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MG Midget and Sprite Technical - Cooling conundrum

The old engine, which would not stop blowing head gaskets, has now been replaced with the spare, actually the original. The spec is exactly the same as before; same head, camshaft, ignition, carb etc. To recap, 948 bored to 998, Metro head with bigger valves, HIF44, Maniflow medium bore LCB. Now nicely run in at 800 miles. Ignition and fuelling settings are as they were on the old engine after the rolling road session.

My first impression was that this engine runs much cooler than the old one. I now have a new temperature gauge which sensibly reads in degrees C. At 50 to 60 mph on A roads the temperature is about 70, which is not enough to get much out of the heater in winter. But at 70 mph it goes up to 90º. I have a thermostatic radiator fan and the switch is marked 85º, so is always on when cruising on the motorway.

Having run these cars since 1971, I have never seen such a big difference in temperature with a relatively small difference in speed. What can be going on?
Les Rose

Are you sure that your fan running the right way Les? It could be acting as an air dam when it comes on.
GuyW

I have found that seemingly two identical engines need different settings. Assuming the thermostat is working normally :-

1)Is is too far retarded or is the advance on the distributor not enough

2)Is the mixture a little lean at top end. Does it need a richer needle

Perhaps another spell on a rolling road would pinpoint the problem.
Bob Beaumont

Is it that's it's still a bit tight if you have only done 800 miles.

Trev
T Mason

Out on the road you shouldn't need a fan to help cool it
my money is on a seized distributor advance or a blocked radiator
William Revit

Quite agree with that William. At speed, the fan can obstruct and reduce air flow through the rad. If the fan is set to come on at 85, with the engine reaching 90, if it is rotating the wrong way it could be creating even more of an air dam! Easy to check!
GuyW

Yep, something odd going on. Fan should only be needed at low speeds.

It happens to the best of them... last time at Peter Bs I ended up getting involved in fixing a Ferrari 308 that kept overheating (amongst other problems!). I was stood in front of it as it was slowly boiling itself. The fans were on full chat and I noticed that my shins were getting very hot! The "specialist" that had fitted the rad fans had wired them backwards, causing issues just like Guy described.

Another thing that seems odd "the temperature is about 70, which is not enough to get much out of the heater in winter" the thermostat should be making sure you get properly up to temperature.

Malc.
Malcolm

Echo last 3 comments.

1. Check fan isn't the problem

2. Check thermostat, spec and function.
anamnesis

Most moderns would run at around 92, so I don't think yours is running too hot as such. But the rapid increase from 70degs to 90 with only slight change in road speed is odd, particularly as Willy says, almost all of the cooling at higher speeds is by air ram effect through the rad. Suggests either coolant circulation issue, or something impacting air flow, possibly when fan switches on at 85, which is why I wonder if it's running backwards.
GuyW

"Ignition and fuelling settings are as they were on the old engine --".

I'd also double check you haven't inadvertently altered something, or something hasn't changed by a fault condition.
anamnesis

We once had an MGF with equivalent symptoms: normal until higher speeds then it increased. Radiator was fine, problem was a build up of debris another point in the system reducing sectional area of the plumbing at that point by 30%.
Paul Walbran

Just to be sure...

You aren't using Evans Coolantless Coolant are you?!
Malcolm

I run Evans in my (K) midget and 1978 Jaguar xj6.

No problems sofar concerning type of coolant.


My friend replaced Evans to water wetter in his V8 mgbgt and solved his issues.

Malcom, what issues did Evans give that you heard of or experienced your self?
A de Best

Thanks guys.

I have had this fan on the car for about 15 years, and it's connected so it's not possible to have it going the wrong way. But as Willy says, it should not be coming on at all at cruising speed.

The coolant is ordinary water and silicate antifreeze.

The cooling system has been cleaned and reverse flushed.

I have had similar thoughts to Willy, and the distributor is away at H&H Ignition Solutions to be checked and set up with the correct advance curve.

I know it's just a rule of thumb, but a plug test shows that mixture and ignition timing are in the ball park.
Les Rose

Les
I've learnt over the years--never take anything for granted--You say the fan is connected and can't be the fault----test it--bypass the sender unit, get the fan running and make sure it's blowing in towards the engine-----------don't forget you were having headgasket issues previously, could this be the cause all along

Other thing is the thermostat---I believe the correct thermostat is 73c which normally runs around 80c on the gauge-----If it's showing 70 on the gauge 'maybe' the thermostat is stuck in a slightly open position giving you the 70 but not opening up at temp causing the rise in temp----The thermostat hasn't had holes drilled through it -- ? I've seen thermostats that were incorrect temp rating(too hot) drilled to give coolant flow to semi make up for their incorrect rating but they invariably run too hot at speed---might be worth checking what you've got there
William Revit

OK Willy, I just tested the fan and it's rotating correctly. As I said, it can't be connected incorrectly as I have male to female connector one side and female to male the other. I've just had the thermostat out to test. I don't use a bypass hose, and the thermostat has four 3/16" holes. It is marked 74ºC and I just tested it in hot water. It opens and closes correctly. The fan switch is marked 95-86º, presumably on at 95 and off at 86. That's exactly what it appears to do according to the gauge and warning light.

I now have to wait for the distributor to come back from H&H. I'm a bit peeved as they quoted a two day turn round and the tracking says it was delivered to them last Tuesday. Yesterday I had heard nothing and phoned them after lunch. Just got the voicemail and they seemed to have shut down for the long weekend. If the problem persists after I have the distributor back in then it's back to the carb.
Les Rose

Lean on me.

There's a debate about this, so feel free.

But from personal experience over 45 years plus, I've found when I run my Sprite too weak, it overheats when I push it.

Maybe something, maybe not.

Are you aware the HIF's have a Fuel Temperature Compensation (Viscosity Compensator)? Weakens the mixture.

If it's not working properly, or too well, maybe that's your problem. It's easy to find reports of people who've had a problem with it. But equally people who haven't.

For those that have, there's a fix. Buy a new one, or modify/defeat the existing one.

I seem to recall I have a thread in the archives about it (or somebody has), and pictures on my laptop showing how it's done.

Something else to check, if you are going to check the carb.

https://youtu.be/Nx_D0VTHBag

Geddit? 😉




anamnesis

Les--Not a fan of running without a bypass but 'tis what it is, that shouldn't make it run hot so not the problem--
Having the holes in your 'stat doesn't work quite the same as a bypass, the bypass hose recirculates warm coolant back through the engine whereas the drilled thermostat sends it off to the radiator, both ways keeps some bypass happening but the drilled thermostat sending the coolant off to the radiator is why it's only showing 70c when it's not under load-
Why it's going to 90c is still an issue--If the dist. was ok I'm leaning towards thinking along the lines of what Paul W. mentioned.
If you're 100% sure your radiator is clear there must be a blockage somewhere----or bottom radiator hose maybe, If that's gone soft i've seen them suck flat with some revs on---?
William Revit

Let me return to where this discussion started. With the identical setup on the old engine it always ran at about 190F or 88C, whatever the load (which was too hot to my mind). Now it runs at 70C up to 60 mph and 90C at 70 mph. I can't work out what is different.

Yes Anam, I know about the fuel temperature compensator and replaced that a while ago. I would always look at lean mixture as a cause of overheating, but the plug test shows the mixture is correct.

The block was cleaned out by the machine shop so I'd be surprised if there were any blockages there.

All the hoses were replaced last year.

When I get the distributor back I'll be interested to see if anything has changed.
Les Rose

I know you say the temp is road speed related, but what about engine speed if you run the equivalent revs in third gear? I'm wondering if the extra heat being generated is to do with oil temperature if this engine has tighter tolerances on the bearings, creating higher 'squish' pressure and temperature?
GuyW

Is the t/stat meant to have 4 holes? My understanding was that if the flow was too great then, although counterintuitive, the cooling isn't as effective because the coolant is moving too quickly so might not absorb enough of the heat. Could that be a factor?
Bill Bretherton

Some out of left field thoughts for an unusual problem:

Holes in the thermostat: In a way the temperature behaviour is typical of what happens when the thermostat is omitted entirely: gets hotter the harder the engine works. Normally, the thermostat just opens further. 4 x 3/16 holes will allow quite a bit of coolant to go past a closed stat and into the radiator as per Willy's post above, and put the system halfway between having a thermostat and not. Were the holes there before or were they added during the rebuild?

Gauge: A new gauge gas been fitted - is it accurate? We test the calibration of every gauge we get through and the fail rate is not a nice one, about 25%. An over-reading gauge in conjunction with the holes could give what is being observed.
(I spent untold effort on my B fretting about overheating when all along it turned out to be the voltage stabiliser failed and the gauge reading high as a result. So assume nothing, check the gauge.)

Radiator: It has been cleaned and flushed. Is that a chemical clean or was it a mechanical clean with the tanks off? We have found it not uncommon for a chemical clean to get only some of the blocked tubes clear.
We also find that if a cooling system had a sealant in it and the radiator wasn't flushed out when it was removed and sat around empty for the duration of an engine rebuild, then it can get tubes blocked/ restricted/ a lining that inhibits heat transfer. So if the tanks haven't been off to check it, that would also be on my list.
Paul Walbran

With holes in the thermostat like that it will be passing a good quantity of water to circulate through the rad, even when cold. At medium engine temperatures it will still pass water through and slow the warm up process. It could be that is the problem, rather than overheating when at motorway speeds, by which time the thermostat has opened anyway, and the fan has come on. It is actually overcooling at mid range speeds. This would give you the unfamiliar 70 degrees at A road speeds. The holes are irrelevant once the stat has opened, but are allowing too much flow when it is closed and ruining the progressive temperature ramp effect over the 10 degrees or so that a stat takes to open.

That doesn't explain the change between the two engines, unless in reinstalling you have flushed the radiator or made some other improvement to the cooling side of the equation.
GuyW

Notwithstanding Guy just stole some of my thunder lol, -- Observations on observations.

Willy and Paul both said; don't assume, don't take for granted.

"The block was cleaned out by the machine shop so I'd be surprised if there were any blockages there."

How well was it cleaned? Yes, you may be surprised, but that doesn't mean there isn't a blockage.

You tested the stat in water that you heated. That water didn't depend on the stat to reach it's temperature. Of course the stat opened. But it's not a real life test, because it wasn't affected by those holes. Those drilled holes, may allow just enough flow to stop the stat opening fully when it needs to, but be open enough at lower temperatures. Put in a different stat, or take it out completely as a test, and go do 80mph to see what temp' you reach.

Where is the temp sensor? Head or radiator?

Irrespective of gauge reading, does the engine behave differently/worse, when the gauge reads 90c?

As Paul said, test the gauge.

The change between the 2 engines, is the BLOCK HAS CHANGED. It's been rebored. New pistons and rings.

800 miles isn't so much in new bores, and can depend on how it was 'run in'. As Trev said earlier, it may still be tight, and that spells heat.

What oil have you got in your dashpot?

The colour of the plugs can be misleading, esp' with modern fuels.


anamnesis

Malcolm mentioned waterless coolant. The problem with waterless is it can mask overheating problems.If your car was cooled by water you would soon find out if it was prone to overheating because the water would boil. If it had waterless coolant it wouldn't boil, it would just get hotter and hotter which on an engine designed in the 40s/50s is not a good idea.
One of the waterless fluids (not sure which one or it could be them all) is in fact 100% anti-freeze (i.e. no water).
Rob
MG Moneypit

Les I think you may be misleading yourself saying the setup is identical because it's not. You have a new engine which will be tighter than the old one and a new gauge which presumably also has a new sensor so not identical to the old setup.

As others have said I also think that 3 holes of that size is too much and is cooling too much at lower speeds maybe coupled with a clean block so increased cooling. My money is still on the fact that it isnt fully run in yet thus causing the heat at higher revs. I would also run it a bit richer for a start too.

Trev
T Mason

I didn't mean to start a hare running, but interesting ideas anyway.

Yes I tested the new gauge, over a range of temperatures, using a cooking thermometer. Exactly correct. Sensor is in the head.

The thermostat is the same one that was in the old engine. I keep repeating that everything is as before except for the engine itself.

If there were a blockage, that would push up the overall temperature. It would not run cooler than the old engine at 60mph and hotter at 70. It's this difference that puzzles me.

Engine seems to run well when at 90C. It is now at 1200 miles (sorry I was guessing without checking the odo). I ran it in using Keith Calver's method.

Correct dashpot oil from Burlen.

Dizzy coming back from H&H tomorrow.
Les Rose

It's a mystery.

Just to be clear. You said,

" The old engine, which would not stop blowing head gaskets, has now been replaced with the spare, actually the original. The spec is exactly the same as before; same head, camshaft, ignition, carb etc. To recap, 948 bored to 998, Metro head with bigger valves, ---".

When you say you are using your spare engine, do you mean your spare was complete with an identical camshaft/head/rockers/pistons?

Either, the new engine is making more heat, or the cooling system is less able to dissipate more heat.

Assuming the distributor comes back unaltered, -- since you haven't changed anything about the cooling system, but you have changed the engine for another engine, then that's what you have to check.

What is NEW, in your now in use spare engine?




anamnesis

Les, while there could well be another cause, the symptom of normal temperature at lower speeds and rising only at motorway speeds is exactly what was happening in the partial blockage case I refer to in my earlier post. Because the restriction was in an unusual place the cause had us similarly baffled.
If the distributor comes back ok and you havent had the tanks off the radiator and the flushing was with a normal domestic hose, that is where my money is on. Testing a radiator requires a lot more flow than that.
Paul Walbran

No Anam the spare engine was dismantled and I only used the block, crankshaft, end plates, sump, and timing gears. I transferred everything else from the previous engine.

I have the dizzy back from H&H. They took much longer to do the job than they said. I have not really done a long enough run yet, but so far the temperature looks more sensible. I followed Keith Calver's procedure for setting the timing, and lo and behold it came out exactly where I had it before. After a 15 mile run with a short bit of dual carriageway the gauge was showing 80 degrees at 50 mph and 85 at 70 mph, without the fan coming on.

Some of you might remember my other thread about unstable idling. This is also better. Previously it would not idle for more than a few seconds before dropping and stalling. I now have to keep it idling at 1100 rpm, which is not surprising I suppose with my 270 degree cam and HIF 44.

I have to say that Keith Calver has been hugely helpful with this, especially as I have not actually bought anything from him. I am not likely to either as he only really sells race components.

I'll need a longer run to see if this is really fixed, but I think I'm going in the right direction.
Les Rose

I just looked at the spec that was applied by H&H. Total advance was reduced to 8 degrees and increased in the mid range. This makes sense. Over-advance at high speed would cause overheating. Curiously this is the reverse spec to the one recommended by David Vizard, which was to reduce advance in the mid range and increase it above 4000 rpm. I'm glad I didn't follow that.
Les Rose

Very good--sounds more like a properly set up dist.
I take it the 8 deg. is distributor degrees so 16 at the crank
Les, can I ask please where you have your idle timing set at--as a guess I'm thinking 13-14 ish giving 29-30 total
Just interested, it sounds about the same as I have in a mates racer MGA, It runs 16 idle-31 total @ 3000rpm with no heating issues

Hope it fixes your issue
willy
William Revit

Well done Les

It does seem as though you are moving in the right direction. Hopefully it will settle down a bit more once fully run in. May need to check the fuelling once that is done.
BTW nice article in Rev Counter!!
Bob Beaumont

Not really so much of a mystery then after all.

"Ignition and fuelling settings are as they were on the old engine after the rolling road session."

Having swapped the block, it clearly isn't the same as the previous engine, something is different, and it needs different ignition timimg.

Now all you need is some nice sunny weather, to drop the top, have some nice long 'cool' drives. 😉.

anamnesis

Willy, my reply to you about timing at idle was posted but disappeared. I just said that I'll need to dig out the timing light, when I have time. :) Yes the 8º is dizzy not crank. It was originally 12.
Les Rose

Cheers Les--no hurry, whenever
William Revit

I have at last dug out the timing light, and the figure at 1000 rpm is 7º BTDC. Still not using vac advance, following Keith Calver's advice. I'm surprised that the fuel consumption seems to be so much higher. I'm struggling to get over 30 miles per gallon. I remember that some 1275 Spridgets had no vac advance fitted. I did a European tour in one in 1968 and the fuel consumption was upper 30s to my misty recollection.
Les Rose

I recently rebuilt the 1275 on the 1971 Midget. It had been blowing head gaskets and generally running poorly due to poor machine work the last time it was rebuilt. With a more aggressive cam and higher compression ratio pistons, the rebuild is running beautifully. It comes up to normal operating temperature very quickly, obviously part of the break in process.

As the radiator had to come out, I removed a booster fan that had been in place for many years with the idea of decluttering the engine bay and going back to basics. The electric fan had been set to come on at about 220°F and to keep running when the car was parked to remove engine bay heat for easier starting.

The new engine is running at 180° where the thermostat opens except in traffic over 90° F. Then it will rise to 190° or so but runs much cooler than previously. My conclusions are:

1) The worn engine was retaining heat that should have gone to the rear wheels due to wear,

2) The electric fan was obstructing air flow, especially at speed. It was noisy and obviously put a heavy load on the electrical system.

I think that you may be better off without the electric fan, especially at speed.
Glenn Mallory

Thanks Glenn

I also think the fan obstructs air flow at speed, but I have had this fan on the car for about 15 years, and fuel consumption has been almost 40 mpg until I had the distributor set up 4 weeks ago.
Les Rose

This thread was discussed between 04/05/2023 and 03/07/2023

MG Midget and Sprite Technical index

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