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MG MGB Technical - Cooling poblems in traffic

The cooling system for my three bearing ī63 MGB engine works perfectly when the car is moving. With a 180F thermostat, temperature rarely passes 190 even when climbing hills and driving very slowly in warm summer weather. Most of the time it stays around 170-180.

But as soon as I stop, the needle starts to climb. I have not timed it precisely, but I doubt if it takes much more than 10 minutes before it reaches 212 F. I feel it heats up faster now than it did a year ago, but again I have not really timed it.

The system is clean, and everything is standard. I have a two year old cast iron County water pump with the right impeller shape and clearance. I also believe the fuel mixture is about right.

Is it normal for this engine to heat up so quickly in traffic? I have not checked the ignition timing for some time - can wrong timing be the cause?



Retarded timing will cause warmer running. Since it only happens when stopped would indicate that the fan is not moving enough air over the radiator. Make sure your fan belt isn't too loose and slipping You may want to try installing a fan shroud if you don't have one already.
John H

I fitted the Moss shroud and that cured my problems in this situation. My car had never boiled, but I did lose my nerve and switch it off once or twice in traffic.
Stan Best

Tore - The first thing I would check is ht eaccuracy of your temperature gauge, particularly at the top end. Sthe second thing to consider, how does the car run when you reach 212°? With a 7lb radiator cap and a 50% solution of antifreeze in the cooling system, 212° is not really an excessive temperature and as long as the engine is not missing or sputtering, I would just keep an eye on the temperature (boiling point for the above combination is 245°) and consider stopping when the water temperature moves into the oil pressure range (if you have the combination oil pressure/water temperature gauge). I assume that you have the original three blade fan installed. If so (and depending on your wish for originality) consider going to the 7 blade plastic fan from the later MGBs and a spacer to move it closer to the radiator. You mightalso consider an electric fan, although I'm not a real advocate of that approach. You may also want to put some kind of air block around the top tank of the radiator, in the gap between it and the diaphragm so all the air is forced through the radiator (see attached picture). Finally, have you installed a new, after market radiator? Some of the aftermarket radiators are less than efficient and have been causing a lot of problems. Cheers - Dave

David DuBois

Hers a pic of the top

Stan Best

and of the side

Stan Best

Thanks for lots of useful input! I have the original three blade fan and the original radiator, but that was re-cored a few years ago. I also use 50% anti-freeze.

When the temperature reaches 212F and above the engine is running a little rougher, so I suppose the temperature gauge is rasonably accurate. I once had it at 230F, and then it was pushing out a few drops through the overflow pipe, but the engine was not missing or sputtering.

The shroud looks like a very good idea. I will also consider the plastic fan and the air block, but first I will check the ignition timing and fan belt.


I have had my 66 GT since 76. It has always overheated in heavy traffic or idling at lights unless the revs are kept over 2000. I have had a high efficieny radiator fitted which helped a bit as did the fitting of a thermostat blanking sleeve. The 3 blade steel fan does not suck enough air at low revs. During the 30 years I had my MG repair business I converted about 15 cars (5 brg) to the 7 bladed plastic fan but that can only be done with a shorter water pump fitted - GWP130 - or the fan gets too close to the radiator. I don't think there is a shorter pump for the 3 brg motor. So that leaves the fitting of an electric fan a necessity. Which I did to 3 other 3 brg cars.
Garth Bagnall

Although not pleasant, you can relieve some of your (in traffic) over heating problem by adjusting your heater blower to maximum output.
Steve Buchina

Is the fan the correct one i.e. sucking air through the rad, with the correct number of blades of the correct size? Is it mounted the right way round? The wrong way will reduce efficiency. Plastic fans usually have the leading edge thicker than the trailing edge like an aircraft wing. Metal usually have a correct orientation as well, with the rounded corner on the leading edge, although that may be more for safety than efficiency.

I'd say the gauge is correct if it is reading correct when running, it is the large increase when stopped that is incorrect, you shouldn't get that with a mechanical fan. You do with electric, but that is because the fan stat is designed to have a higher switch-on temp than the typical running temp which is controlled by the thermostat.
Paul Hunt 2

Reviving this thread as I learned something about the Moss fan Shroud today. At low tempuratures it can bend and contact the fan blades. The car sounded as though it was about to expire with a rocker assy or timing chain problem, but it was just a fan tip hitting the shroud. It was as the engine moved around on its mounts, it was fine not running. Attempts to adjust it failed and eventually the engine bay warmed up and it stopped.
Stan Best

1. If you want to keep it original, fit the piece of foam at the top between radiator and radiator shroud. There was one originally, bit mostly this got lost through the years.
2. You can also fit the seven-blade steel fan that was available from MG for tropical climates - and you are still original which will not be the case with the plastic fan
3. do it properly and install an electric fan with a thermoswitch.
I had a screwfitting for a Volkswagen-thermoswitch soldered into the lower radiatorbox and a SPAL-fan blowing from the front. I also removed the metal fan complety.


A few points. Make sure the ignition is properly timed. If it's too advanced it will get even more advanced at idle,causing overheating. "Mixture about right". Weak will cause overheating, try "a flat" of enrichment. Use more water, less anti-freeze......if you can!!!
Having a mechanical fan is counter intuative, i.e. it runs at it's slowest and moves least air when you need it most! Bin it, and go electric. They are more efficient, run only when you need then and are quieter. My GT had one of the later, multi bladed, plastic efforts on when I bought it. It sounded like there was an RB211 under the bonnet!!! My electric has a therm controlled switch and an over-ride switch, the former is there for insurance but seldom cuts in most of the year, the latter is useful to use in the odd heat wave in heavy traffic!! Or if I want to leave it idling for some reason. The last advantage of electric is it releases an horse or two consumed by an engine driven fan.
Allan Reeling

"If it's too advanced it will get even more advanced at idle,"

That can only happen with manifold vacuum, which didn't happen on North American spec cars until 1971, and UK cars until late 76. Before that all cars were carb vacuum, which gives zero advance at idle.

North America got the 7-bladed fan from 1970, elsewhere from 1974, although Germany and Switzerland reverted to 3-blade in 1975 for noise reasons.

The factory electric fans probably caused more paranoia about 'overheating' than anything else, as the temp goes half-way from N to H and back all the time in anything other than running in free air. My 73 mechanical fan keeps the temperature stable unless stuck in traffic for a very long time on a very hot (UK terminology) day, but even then when it did finally reach the H zone absolutely nothing untoward happened.
PaulH Solihull

The engine bay looks totally original with the 3 blade fan and the shroud. Cooling is fine with this arrangement. I just wanted to share a bit of newly aquired knowledge with the BBS.
I was gratified to see how the blanking cardboard was sucked against the radiator while the engine was idling. We have discussed this before as well
Stan Best

I have a 65 with the 3 blade fan. When I lived in California I would get the same heating described by Torr. I installed an aftermarket pusher electric fan similar to the one Joern uses. I only have a manual switch in the O/D hole (sadly no O/D on my car). I turn it on in traffic when the car would heat up. At higher speed or with the temperature under control I turn it off. It has worked well. I can keep the temperature around 212 deg F or so in traffic on hot days. With a 7 psi radiator cap and 50% ethylene glycol coolant this gives a reasonable safety margin from boil over.

Now that I live in New Hampshire the generally lower temperatures and less traffic means I use it seldom.

Robert McCoy

Yes Paul, got the dizzy vacuum thing mixed up but stand by the rest. "Factory fans caused paranoia"? Most MG's are followed, most of the time.......ho, ho!! Stillsooner stay electric, gain a few BHP and cut down the racket.
Allan Reeling

Joern M From Germany, Sorry, could not help noticing what a nice oil filler cap you have on your rocker box. Not standard, but lovely just the same, where did you come by it? Mike
J.M. Doust

You're not saving HP by using an electric fan. The power to turn the electric fan has to come from somewhere. In this case it is coming from the alternator or battery. There's simply no free lunch. RAY
rjm RAY

>You're not saving HP by using an electric fan.

When it's not running it uses infinitely less power than a mechanical fan. ;-) And it should only be running when the car isn't moving, meaning the parasitic loss wouldn't really be noticed. At speed there's enough air moving through the radiator that the electric fan should not be running, so you DO get the HP back.

I forget the amp draw on my electric fan but I ran the numbers and seem to recall it ended up being about 1/2 hp, even after considering the alternator being about 50% efficient. Anyone know what a mechanical fan draws?
Rob Edwards

Mike, thanks for the interest in the filler cap!!
Actually itīs a filler cap that came with an aluminium rocker cover. I bought this about 20 years ago at the Technoclassica Essen from an english exhibitor and have never again seen a cover like this - neither at a dealer nor on a car.


Ray, you're wrong. An electric fan only runs when necessary.In other words very little! and modern fans, unlike OE ones, draw little current and therefore put a smaller demand on the alternator. An engine driven fan is always running and the faster the engine runs the more work it's having to do. Work has to be paid for. Some course, multi-bladed fans can consume at least 3 BHP.
Allan Reeling

I'm thinking of converting to an electric fan before the summer :) Which size should I use? would 12" be about right or could I get away with a smaller one?
R Fowler

I find this thread surprising. I've owned my 1965 5-bearing Mk I MGB for 42 years and have personally covered over 200,000 miles in the car. For many years it was my daily driver, used in stop/start traffic. It gets an awful lot hotter here in Australia than in many places listed above, such as Norway. Despite this, never has my car even hinted at overheating in heavy traffic (nor elsewhere). I would have to question then if there might be some other issue/s in those cars that are running hot.
(My MGA, with its cramped engine bay on the other hand did have problems such as you describe. A radiator shroud made a HUGE difference to coolant temperatures at idling and in heavy traffic. Ultimately with the MGA, a custom made aluminium radiator solved my remaining hot-running problems in my MGA).
T Aczel

Trying to upload an image of my MGA with aluminium radiator and (Moss) shroud.
A shroud ensures that the (round arc of action) fan sucks air through the entire (rectangular) surface of the radiator. It also stops air in the engine bay wastefully getting drawn through wastefully by the fan again. You can feel and here the additional air getting drawn through.

T Aczel

The problem, that I have encountered, with the radiator shroud that is currently on the market for the B is that there is a huge gap at the bottom that allows large amounts of air to be drawn in from underneath the radiator. It defeats the whole purpose of a shroud. After two years, I removed mine due to the fact that it made it almost impossible to get to the supercharger belt tensioner. It made no difference in the operating temperature of the engine. RAY
rjm RAY

This thread was discussed between 18/06/2007 and 11/03/2012

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