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MG Midget and Sprite Technical - Overheating?

I'm enjoying a break in north Wales in the midget, but getting a bit concerned as to why or what is causing it to overheat in certain circumstances.

If I am driving at 50 - 60 the gauge is showing 160 - 180 ish and the engine seems to respond ok. However when I am driving at slower speeds around tow or in queuing traffic the temperature starts to rise and the engine runs lumpy and does not respond well.

I generally stop before the temperature gets above 200! When it cools down I need to top up the radiator with about half a pint, but I cannot see any physical leaks of coolant.

Any ideas on what is going on?

Cheers

Tony
Tony Brough

Hi Tony,

Have you checked the exhaust colour when it's hotter.

Also have you checked the concentrations of the coolant and are you topping up with more coolant or water?

Have you checked the front of the radiator to see if the fins are not damaged or blocked.

Kind regards

James
James Paul

bit of an odd one, but I had this because PO had fitted a fan from a mini, which blows air out of the front...

so stopped was fine, fast was fine, but slow resulted in the temp heading for the sky pretty rapidly.

or could be fuel vaporisation - will it idle OK?
Rob Armstrong

Thanks for the replies.

James

the colour doesn't seem to change and the fins look OK with no areas of damage. I am currently topping up with water as I don't have any coolant with me and intend to change it once I have solved this issue.

Rob

She idles ok when hot but any attempt to accelerate and she runs lumpy, that said she may not run that we'll when cool!!

I have attached a picture of the fan, which I believe is the correct way but am happy to be corrected.

Could it be anything to do with the thermostat?




Tony Brough

fan's correct :)
Rob Armstrong

The lumpy comment is what gets my attention

Id say check your timing, you maybe to advanced, id also look for a vacume leak, causing it to run to lean

Thermostate seems good to me...

But it could be weak coolant...a pint over and over is a lot of water

Is the choke sticking, you have a heatshild in place to protect the carbs from heating and vaporizing the fuel

Lean conditions will make an engine run hotter and be more irradic

Prop
Prop and the Blackhole Midget

Fan's correct, but if you have an 8 blade, I'd put that on.

How's the state of the engine tune? Timing, mixture?
Lawrence Slater

The mixture was good before I set off and the timing may be a little retarded, I will try to adjust tomorrow before I set off on the journey home. I appreciate that using water to top up is not ideal so I will change the coolant when back home.

I am quite a novice so all suggestions are welcome.

Tony Brough

Tony,
The most effective air flow through the rad is from the forward motion of the car. So the cooling system does work best at higher speed. Conversely, the fan has little additional impact above 20mph. In fact the fan blades possibly impeded the flow of air being pushed through the rad by the car's movement. But that isn't critical. Below 20 mph it is the fan that does the cooling. This is why these cars often run hotter at low speeds, especially in traffic.

Even so, it shouldn't be overheating. As it runs smoothly at higher speeds (50 +) I doubt that the timing is badly wrong. Have you checked the fan belt isn't loose and that the sides of the belt have not worn (belt goes hard and polished on the edges) A slipping fan would cause overheating at low speeds.

When you say you have to keep topping up the radiator I presume you are topping up the expansion tank. Are you aware that the tank should be only just over half full when cold. This is normal. Anything more than that and it will expel the excess out of the overflow pipe so next time you look you may be thinking it needs topping up again.

200F is not excessively hot. The car runs a pressurised cooling system I forget the figure that this raises the boiling point to but it is around 220deg F

When the engine is faltering and not pulling properly, check to see what the rev counter needle is doing, The irregular running is probably fuel, or fuel temperature related, but if the needle is flickering then it is an electrical problem that you have.
Guy W

I believe that for any given system, glycol/water mix is about 10% less efficient at transfering heat than water alone, so only worry about the glycol concentration when the ambient temp drops below freezing.
M J Chapman

water's not so good at preventing corrosion

if it's not a leak, obstruction to rad, fan belt loose,
timing, HGF, brakes binding, lack of coolant

then have you fitted any new parts recently?

I notice that the heater (rubber) hose joint has cracks on it (could be a new hose then or very old)

I'd suggest when you get home you do the thorough clean and flush of the cooling/heating system even if the cause is elsewhere

I've got one convert who will tell you it's value

have a look at another overheating thread where I've put my simple thorough method (I'll find the thread and post next)
Nigel Atkins

Guy.

Thanks for the info. My car is a mkii '66 and doesn't have an expansion tank. I thought that 200 was too hot and that's why I generally stop before then, but I will try to stay calm when it is in that region and see how it goes.

I have a heat shield fitted between the manifold and carbs, the only none standard thing is an inline fuel filter just before the carbs.

The rev counter has started to be very erratic, but this seems to be all the time and not just when it gets hot.

MJ

Good to hear that having a weaker mixture of coolant for the time being as. Will change it back to standard when back home.

Tony Brough

if you can keep at 40mph in fourth gear on flat roads you can can usually limp home especially at night

see my previous post

(also just thought have you got the correct rad cap and checked it's seals and spring)

the thread is further down the Technical page called ‘Thermostat help’ by Brad (BK Dyson) – I go further than the post I put in that thread let me know if you want the extra details


Nigel Atkins

Nigel

Thanks for the post. Can you pot the link you mentioned.

The fan belt is tight but may be a bit old so I will change it when home.
Tony Brough

Tony,
the links within the site never seem to work for me but try this - http://www2.mg-cars.org.uk/cgi-bin/gen5?runprog=mgbbs&mode=thread&access=&subject=97&source=T&thread=201307262133488351

if it doesn't work then if you come out of this thread and look towards the bottom of the threads list page (Technical) it's there - the thread is further down the Technical page called Thermostat help by Brad (BK Dyson) I go further than the post I put in that thread let me know if you want the extra details

for setting the car up follow this order -
tappets, CB points, plugs, timing, carb mixture
if you adjust any item in that chain then you need to check and adjust if required all the items that follow in that chain

when you get back remind me if you want it and I'll send you my extended easy thorough coolant cleaning details

did you check rad cap?

did you fit any new parts before going away?
Nigel Atkins

Nigel

Thanks for the post. The rad cap is a bit old so will get changed when I am home.

I will reply when I get home and check out all the items mentioned above.

Cheers

Tony
Tony Brough

Tony wrote....

******The rev counter has started to be very erratic, but this seems to be all the time and not just when it gets hot*****



Okay....space station we HAVE ignition, its ready to detonate

Yeah thats were we need to focas... it well could be electrical

Bright side... your on your way, down side it may catch fire any time.

Got hot dogs ?, I got marshmellows

Prop
Prop and the Blackhole Midget

Tony,
just a quick note on the rad cap - for the vertical flow radiator you should have the 7lb ‘long neck’ rad cap, 34mm overall height
(the 'short neck' is 25mm overall height)

Known part numbers for the 7lbs ‘long neck’ rad cap :
GRC101
FC41A
FRC64
CHT105

that other thread seems to have disappeared
Nigel Atkins

I agree the cooant is more efficient when it isn't pure water, but I doubt the coolant mixture strength has much if anything to do with this. I've had pure water in mine before and don't have a very strong mix now, and don't get to 200F. But I have had the engine that hot before, and I too found it doesn't like as much as when it's around 170/180.

How's the water pump? The fan belt turns that too. If it's not up to muster, it won't pump well at low revs, and you need it to.
Lawrence Slater

Rethinking the evidence, I think that you might have early symptoms of a headgasket failure.
Loosing water; misfiring at low speeds; overheating at low speeds; smooth running at high speeds.

If there is a slight water jacket leak into one or more cylinders, the injection of water into the cylinder at higher engine speeds will have little or no impact. In fact it could make it run better! But at low speeds it will disrupt the detonation and cause it to run roughly. And of course it will gradually loose water and overheat.

Just speculation, so it needs more tests. I would take the plugs out to inspect. If you do this first thing in the day before starting the engine and after it has stood overnight, if there is moisture in the cylinder you will see rust developing on the plug tips.

With the engine running, remove the filer cap and look for air bubbles continually rising in the coolant. Not just turbulance in the water, but a distinct emission of air, especially when you rev up a bit. Could indicate the system being pressurised with exhaust gasses.

Having now worried you .. for a happier and more optimistic alternative take on the fault, when did you last check the contact breaker gap? If its too close the car will run roughly at low speeds, and the spark will come early so it will be prone to overheating. Rev counter needle will flicker. But would probably run fine and smoothly at higher speeds. :-)

Guy W

Thanks for all the ideas on what may be going on. I have driven back from Wales without any dramas and tye temperature never rose above 170. We stopped for a break, when I went back to the car there was a pool of water that appears to have come from the radiator overflow.

I will go through the ideas in the next couple of days and come back with my findings.

Tony Brough

$10 dollar Vacume guage will tell you if you have a blown Head gasket....that would be the easist

I like Guys thinking and theory...it makes good scence
Prop and the Blackhole Midget

Hi Tony,

As Nigel added my thread, adding water doesn't help at all!

I had some over heating problems caused by just adding water and air pockets in the system. Regardless of all the theories around major issues above, start with a drain/flush/refill of the system (with coolant) and then go from there. As of 2 weeks ago I hadn't done a flush, having done it, it's easy and actually good fun!

Brad
bk dyson

Well it may need a thorough radiator drain and flush, but I certainly wouldn't start with that. Maybe I am just lazy but if I was holidaying in Wales and had these symptoms I would do the easy things first. Not much is easier than:
1. Sitting in the car and noting the flickering rev counter needle and deducing that there is an electrical problem - though this could be a cause or an effect of overheating.
2. Measuring the radiator cap neck as Nigel has supplied the measurements. A short cap version will work OK until the engine begins to get tested on a Welsh hill when it will overflow part of the coolant prematurely, and thereafter overheat because it is low on coolant and not pressurised.
3. Flip the dizzy cap off and check the points gap. If it has closed down it will run roughly at low revs and will be over advanced and liable to overheat.

4. Got dirty hands yet? Remove and check the plugs for rust crystals which may indicate where the water is going and suggest what may need doing when you get home.

These simple checks may cure the problem, or may clarify what needs doing when you get home. Draining and properly flushing a cooling system isn't really a thing to do whilst on holiday unless it becomes really urgent. That said, I have changed a clutch and replaced head gaskets whilst on holiday but only because they did become really urgent!
Guy W

Tony's back from holiday Guy, I agree with you the thorough clean and flush and change of coolant is in addition to checking those items - and as you know I have a very thorough easy method that helps to keep the engine cool and cabin hot when required

just for once I was being diplomatic and HGF crossed my mind but I slipped it in with other bits, unlike you I forgot to put about points gap closing as the car was adjusted before the holiday

Tony,
now you're back reread this thread and Brad's thread and consider all the possibilities, you can get paper and other types of testers to see if there are gases in the coolant suggesting HGF or just look as Guy has described

email me if you want details of my 'method'

Nigel Atkins

I completely have to DIS-AGREE with guys last post

Your on holiday, outside your home country...id go for the complete over haul and rebuild...perfably in a parking lot grocery store

Yes it will suck, it will be ungodly expensive, heck you may even get mugged

But in 10 years ... think of the great story there is to tell, provided your still not in a comma from the beating You took from the mugging

Sights sounds and smell....eh over rated, thats why we have the internet

Rebuilding an engine 1000 miles away from home while on holiday... now thats a mastercard commercial all by its self

Prop
Prop and the Blackhole Midget

Progress update.

Sorry prop I was already home before I got your post!


Fan belt is tight and in good condition and does not appear to be slipping.
The points gap is ok.
All 4 plugs are a greyish brown colour with no signs of rust so it looks as if the mixture is ok and hopefully the lack of rust is a good sign.

I did a compression test and the result are the same as they have always been:

1 = 190
2= 190
3 = 190
4 = 170 ish

As Guy suggested I left the rad cap off, I don't believe I could see any bubbles just the ripples.

I had a look under the rocker cover and didn't see any signs of "emulsion" just oil.

The rad cap was quite old so I have changed it for a new one. Both the old one and the new one measure about 38mm so must be the correct type.

I will get some more antifreeze tomorrow and get ready to replace the water..

What is the best way to flush the block?
Tony Brough

Ah, I thought Tony was still in Wild West Wales, so my comments were based on that assumption. A proper flush of the cooling system needs access to a hose and lots and lots of water and I know that Wales is desperately short of the stuff. And I cannot believe that Prop should DIS-AGREE with me. That's just not right!

Flushing the cooling system. Very easy to do intuitively as it is fully explained in those 4 words. But Nigel will provide an excellent written explanation which is a very effective and thorough method. And I mean no cynicism by that Nigel. Its a genuine compliment to your methodology.
Guy W

Tony,
if you want my notes just email me, (see my address on an earlier post here)
regardless of that I'd pre-mix the coolant prior to putting it in that way you know it's at the correct ratio and so will be the excess if you need it for top-ups later, take your time filling so as to reduce the air bubbles going in, I've always just followed the Handbook directions for refilling and never had problems or need to run with the cap off (other than when topping up because of HGF just at the start of a weekend away tour)

the main thing is to take the block drain plug (and washer) out and scrape the crud out of the drain hole with some thickish wire and keep scraping as you flush, when you think it's all cleared out stop for a cup of tea then repeat, as Dave Squires will tell you if you done the same next year you'd probably still get more bits of crud out from there


Guy,
there's a bit more to my four words - thorough cleaning and flushing - and I only put the shortest of the three stages in my posts now

flushing the cooling system is quite intuitive but it's improved on by following written directions from the good book - and then further enhanced by my experience :) - but none of it is anywhere near rocket science

it is however more pleasant to do during this very extended period of hot dry weather (well here at least)

as for refilling the 1500 system without getting airlocks the relevant good book helps there too - and good news for Tim, Dave and many others, a friend gave me an original copy of the 1500 good book - I'll now be able to compare and swap page numbers with 1500 owners, oh the sheer joy for all
Nigel Atkins

This thread was discussed between 16/08/2013 and 19/08/2013

MG Midget and Sprite Technical index

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