XPAG Overheat – an unexpected but common cause

Engine overheat is a common problem within our T-series cars. It has been discussed on many forums and there is a logical decision tree to go through to help trouble shoot. However, it is not uncommon for the problem to remain after exhausting the list of common sense things to check. First, let us review the obvious and then reveal the not so obvious solution.

Radiator – Flush your radiator and block periodically. If you have your radiator off, you can check if your radiator is plugged by filling with a garden hose and see if the flow in from the top is equal to the flow out at the bottom. If blocked, have it rodded out by a professional. Proper cooling fluid is mandatory and the additive “Water Wetter” may also help.

Water Pump – Understand that the water pump pulls cooler water from the bottom of the radiator and forces it into the block to circulate and cool the block and head. Then the hotter water exits the head through the front water outlet, up through the thermostat, and on into the top of the radiator to recycle and cool down through the radiator to repeat the flow. If pump is bad there will be no flow, but this is not the common problem.

Thermostat – The thermostat is closed at start so the engine can warm up to operating temp. While it is closed, no water goes to the radiator top tank. Instead the water coming out of the head returns to the engine block via the bypass hose and into branch pipe to the water pump and then block. This cycle continues until the engine is warm and then thermostat opens allowing hot water to go to the radiator top tank to cool. There are 2 scenarios involving the thermostat to be the problem. First, it could be plugged or frozen. If so, water will never get to the radiator and over heat will occur. Second, overheat could be caused by the bypass port being full open and thus the water is bypassing the radiator. The outlet on the side of the thermostat housing should be partially blocked, which will deny full flow of water to bypass cooling and return to the block even with the thermostat open. Original thermostat housings automatically blocked this port when thermostat opened. Modern replacements have this port partially blocked for same purpose.

Fan blades – Fan blades can be installed incorrectly. Install originals with the rear blade having the offset holes and the reinforcements facing forward. A common change for more cooling is to install the 7-blade fan from the MGB. This will give more air flow and may help. However, if you have an overheat problem you should find the source of the problem and not mask the problem.

Block – Engines that have been stored for a long period tend to be full of corrosion. You may be able to chemical flush the block while in the car but the best way is to clean during engine rebuild.

Carbs – If the carbs are adjusted too lean this will cause the engine to run hotter. Making them a little richer will help your engine to run cooler. However, check your plugs first. If they are dark with carbon, your carbs are already on the rich side and the carbs are not the cause of overheat.

So, the above are all the commons things to check. While these topics are all common sense there is often a root cause that is not on the list – the distributor.

Distributor – Engine timing can greatly affect the operating temp of the engine and a worn distributor can have a serious impact. The engine is normally timed at idle at 0 degrees TDC. However, with modern fuels you should set the timing on the advanced side.

But what happens with a worn distributor at high rpm? If the shaft and bushing have end play the dizzy gear will ride to a higher contact point on the cam gear. This retards your timing at high speed and can cause over heat. Your advance weights and springs can also be the culprit and add to the problem. There is a simple check you can do for your dizzy. Remove the cap and rotor and pull on the rotor shaft to see if you have any end play (up & down movement). If you do, have your dizzy rebuilt or replaced. Although this is the last item in this article, consider this one of the first things to check. The dizzy’s are 60+ years old and are tired and need attention.

FTFU will be glad to assist trouble shooting your over heat issue. We also offer every one of the above discussed items to include rebuilt distributors. Don’t run it hot, stay cool.

Doug Pelton

480-588-8185 www.FromTheFrameUp.com


2 thoughts on “XPAG Overheat – an unexpected but common cause

  1. Dughall Leask says:

    I have recent personal experience with a 1250 TF which suffered an internal failure of the distributor – causing dramatic retarding of the ignition and subsequent excessive overheating. Subsequent to the overheating of the engine, the modified rear crankshaft seal (fitted with the original type oil seal) suffered ‘relaxation’ of the lip on the seal and began to pass copious volumes of sump oil. This brings two current discussion matters together!
    So there appears to be a motto – Do not let engines fitted with modified rear seals be allowed to overheat.
    NB The engine is being re-assembled with the new narrow VITON seal.

  2. Nick Proferes says:

    I live in a hot (40C in summer) climate in Australia and have owned T-Types since the early ’70’s. Having worked as an engineer in industry and in refrigeration design, I got involved looking into coolants used in manufacturing processes. Modern automotive coolants are essential to reduce the risk of overheating (in addition to all those issues mentioned in the article). They do several things, improve the thermal conductivity between coolant (which carries the heat from engine to radiator) and block and then between coolant and radiator. When overheated, they foam thus maintaining a small measure of heat extraction from the engine and reducing the risk of damage. They also contain an anti corrosion agent to inhibit corrosion throughout the system. Most are based on ethylene glycol (though propylene glycol can be used) with a corrosion inhibitor such as sodium benzoate. The corrosion inhibitor will degrade over time in doing its job so it is important to replace coolant in line with manufacturer’s recommendations but I currently use a coolant which has a 3 year life in the engine. These can be purchased pre-mixed or in concentrate form which must be mixed in line with recommendations with distilled or de-mineralized water. The mix is designed to give optimum balance between boiling point and thermal conductivity.

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