Overheating diagnosis and bleeding procedure

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 In MGF, MGF Engine

Overheating in the MGF engine has the same range of causes as any other engine. It is often read as being a failed head gasket, but the most common cause of overheating is the coolant ceasing to circulate due to air entering the system.   In this respect the F is more vulnerable than other installations as there is quite a long cooling tract for the pump to circulate so any air makes a hiccup more likely.

Gasket failures associated with overheating are nearly always a response to overheating rather than the cause, so I suggest the following procedure to determine whether the head should be removed – and if not, what alternative action to take.

Diagnosis

  • Check the oil for water contamination – if yes, then head gasket.
  • Check the coolant for oil contamination – if significant, then head gasket, if odd drop or 2 then I suggest you leave & monitor.

If the reason for suspect head gasket failure is just that the car has overheated, and neither of the above symptoms are present, then gasket failure is unlikely.  Check the cooling system for leaks (see below) then refill with coolant, bleed properly (also see below) and continue to use the car, monitoring for any repeat overheating incidents.

If the cooling system has had substantial oil contamination then it will need a through clean out, including the heater.

Gasket failure

If the head gasket has failed, it is essential to check the engine for damage from any related overheating, in particular

  • Check the head for hardness as a soft head will collapse around the fire rings leading to another failure.
  • The liners should be checked to see that they have enough protrusion (0.003”) above the block as overheating can also lead to the block collapsing at the liner seats.

(The overheating necessary to cause either of these is major, such as continuing to drive the car for a significant distance with the coolant having dumped.)

Minor-moderate softening of the head can usually mitigated by the use of a head saver shim which spreads the load from the fire rings.  These shims are included with the multi layer gasket or available separately for the original style of gasket.

Cooling system checks

The age that these cars are now is such that cooling system hoses will be approaching the end of their service life and could fail.

Also vulnerable are the steel coolant pipes – one set under the car going to & from the radiator, the other runs from the thermostat (behind the water pump) around the back of the engine.  If the coolant hasn’t been kept fresh it will lose its anti-corrosive properties, or worse still, straight water is used.  These scenarios are often the case in cars imported second hand from Japan.   Either of them lead to internal corrosion and failure of these steel pipes.  The most common place for the one round the back of the engine to fail is where it joins the thermostat.

Bleeding the cooling system

The inverted nature of the MGF cooling system (radiator at front, pipes under the car) means that it traps air in it and will require bleeding to get the air out.

To bleed, simply release the bleed screw until air ceases to come out and only coolant is discharging.

There are 3 bleeding points:

  • At the top left (passenger’s side) of the radiator
  • On the coolant pipe around the back of the engine
  • In the heater circuit.

Important!  The radiator tank and bleed screw are plastic and easy to break! The seal between the two is by o-ring, so excessive force is not required.

If the heater hasn’t been drained, then it is usually unnecessary to bleed it.

If you can get the car on a steep hill or jack the rear of it up a couple of feet, it is usually unnecessary to bleed the coolant pipe around the back of the engine. Assume you don’t need to bleed that point then monitor the car for any temperature irregularities after start-up.   (Note:  It is not uncommon for the temperature to fluctuate for a short time them settle back to normal.)

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