Click images to enlarge

XPAG Overheating

This article can also be read in conjunction with :

XPAG Water Flow–Cooling.
Overheating or Vaporisation.
Warming Up times.
XPAG Engine Temperatures and Cooling.


There seems to be a lot of owners of MG’s powered by the XPAG engine, who are complaining of their engines overheating. In my experience the MOST COMMON cause today of the engine overheating, especially whilst idling or slow driving in heavy traffic, is the fitting of a modern thermostat and FAILING TO BLANK OFF THE THERMOSTAT’s BYPASS.


This was the problem with ‘Little Gem’, the YB the MG Car Club now owns. It had an after-market alloy thermostat housing fitted with a modern Waxstat. The Waxstat had been removed in a vain attempt to stop the engine boiling even after just a few miles.


XPAGcoolingdiagrambStudy the sketch of the original Morris expanding capsule thermostat in the diagram. It has a sleeve that closes off the bypass as the thermostat opens. The idea of this bypass is to get the engine to warm up rapidly by allowing the water to go straight back into the water pump, and not through the radiator. Closing off the bypass with its sleeve as the hot water expanded the capsule made the water go into the header tank, and thence through the radiator to cool.


Many people have fitted after-market thermostat housings but have NOT blanked off this bypass. Hence their coolant does what the designer intended, it goes via the easy route back into the pump inlet and not through the radiator. The modern thermostat has no sleeve to close the bypass off. Hence all these boiling engines because very little water is actually being cooled.



The cure is simple.


XPAGhosesbMake up a little alloy gasket to go between the thermostat’s housing and the bypass elbow. Drill a tiny 1/16″ hole in its centre to relieve any air locks. Also ensure any modern thermostat you fit also has a tiny hole in it to let water through, or it will not sense the hot water soon enough ( most already have this hole, some even with a little non-return ball valve). The tiny hole in the modern thermostat is also VITAL to let the system fill with water, or again you get an air-lock.


The radiator on the XPAG cars is over-engineered, being able to cope easily with its job. Old age will mean things like the water pump and the radiators core must be in good condition and the air vanes free of dead flies and leaves. With modern petrol running hotter than the old TET stuff,if you are overheating and you have a blank fitted to the bypass with a modern thermostat, then you need to look at other causes.


Other causes of over heating are legendary.


A blocked radiator core, blocked radiator air vanes, corroded up water pump impeller, failed thermostat that is not opening, ignition timing out, camshaft timing out a tooth, too many badges on the radiator grill, not turning the tap off in summer on your heater if fitted (again the water is not going through the radiator), slipping fan belt. All XPAG engined cars (not the TF) are not pressurised, the cooling system runs at the ambient air temperature. When your engine overheats, it will blast all its coolant out in a massive steam cloud, otherwise you are just running a bit hot. Such cooling systems NATURALLY lose water by evaporation and normally need topping up every week, and every day in hot weather. Your modern car with its sealed cooling system running at about 15psi should never need topping up. The new MG Y Register’s website has quite a bit of XPAG information, go to .Look at the Technical Information Menu and The XPAG Files.


Beware the ‘Radiator Specialist’ who has just washed your core out with some limescale remover, cleaned up the radiator and resprayed it matt-black. He will tell you its is a new core, and charge you for one. You will probably still overheat….


Neil Cairns.( from a recent (2011) letter to Safety Fast )