M20 cooling problem

[Mne666]

I have a cooling issue , it has been creeping just above central on the gauge since the engine was swapped but normally a quick blow of the heaters gets it straight back down or just blipping the throttle speeds the fan up and it lowers itself, it only creeps up when sitting, it's fine when driving.

After a while it had cooled off and I was about half a mile from home so I drove back slowly, temp started to increase again and fams were cold and as I was about to pull over the temp all of a sudden dropped back down and the fans blew red hot.

Everything was ok but as I pulled onto the drive the fans went cold and the temp started to rise again.

Any ideas?

[Brianmoooore]

You had a fault which you knew about, chose not to sort it, and now it looks like it might have cried 'enough', and real damage has occurred.
Original problem may have been something as simple as a duff viscous fan coupling, but now it looks like the cylinder head may have cracked, or, if you're very lucky, the head gasket has failed.
The heaters going cold thing is an airlock in the rear of the cylinder head, but airlocks don't occur once all the air has been removed from a system, so it seems likely that the 'air' is gas from combustion.

[Mne666]

It had a new coupling about 100 miles ago, the same time It had the new engine. So I didn't ignore the fault, I just drove it home and then found it wasn't right... Took it for a drive after topping it up and then that happened.

Which was about 1 mile from home

[SGP]

Maybe the system wasn't bled properly after the swap ? Could it simply be an air lock ?

[Mne666]

I filled it up and bled it last night , ran it on idle for twenty mins with no issues and then drove it.

We are going to change the radiator as it's quite old and to make sure that it wasn't leaking from around there.

[Mne666]

I filled it up and bled it last night , ran it on idle for twenty mins with no issues and then drove it.

We are going to change the radiator as it's quite old and to make sure that it wasn't leaking from around there

[Brianmoooore]

You didn't indicate that the engine swap was as recent as that! After just 100 miles there's still likely to be a small amount of residual air in a cooling system that was filled from empty.
It's entirely possible that you've trapped just enough air to cause an total airlock at low pump speeds, but higher pump speeds produce enough pressure to enable some flow. I've experienced this on one or two occasions.
Note, however, that with an airlock, the head can crack in as little as ten minutes. You cannot take liberties with the cooling of a M20B25.

[SGP]

I filled it up and bled it last night , ran it on idle for twenty mins with no issues and then drove it.

We are going to change the radiator as it's quite old and to make sure that it wasn't leaking from around there

When did the coolant level start dropping ?
If the coolant is dropping and there's no obvious leaks, it's quite possible that the head gasket is blown (at best) or the head itself is blown ( at worst )
If so, the coolant goes out the exhaust making lots of white smoke. Unless it's just water.
Which was in my engine btw.

[Mne666]

I washed the car and them on the drive after I opened the bonnet and I saw some water dripping off the fan blade, at the time I thought it was just from washing it but now it makes sense that the rad is possibly leaking. I don't know of it sealed itself now as it's full and I can't see any leaks and the level is steady but I'm going to replace out of caution.

There is no gunk in the oil gap or smoke from the exhaust

[Brianmoooore]

If there is air trapped in the engine, the coolant level will drop as it is expelled. Also, on every M20 I've come across, the level in the bottle stabilizes at about 10mm lower than what is supposed to be the correct level (the seam across the bottle).
The source of coolant leaks can be surprisingly difficult to pinpoint. The ideal way is to pressurise the cooling system with compressed air and wait. Since I've had the kit to do this, I think every car I've tried it on has had at least a weep from somewhere.
Radiators are a 100,000 mile service item, IMO, so a new one wouldn't hurt, and check that your coolant pressure cap is the post 1997 modified type.

[Rallyprep_UK]

Check the heater matrix pipes are not the wrong way around

[HairyScreech]

I have found the best way to bleed these things up is to fill them while squeezing the top hose, fire it up and then massage the top hose while topping the coolant up as it drops, so far this has never failed.