MGF overheating – causes and consequences

 In MGF, MGF Engine


The K Series engine is maligned for head gasket failure, yet this is almost invariably the effect rather than the cause of a problem – gasket failure is usually the result of the engine being seriously overheated. (Exception: early engines had plastic dowels between the head and block which were prone to failure; later engines/replacement gaskets have steel dowels) On many occasions, head gaskets have needlessly been replaced, or cars even scrapped, because the mechanic (or owner) has leaped to the wrong conclusion because of hearsay.

Causes of Overheating

Low Coolant level

The coolant level on the MGF is critical. If it gets too low, (and it doesn’t have to be all that low) then an air lock develops which stops the coolant circulating through the radiator – the engine can be imitating a kettle, pushing coolant out everywhere yet the radiator remains stone cold as no coolant is going through it. The pressurising in such cases is simply good old fashioned boiling. Heaps of pressure, coolant coming out everywhere, but just driven by boiling rather than compression pressure.

Thermostat failure

A further common cause of MGF overheating is the thermostat failing to open, either because of failure or because of the marginal recirculation system which keeps it open. Its position at the intake to the water pump means that it has cooled coolant from the radiator flowing over it, so it is very reliant on this bypass flow. For a reason I have yet to fully understand, this seems to go on the blink in a few cases.

Split hoses

At the age the cars now are, cooling system hoses are reaching the end of their life. It is sound, sensible pre-emptive maintenance to replace them before they fail.  The hoses aren’t cheap, but are way cheaper than a cooked engine.  If budget is stretched, replace the “hot” hoses first – the one taking the coolant from the engine to the under-floor pipes and the pigtail shapes heater & bypass hose.   Both these hoses join the engine at the left front of the head (as viewed in position in the car).

A number of split hoses are triggered by overheating arising from gradual cooling loss from a slow leak, leading to air lock, circulation failure, boiling, excess pressure and thus a tired hose gives up.

Corroded Cooling system pipes

The pipes under the floor which conduct the coolant to the radiator and the return pipe on the engine leading to the thermostat and water pump are made of steel and so vulnerable to corrosion and consequent leakage if the coolant isn’t kept up to spec.  It is surprising just how often changing the coolant is neglected, or that water is used instead of the specified mix.  Both of these are false economies which will cause major damage to the engine.

Sudden Coolant Loss

A temperature gauge which shows normal when the engine is very hot indicates sudden coolant loss, as the gauge monitors the coolant temp. With the coolant gone, it has to wait for conducted heat from the head, and there is usually a lag in this showing up. This is a strong indication that the failed hose caused the overheating rather than vice-versa. The gauge could of course be faulty, but the lag in reading is very characteristic of sudden coolant loss.

(I might be showing my age, but the first time I experienced this was when I was stranded in the dark out in the countryside in my old Morris Oxford van when the heater suddenly when cold while the temp gauge still went normal … )

Use the correct coolant, and keep it fresh

As with all modern alloy engines it is essential to preserve the coolant’s corrosion inhibiting properties.   Coolant should be should be changed every couple of years, and should be 50/50 antifreeze and water – EXCEPT that engines with the multi layer head gasket should use OAT coolant as the coating on this type of gasket can be dissolved by conventional antifreezes.   NOTE:  Before OAT is introduced to the system, it should be throughly flushed through with water as OAT can react negatively with the original style anti-freeze.


Catch it in time!

If overheating is caught soon enough, no damage occurs – simply top up, bleed the air out of the system (most important) and off you go. We’ve done this successfully to a few cars now which we were confident had not gone past the point of no return in coolant loss/temperature rise and have monitored the resulting job over a significant period time. No further problems have evolved.

If you see a car (any car) has its temperature gauge needle in the red, STOP.   It may be inconvenient and need a tow, but it will save a big repair bill.  Whether a modern alloy engine or an old cast iron one, continued driving on an overheating engine will usually terminally kill it.

If the engine is reading higher than normal but not in the red zone, it is usually safe to continue to drive keeping a careful eye on the gauge.  If it continues to rise, you should investigate to see why.  If it gets to the red, make sure you stop.

There is a bit of leeway.  To give you an idea of how much, we once encountered a black dog standing in the middle of the road one dark, rainy night.  We didn’t see it until it turned its head towards us when we were only about 3-4 metres away.  A sickening thump, we stopped to check out the dog – it had gone – and didn’t notice thans to the heavy rain that the contents of the radiator were pouring onto the road.  We drove for about 3 km I think before the temp gauge suddenly shot up (with no coolant there is quite a delay in the sensor getting the message).  Got towed home by the K Midget (caused a few looks!).  To my surprise, investigation revealed that everything had survived OK.

But if not …

Unfortunately sometimes people don’t notice the high temperature in time, not the least because the engine is behind you so there is no telltale trail of coolant coming out of the bonnet onto the windscreen.   Astoundingly, some drivers, having noticed the engine is hot, continue to drive.   Either way, the result is all/most of the coolant get boiled off, and the head overheats and softens, leading to a loss of clamping pressure. No headgasket can cope with a collapsing head.

The give-away is the fact that significant indentations occur where the gasket’s fire ring contacts the head.  It is is ususually worst round the exhaust valve, where it is hottest. If you want to confirm, get the hardness of the head checked. Starts off Brinell 100+, serviceable above 90, (though the gasket makers say 75). Soft heads can be down in the 50 range in the worst spots, possibly lower.

We have checked out quite a number of heads now, and have yet to find fire ring indentations on any head still retaining its correct hardness, regardless of mileage. The softer the area of the head is, the deeper the indentation goes. And the only thing that causes the softness is serious overheating.

The other serious problem that overheating causes is the block collapsing around the liner seats, resulting in the liners dropping and preventing the head from getting the correct clamping pressure on the gasket.

A good guide as to how hot the head got is to check the inner cam belt cover – if it has melted (and I’ve seen that on a quite a few!) then the head got seriously hot.

The engines aren’t fragile!

Giving the car a good thrashing, even  around a track, won’t cause a problem with overheating so long as the cooling system works properly and carts the heat away as it should – the villain is coolant loss. Without coolant, nothing stops the head from reaching seriously unpleasant temperatures.

As I noted before I’ve seen high mileage heads which show no problem, and every head I’ve seen with fire ring indentations has been hot at some stage. A lesser degree of cook-up can result in the damage which takes longer to appear, but it’s always instant when the overheating is major.


We have successfully re-hardened soft heads here, the guinea pig head being the current one on our K Midget which has done a couple of years really hard graft with no ill effects. (There was nothing wrong with the old head, just wanted a good test bed to verify if we could sell such a process to our customers with confidence, and the K Midget works harder and is easier to work on than our F)

However, the process is not cheap, and it is often more economical to get another head.

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