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MG MGB Technical - overheating at highway speeds

Hello,
My 79 mgb gets dangerously close to overheating when driven at highway speeds. The car will run at normal temps in stop & go city traffic. Themostat has been checked and is working properly. Electric fan works all the time (temp switch is bypassed). I suspect the water pump and will be replacing it this weekend, but I would like to hear some other theories in case I am wrong (wouldn't be the first time!). Any advice would be greatly appreciated. Thanks,
Guido
G.D. Perrone

Guido. How have you determined that your car "gets dangerously close to overheating"? Old wires, old temperature gauges and old sending units may not read as accurately, nor as reliably, as they did when new. The last time I helped someone troubleshoot is "overheating" problem, the actual cylinder head temperature was 183 deg F in spite of the fact the dash gauge was about 3/4 towards the hot end. Before you throw money at this "problem" make sure you know what the real problem is.

Les
Les Bengtson

Hi Guido.

I encountered a similar overheating case recently where the thermostat did open when put in a pan of water on the stove, but it simply wasn't opening far enough.

Wax pellet 'stats can lose wax when they are overheated, and never open fully after that.

It would be useful to know the opening distance of a known good stat, has anyone measured and recorded this ?.. I would estimate it at around 3/8" from memory.

It is unusual (but not unknown) for a water pump to fail in a way that stops them from moving water efficiently.

I found that moving the front number plate (I don't know if you have one) up to the front of the rubber bumper, so exposing the orifices below the bumper, helped the cooling on my RB significantly.

Don
Don


I'm with Les on this one. A good way to verify temperature is to open the radiator cap, insert a kitchen thermoneter,start the motor,and blank off the radiator with cardboard or some such to the point that the high temperature reading is reproduced. Check the reading of the thermometer to confirm overheating. When my 1970 MGB is 3/4-7/8 towards the 'H' marking, I'm only running 195 degrees--- no problem. I suspect that you are not really overheating

As a long shot, check the lower radiator hose. If the overheating occurs at high speeds, it may be that the lower hose is collapsing, impeding coolant flow.I've read about this condition for years, but have never really seen it in practice.

Automobiles need maximum cooling when idling, since there is no air flow to cool things down.


Good luck


Peter


Peter VanSyckle

How old is the rad? Old rds build up corrosion and are less efficent; its a rate of heat transfer thing.

If the rad is old, you may try to boil it out, but I never had any luck with that. A recore or new rad may be the answer.


Pete
Pete

This is reverse to the normal situation with electric fans on the MGB. Stuck in traffic the temp gauge should move between about 1/2 the way from N to H and 1/3rd the way from N to H as the fans cut in and out. Under way the temp should be more or less on N depending on ambient and engine load. If the gauge is indicating as it should in stop-go, but higher on the open road, then I'd say that on the open road the engine is generating too much heat (possibly over advanced or too retarded timing) or the radiator can't get rid of it fast enough (possible blockages in the flow). However it could be that the thermostat is stuck open which would result in lower than normal readings in traffic, but if this were combined with an over-reading gauge i.e. faulty stabiliser you could well get the onditions you describe. The first thing to do is to get an accurate indication of actual coolant temperature for both 'normal' and 'high' readings on the gauge, as Les says.

Finally an engine is overheating only if it is emitting steam or coolant, usually temp gauge indication right up to H are quite safe, *if* the conditions you are using it under are likely to generate lots of heat i.e. lots of load in high ambients. Rubber bumper cars do tend to run hotter than chrome bumper, but both are run in desert states with no problems.
Paul Hunt 2

Paul, do you think the fan running all the time is impedeing the air to the radiator at high speeds?

maybe a compination w/ a collapsing lower hose.
Luigi

Guido, I agree with the others about possible and most likely causes, but I've also had another cause with my car that acted similarly. That was that the timing was retarded from spec. Ran ok around town, but got hot really quickly on the highway. In my case it was caused when I replaced the timing chain and forgot to reset the distributor. A bad vacuum advance unit might also cause this.
Bill Young

Luigi - I wouldn't expect the fan running all the time would have any effect whatsoever, in fact a stationary fan is probably more of a resistance than a turning one, and probably all electric fans are rotating to some degree in the airflow of forward movement when they are not being driven. As to a collapsing lower hose, I've heard tell of this but never experienced, it. Quite the opposite, my V8 lower hose swelled in normal use due to coolant pressure which moved it closer to the fan belt, and was eventually cut through at motorway speeds. Whilst some cars may need a spring supported lower hose if they have very high throughput pumps I doubt that applies to the MGB or it would be common knowledge and universal.
Paul Hunt 2

Wow, what a fantastic response. You have all given me a lot to think about and a lot more work to do! The coolant is drained and the old water pump is off the car. I just checked the thermostat again and it opens fully at ~185 degrees and closes as it cools, so that's not the problem. I will put the new pump in and then try some of your suggestions.
Thanks again, I will keep you posted.

Guido
G.D. Perrone

I had a problem with overheating at speed. Apparently a previous owner had thrown some stop leak stuff into the radiator and it had clogged the heater valve and, for all I know, other tight spots in the coolant flow as well. A good flush might work if that was the problem.
JMcHugh

I've heard of that as well, a friend had to have his rad opened up and rodded out after using Barrs Leaks. Another possible cause is the incorrect water pump, I've heard that two will fit some engines but the incorrect one results in much less flow.
Paul Hunt 2

Paul, I haven't heard about any MG water pumps that fall into this catagory, but am aware that there are several makes which changed from a clockwise rotation pump to a counterclockwise rotation when they went to the serpentine belt accessory drives. Pumps look the same but definitely won't work well if you have the incorrect rotation pump.
Bill Young

This thread was discussed between 12/05/2006 and 17/05/2006

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