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XPAG Overheating or Vaporisation?

This article can also be read in conjunction with :


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


MG classic petrol vaporisation problems.


Some believe this is caused by the engine overheating. An overheating engine and fuel vaporising are two completely different things. Fixing them is very different as well. It is possible for an engine to overheat and not vaporise its fuel, and the vaporise its fuel when not overheated!


Classic signs are stalling at traffic lights and not being able to re-start. A very ‘quick’ temporary cure is to pour cold water over the carburetters to ‘condense’ the fuel. The fuel pipe that runs behind the engine block to the pump needs insulating.


XPAGcoolingdiagrambAn overheating engine is far more dangerous, and one VERY COMMON cause today is where people have fitted a modern thermostat. The old ‘Morris’ thermostat was a set of bellows with a sleeve that closed off the by-pass when the thermostat opened to STOP COOLING FLUID GOING BACK INTO THE PUMP and not through the radiator. Once this sleeve covered the by-pass, all the water had to go up into the radiator. If you fit a modern thermostat, it has no sleeve, so the by-pass is open ALL THE TIME. This by-pass is only there to help the engine warm up quickly. If it is left open under normal running, water will take the route of least resistance, and not bother with going through the radiator. It will instead simply go back to the pump inlet and back through the engine uncooled. This will inevitably eventually lead to the engine overheating.


It is easily cured by fitting a blanking plate under the by-pass connection to the thermostat housing. Drilling a small hole in the plate (0.625″) will assist the system to fill easily and release any air-locks. Some of you may have heard of ‘Little Gem’, an MG YB left to the MGCC in a Will. Kimber House now run the car but only after I had to cure its terrible boiling problems. It had no blanking plate on its by-pass.


I hope that is clear as the recommendation not to fit a blanking plate is a very dangerous path to take. If you have no blanking plate fitted with a modern thermostat you may run for ages with no problems, but it is just waiting for that very hot day on a fast run, and once that engine boils you will lose all your coolant and probably seize up solid. Making a little plate only takes five minutes.


Overheating engines can be caused by the radiator’s matrix being blocked up by dead insects, leaves and muck; the water tubes inside the radiator furred up with lime; a faulty thermostat; ignition timing out; to weak a mixture; a water leak (usually from a rusted out core-plug, at the back of the block where it cannot be got at!)


On the pre-TF cars, you will soon know if you overheat as the water will boil and there will be steam everywhere, the cooling system is not pressurised. That of the TF is. When the engine is getting near boiling you can hear the water banging and bubbling in the cylinder-head.


Neil Cairns.