Why did my MGF overheat, and what should I do now?

Ok so today I had to travel a 250 mile round journey.

This was for work, not a lockdown jolly up!

So before I left I checked the oil and coolant, and both were fine.

Got about 90 miles into my round trip absolutely fine, and pulled into the services for a jimmy. Come back out, started the car up, and I noticed the cooling fan come on momentarily (not sure if it was the engine bay one or the rad one)

Anyway, pulled out of the services, had to floor it a bit because of an awkwardly positioned lorry, and got back onto the motorway, doing a steady 70. About 2 minutes later I glanced down and noticed the temp gauge was right at the top, I immediately pulled onto the hard shoulder and switched off. At this point steam was billowing out of the top engine vents. So I thought then the head gasket will be toast.

RAC guy turned up, looked in the now empty header tank and agreed the HG was likely now damaged. However he suggested we refill the tank and try starting up. Amazingly it started up sounding perfect. Nice and smooth, no misfire.

He then asked me to drive slowly down to the next junction so he could have a proper look off the motorway. This was about 4 miles, and he said to stop immediately if the temp climbs and keep my speed under 50. Did that, it drove fine, got off the motorway, and he rechecked the coolant and it hadn't lost a drop. 
So he was at a loss as to why it had lost coolant before. He suggested putting in some Kseal in (just to get me home) in case there was a leak anywhere. I was in two minds about this but before I knew what was what he had put it in. 
I was told to carry on with my journey, keep an eye on the temp, and stay under 60mph.

I did, and got to my destination fine. Checked the coolant when I got there and it was fine, then drove home (staying under 60mph) and everything was fine. Overall after it overheated I was able to drive about 160 miles.

So I am really confused.

What could've caused it to overheat in the first place?

What do I do next? Could the HG still be damaged? 

What can I do to check there isn't some sort of leak?

Presumably it would be a good idea to get the cooling system flushed now it has Kseal in it?  

Get that K seal out before it blocks the narrow water ways in the block and radiator of the K series engine.

I had a similar problem on a trip back from Naples.

The car overheated and took 6 litres of water at the top of a Swiss mountail pass.
The 80 miles later it did the same climbing the moutain in Lichenstain.

The next day I topped the oil up from just above minimum to the maximum mark.

Once back in the UK replaced the radiator as half the fins were mising.

This was in 2016 and the car has been fine since including a run down to Rome the following year.

Don't assume HGF if the car is OK after topping up the coolant.

I'd guess it was a dodgey coolant filler cap, theses are notorious for failing...... get 2 and always carry a spare.
I'd say you were lucky enough to shut it down before any damage occured.

Look or any sign of oil in the coolant or coolant in the oil.
Globs of what looks like tockley snot in the coolant will be oil.......not good!
And snooty looking shit on the dip stick and the under side of the oil filler cap........also not good!

A must do for these cars is fit a coolant level sensor kit

The one pictured is the one I use, the Brown & Gammons kit, very easy to fit and use.
there are others out there buut IMHO this is by far the best.
The indicator goes off when your coolant level drops by @ a litre or so, this buys you time to shut the engine down and look for the cause before any risk of damage occurs, giving you much piece of mind.
Don't whinge about the cost of the kit, compared to the cost of a new engine.....it's nothing!
Anyone with an MGF / TF not using a coolant level sensor is insane and needs locking up for their own good!

Beg, buy, borrow, bludge, hire or steal a compression test gauge set

and do a compression test. The important thing isn't so much the actual gauge reading as the variation betwen readings of the varoius cylinders, you don't want any more than about 10% variation between cylinder pressure readings.
Whilst your doing this inspect the spark plugs (as you will have them out to do the test)  if one of then is much cleaner than the others then that is probably going to be the cylinder with low compression, it's probably clean as coolant leaking into the combustion chamber will have steam cleaned the plug.

The next trick is to beg, borrow, etc. a radiator pressure test gauge set

Now as per the instuctions provided wit the kit replace the filler cap with one of the adaptors and pump the pressure up to @ 15 psi as indicatored on the pump gauge what you want is the pressure to stay at the same level over a period of time. If the pressure drops you system has a leak.
While the system is under pressure  do a bit of and inspection... radiator, hoses pipes etc. to see if you can see any coolant leaking out.

Now here in Oz we don't have Kseal, but I assume it a compound that you tip into the cooling system that will sometimes fill a gap in the head gasket or fill a crack in the head or block,
We have various products that do much the same, Irontite, Chemiweld, Bars Leaks etc.
I doubt these will do much harm, I doubt they will do much good either. I would flush the system.

 

Thanks both. I'll invest in one of those pressure guages and the coolant alarm.

I've had a look at the oil and coolant this morning. Oil looks fine on the dipstick, but the coolant has a bit of a film on the surface and "bits" floting in it.

I've tried to take a picture but it doesn't really show it. I'm not sure if its oil or the Kseal

Those bits could be the Kshit, sorry Kseal.

K Seal always reminds me of the old way of sealing a radiator,...drop a raw egg in it. That didnt work very well either!

Thanks everyone for the replies.

I'm going to try and get the car booked in at the local MGR specialist next week and get the system pressure tested to start with

Whilst I was out driving this morning (at about 50mph), the EML light started flashing at me. Its never done that before. I'm concerned now that its related to it overheating.

Also, I've replaced the coolant cap. However after driving this morning I left the car about 3 hours to cool down, then went to check the coolant level. When I took the cap off the exapansion tank released some pressure. Surely there shouldn't be any pressure in the tank once the engine is cold?
 

Pusb wrote: Also, I've replaced the coolant cap. However after driving this morning I left the car about 3 hours to cool down, then went to check the coolant level. When I took the cap off the exapansion tank released some pressure. Surely there shouldn't be any pressure in the tank once the engine is cold?

 

Yes, there could still be pressure after 3 hours, probably very little though.