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MG Midget and Sprite Technical - Overheating Frog

Last summer in hot weather I found the Frog was overheating quite badly - air temperatures of 28 - 30 C caused the temp. gauge to head round to 210+ when cruising at 70 ish and hills at quite a bit less speed.
The engine is a 1275 (+fast road head, HIF44, long branch exhaust - 90 odd bhp according to Peter Burgess), with cross flow rad (3 years old), electric fan, heater, no thermostat (drilled blanking sleeve instead), silicone hoses are in good condition. Its fine at cooler air temperatures.
I think my first steps are to give it a proper flush, and would appreciate some advice: I understand Nigel is the man with detailed instructions, which I don't have. Also, which make of engine flush/cleaner to use.

Stay well,
J.N. Williams

A couple of questions:-

Why did you change the radiator and was it OK until last summer?

What coolant/antifreeze are you using?
Dave O'Neill 2

Food for thought, follow links below for a full read.

" --- it should be understood that a blanking plate is normally intended for racing use. On a street machine, installing a blanking plate without including a thermostat while leaving the pulleys the original diameter usually results in HOTTER running, ----"

This is due to the fact that without the flow restriction caused by the presence of a thermostat, the coolant circulates so rapidly that it does not have time to absorb heat from the engine, nor does it have time to release heat into the cooling matrix of the radiator.

Thus, if you have chosen a camshaft ( my edit, or a fast road head ) which causes the engine to be normally operated at a higher average engine speed (such as a Piper BP285), then it would be wise to install a larger-diameter pulley wheel onto the coolant pump in order to reduce the pumping speed of its impeller. This will assure that the coolant has sufficient time to absorb heat from the engine and release it into the radiator matrix.

Too much coolant flow can force the coolant through the coolant passages too quickly, causing it to not accomplish maximum heat transfer. This condition can also prevent the coolant from having enough time inside of the radiator in order to allow efficient heat transfer.

One of the greatest or perhaps worst cooling system myths is that you can remove your thermostat to eliminate overheating. This will only add insult to injury! When coolant never has a chance to give up heat via the radiator, it gets hotter and hotter, especially if youre stuck in traffic. And even on the open road, coolant never has a chance to park in the radiator long enough to give up heat energy to the atmosphere.


But on the other hand ---- albeit not an engine, but the same principles would apply I would think. Wouldn't they?

If you have a liquid-based cooling system, like the loop in the picture below, does the fluid speed actually matter?

Coolant Flow Rate
Looking at the previous expression, we can see that slowing the coolant down is the wrong way to go.


I think that pump cavitation is a much more like likely problem on faster running engines. The pump then fails to pump fast enough, rather than too fast. Certainly slow water circulation would decrease the cooling deficiency.

Simple things first though. Correct pressure cap, seating properly? There are some poor ones around with an undersized sealing washer. Clear waterways? Fan blades on the right way round?

I would fit a thermostat, if only because you want it to warm up to operating temperature as quickly as possible to minimise engine wear. And with winter coming that can take a while without a thermostat in place.

As I understand it, the sleeve is to blank off the bi-pass, to prevent hot water circulating back into the block, and avoiding the radiator. So why drill the sleeve, when you want all the flow into the radiator for cooling?

Looking at the modern thermostats, the type I have fitted, I can't see a blanking sleeve. So even when open, does an ammount of hot coolant flow back into the block and not the radiator? Should then a blanking sleeve be used with modern thermostats?


Dave makes a good point (not that Iím saying the other posts donít!).

With engine and cooling system in that configuration has it ever been ok cooling wise in the same situation? If it has then something has gone wrong or blocked up. If it has never been right then the set up of the cooling system is wrong.

The good thing is that you shouldnít have any problems at this time of year but the bad thing is that it will be a long time before you can test any work you do!
John Payne

Maybe last summer was just exceptionally hot and in prior use it never got to be that stressed. It sounds pretty marginal and for most of the year probably not a problem. I still think the lack of thermostat and slow or insufficient warm up is likely to be more of an issue for more of the time, but then I am not in Sunny Surrey!

I have almost the same cooling set up in my Frog but use original vertical flow rad and 6 blade plastic fan. The engine has a similar spec but with 1.5 SU carbs. I also don't have a stat but the blanking sleeve.Its generally fine in 30c plus except waiting in traffic! If its just happened it may be a head gasket problem? Worth a comperssion test?
Bob Beaumont

One thing Iíve noticed since driving my Mk1 with standard cooling system is how much better it is with an engine driven fan fitted. You donít get any of that raising and lowering of the temperature all the time, itís just fairly constant which Iím sure is better for the engine. Obviously an electric fan is a good idea in some situations but I think if I ever decided to fit one Iíd keep the engine fan and have the electric as an extreme safety measure.
John Payne

Hi Nick,
you're very welcome to my very simple, unoriginal but thorough cooling/heating system cleaning notes. They're really as preventive servicing maintenance rather than repair but they can help with repair simply as they give a chance for a good look over whilst doing the work, and maybe all the repair required as often repairs are just clean and lubricate, as with the other thread on the boot lock (the new fresh antifreeze is the lubricant in this case).

Email me and I'll send you the notes with pleasure.

210+ sounds hot, subject to the gauge being anywhere near accurate and there could be many reasons or combination of reasons for this. Giving the system a thorough clean will only help and not hinder.

So far you've not really mentioned that your car is really overheating, it hasn't unless you've see steam or smelt the coolant or other items getting very hot.

But before you start 'my clean' I'd suggest you take as many photos as you can of the front of the car (with bonnet down), inside engine bay from and rear of rad showing cooling fan, rest of engine bay, hoses, heater. In case we can pick up any points that may help with cooling more.

Check all items mentioned here (my notes cover items too) and that the cooling fan operates in the correct direction by using a smoke test to see which way the fan blows (or sucks).

Also bear in mind the antifreeze element of the coolant can last longer than corrosion and lubricating elements which is why it's best to keep to regular changes.

You should also consider as mentioned other causes of getting too hot (or overheating) er, running too lean, stuffed exhaust, brakes dragging, blockage to rad, fan belt, er, can't think of anymore other than coolant.

Nigel Atkins

just thought - electric fan, what's the switch for it, set to what, probe/pickup fitted where, engine fan still fitted/removed? (photos would show a lot of this).

Also what temp readings for car when at cooler air temperatures and going along steadily on flat ground at say 40mph in fourth gear on open road?
Nigel Atkins

it also depends on when you use the car, if you don't use the car during the cooler and winter months then sticking with the engine fan is a good idea, and not to bother adding an electric fan.

Do also bear in mind the differences in engine and cooling systems between the Mk1 and Mk3, plus if with uprated engine. ICE not the most efficient engines but very good at producing heat.

Having a more practical Mk3 that gets used all year round the electric fan has it's uses.
Nigel Atkins

ETA: HGF, I meant Bob had already covered with compression test.

another thing, sorry I have a drip memory, is your bonnet/front shell metal or fibreglass?

Any vents in it?
Nigel Atkins

Many thanks for all your very helpful thoughts and comments: as ever this forum is full of good advice.

Dave, the cross flow rad was fitted by the PO and about 3 years ago seemed to have had it, so I replaced it along with fitting silicone hoses. Its always run a bit hot in summer. Antifreeze is bluecol - it needs changing, so a good flush at the same time makes sense.
Anamnesis, the sleeve I've got is the round copper one with a couple of holes drilled in it (Moss) which replaces the thermostat. I might well go back to a thermostat when I flush it all.
Guy, the pressure cap is old stock and seems fine.
Bob, I don't think there has been any deterioration in performance - still zippy - so no obvious engine problems.
Nigel, thanks for the offer of your notes along with the other comments. I've sent you an e-mail.

As I said to Nigel, I'll start with the basics - a good clean out and maybe fit a thermostat (I do use it in winter, so that makes a lot of sense) along with checking to see that the fan blows the right way. If that doesn't cure it, maybe fitting the standard fan as well.
Thanks once again,
J.N. Williams

If it's lean, it will run hot - and wreck your exhaust valves. Richen it out a little and see what happens.

there'll be a thud on your electronic doormat.
Nigel Atkins

I haven't got a frog, but seem to think I read somewhere (or imagined it) that they run hotter under that big bonnet, than later mk's, which themselves suffer from heat that can't escape fast enough. Originally smaller engines in frogs coped, but as Nigel remarked, 'ice' are heat inefficient, so a bigger engine and more heat to dispel under the same bonnet.

If all the other options fail to improve it, perhaps cooling the oil might tip the balance back in your favour.

Has the radiator got a shroud? The vertical flow had that substantial built in shroud, but the crossflow doesn't. You maybe pulling air from the sides, and not through tbe rad. But at speed that shouldn't be such an issue.

Overflow tank, with correct pressure rad cap on it? Correctly filled? Crosflow is filled via the plug in the rad, and not via the expansion tank.

When flushing, make sure you flush the block well, as well as the rad. Remove the rad and flush in reverse as well as forward. For the block, I also flush in both directions, and back through the block drain hole/tap too. A LOT of sludge/sediment can remain trapped in the block.


I run a Frogeye based Sebring with a 1275 (88bhp) similar spec to Bob with twin 1.5 S.U's which I consider, with my spec, give more torque.
It uses a vertical flow radiator, six blade fan, & thermostat (74 deg) and runs uphill and downhill throughout Europe and U.K. with no overheating.

Alan Anstead

Blimey Anam you lightened your engine taking that lot out.

I remember someone posting a photo on here of big globules glunk from a mixture of antifreezes IIRC but you have it beat for dry debris.
Nigel Atkins

Thanks Nigel - the docx has landed!

Now to try to find time to do the work: it seems to fly under lockdown.
J.N. Williams

No problem Nick, I hope I wasn't too concise and I only notice the typos after sending and yet again I put "can" when I meant "can't" but I sure you'll get the gist, my typing finger moves faster than my brain
... and my typing finger moves slowly.
Nigel Atkins


I had overheating on hot days. I changed a lot (Water pump, fan etc..) When I changed the 100% antifreeze coolant to 50% coolant/ 50%water the problem was solved. Glycol coolant can not contain and transport as much heat as water.

Flip BrŁhl

This thread was discussed between 27/11/2020 and 28/11/2020

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