Overheating MGF – Don’t Just Assume – Check!

Of all the varied technical queries that are directed to the Technical Help line cooling system issues are perhaps the most common. Now we all should follow the normal routine of regular checks of fluid levels and in the event of seeing any unusual drop in level look to find the reason why and rectify the fault. One thing is absolutely certain and that is it will not go away on its own and it will creep around behind and bite you.

The cold of winter always brings with it a surge in the queries related to head gasket failures on MGFs. This is odd because it would seem more likely that the summer months with hotter temps and therefore more opportunity for overheating would be the most common season for problems. Nevertheless this is a trend we experience every winter. A very common problem surrounding the head gasket failure issue is too narrow a view, and an inadequate examination to the root cause(s) that go beyond the simple gasket failure. Remember that gasket failure is often a symptom of another problem and if this is overlooked and not rectified then the head gasket can fail again.

The drive side when the impellor came loose

Impellor slipping on shaft

Impellor right off

Recently a head gasket failure with a less common, but far from unknown problem, presented itself in an early MGF. It started with an external coolant leak from a hose and although this was noted it was not rectified. Mild overheating was the main problem that followed, this was progressively getting worse as higher revs were used, then gradually the overheating occurred at lower and lower rpms. The inevitable result was that finally the engine expired in a big cloud of steam, and when the cooling system was refilled a jet of water was seen coming from the head gasket line below the exhaust manifold.

OK so now it’s time to take the head off, and oh dear, it now has a seriously ridged head face from the gasket fire rings, plus it was warped. This now ‘Soft Head’ was scrap, other than using a Payen Head Saver shim, or similar. The simpler solution was to use a readily available spare head, which was refitted, the leaking hose tightened up, (yes it was just a loose clip) cooling system filled, and then carefully bled with the engine running. However, within minutes it started to show signs of boiling with cold rad and bypass hose, so the assumption was an airlock. There then followed a detailed re-bleed and confirmation of an air free system followed by the same rapid problem again. Time to stand back and reconsider all the previous circumstances and current conditions , the final conclusion was poor circulation.

Drain cooling system, remove belts and water pump. This was visually OK aside from being a little corroded around the impellor confirming the other signs of the engine having been run for some time with no anti-freeze, not a good thing for any engine but especially so for the all alloy K series. Then a specific check of the pump immediately confirmed my suspicions and the images tell even the least mechanically minded what the problem was. Yes, the impellor was loose enough on the shaft to simply fall off when the pump was turned upside down. Over time the impellor had started to slip, first at only higher rpms then this got progressively worse until it was doing no work at all even at idle rpms.

So as the MGF, specifically those made up to 1999, are especially vulnerable to head gasket failures caused by cooling system faults, the condition of this water pump should send the message out for all owners to add this to their consideration list if their cars are suffering overheating issues that tend to show lack of circulation, which is also the classic sign of a thermostat not opening. Earlier MGFs are certainly old enough to suffer similar, and loose impellors are not unknown on older MGs too.

Roger Parker

Deceased K series head showing obvious residues of serious overheating and a clean indent that is approx 1mm deep where the gasket fire rings have sunk into the head face which has distorted and the alloy ‘gone soft’. Result – a scrap head.