
I would get them to pressure test the system after reverse flushing the radiator and the engine and heater. Have the radiator cap checked as well it may be failing to hold the right pressure.

If the radiator is hot then the thermostat is probably OK, but removal and checking with a thermometer and hot water on the stove will tell you for sure.

Hope this helps.
Post again if there are still problems.
Enjoy the trip to Scotland and have a glass for me!
Cheers, Pete, Queensland, Australia.
Peter Thomas

If your car maintains a good temperature at slow speeds, but boils over at high speeds it could be time to replace the lower radiator hose (the top hose while you’re at it). The hoses can soften with age particularly if oil drips on them. At high speeds the suction of the water pump can cause an old oil soaked lower hose to collapse restricting the quantity of water circulated.

Other things may also reduce the coolant circulation such as the water pump impeller rusting away or a belt slipping because it is loose or glazed. These things usually cause overheating at low speeds as well.
George Champion

Thanks a lot, gentlemen! While waiting for replacement radiator hose, I will try the reverse flushing. The radiator itself was probably just lying around when the PO put it in, and full of rust and dirt. I have also noticed that the rubber items on the car have been in bad condition, so new hoses may be appropriate. While flushing the inside, I might as well give it a hosing on the outside too, to clean out rodents, spiders and cats hiding between the ribs.
OYH

Sir:

Another cause of overheating could be a blown head gasket. If the head gasket is shot, and hot exhaust gas is being pushed past the head gasket and into the water jacket, the engine will overheat. Your cooling system is not equipped to cope with this extra heat and will rapidly overheat. In your post you said it overheats during a high speed run, which is when your engine is working hard, so the gasket could be the problem.

As far as how to detect if this is the problem, It’s my understanding that if you have foam in the radiator this could be the reason. Maybe somebody else with more experience could contribute here. Also, radiator shops have a probe that will detect exhaust in the radiator.

Good Luck!

Bryan
Bryan Prindle

I'd bet the radiator needs rodding out or a new core . keep flushing it untill the cooling system stays clean then see if it's any better .

Enjoy the whisky ! .

-Nate
Nate

Retarded timing and a poor vacuum advance can also cause overheating.
George B.

Another basic cause of overheating is a bad pressure cap. Each pound of pressure raises the boiling point of pure water by 3 deg F. Thus, a 7 psi cap raises the boiling point by 21 deg F. Do not know about the 76 model, but the 77 and later models run about a 15 psi cap. Then, there is the problem with coolant. Water has the best thermal efficiency (rating of 1 for reference purposes) while antifreeze has a rating of .6 and any mixture of water and antifreeze will be a combination of these two figures. I run 75% water and 25% anti-freeze which gives a thermal efficiency of 90% as compared to pure water and 80% as compared to the "recommended" 50-50 mix. But, some anti-freeze or something like Water Wetter is needed because it modifies the flow characteristics of pure water and allows it to flow more efficiently. If it were me, I would flush the entire cooling system (which also helps clean the inside of the block and head), install a new radiator, new hoses and a new thermostat and a 75% water-25% antifreeze mixture. For use in winter, the traditional 50-50 mix is better. I would also have the head gasket checked which can be done with either the exhaust gas detector (as mentioned) or a radiator pressure checker. Les
Les Bengtson

Oystein-
To check the thermostat, heat some water in a pot on the stove. When it reaches the temperature at which your thermostat should open, simply lower the thermostat into the hot water with a wire coat hanger and watch closely to see if it opens smoothly.
Steve S. Virginia

If your radiator hoses need replacing, it might be time to also check the less visible and oft forgotten brake line hoses. This is one thing I have done as soon as I purchase an older car as well as any less than new looking radiator hose.

Safety Fast
Peter
Peter Vann

Gentlemen,
Again, thanks a lot for the feedback. Radiator has now been thoroughly flushed (reverse) and on the outside - full of insects, dust, etc. I noted the lower radiator hose seemed a bit soft, and I will see a mechanic later today (carbs need some adjustment, should probably go over the ignition as well). Radiator has now been filled up with distilled water, haven't bothered with antifreeze, since I reckon I will be at the radiator again shortly. Temperature seems to be fine. Also tugged at the water pump axis, but no slack there. Coolant does not foam, some bubbles after driving only, and it does not lose any water (well, since last night, anyway).

Yeah, I'll enjoy the whisky all right. Springbank, all 235 litres of it. Won't get through all that in just a week, though.
OYH

Oystein,

Glad to hear the heating problems are under control.

If you need any help with some of that 235 litres of Whisky I am sure some of the people posting on the BBS will be only too glad to come there and help ant way they can.

Gee what a great bunch. No need to say thanks...just the sort of people we are!
Peter Thomas

This thread was discussed between 04/06/2002 and 05/06/2002

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