NA Overheating diagnosis

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: _Eunos 1991
  2. I’m based near: _Windsor
  3. I’m looking for technical help or recommendations on: Diagnosing overheating issue.

Car overheated whilst driving on M40 on way to MX5 rally. After letting it cool down on slip road car was fine and didn’t overheat queuing to get into Compton Verney. Car drove the 75 miles home with absolutely no problems (once i had topped off the coolant that had boiled out!). Never had any issues with the car despite thousands of miles of spirited driving.

So took a look today and i need some help to pinpoint what to do next. Here’s what i checked:

The good bits:

  1. Fan works when grounding through diagnostic port
  2. Fan kicks in when grounding thermostatic switch
  3. Fan kicks in no problem after about 20 minutes of the car idling
  4. When turning the AC on (compressor has been removed though) and the heater, both fans turn on (as expected)

Not good bits

  1. No hot air coming out of heater matrix though hoses coming in/out of bulkhead are hot (not sure if this is related or just a flap issue)
  2. When i did the idling fan test, the radiator was stone cold - like 22celcius cold … the top radiator hose was about 80 ish degrees hot which i can’t understand
  3. The bottom hose was probably only about 40 Celsius - sure that’s not right

i am thinking that the radiator is blocked or its a thermostat issue? but how can it be the thermostat if the top hose is really hot?

any help greatly appreciated

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Update … took the thermostat and ran the car upto temperature with no thermostat … bottom hose gets hot now … must be the thermostat … cap is also steaming so will change that too … as for the heater box, i think its the cable and will have a look after sorting this first problem out

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All sorted now. Thought i would comment on my own thread so its here for anyone else in the future. First of all the heat not coming into the car was because the cable had slipped off the flap (easily fixed) and was not related to car overheating.

As for the overheating - a mechanic friend told me i was also better off changing the radiator in case that was an issue due to rust and reduced flow (coolant was a rusty colour).

Steps taken (i took the thermostat out of the housing for all these following steps) … i did this as without a thermostat the cooling is certain to flow around the whole system regardless of engine temp .

  1. Drain old coolant through drain valve in rad
  2. Removed radiator
  3. Ran hosepipe through top hose until water came out clean out of bottom hose
  4. Swapped fans from old radiator to new
  5. swapped radiator mounts from old to new (see youtube for this and remember you will need a couple of “e clips”)
  6. Refitted radiator
  7. Add distilled water and ran engine to check for leaks (and to flush it some more)
  8. Added holts cooling system flush as per instructions
  9. Left car overnight to cool down
  10. Drained cooling system again using drain plug

I then re-added the thermostat and new gasket

  1. filled system with coolant (G30 spec from Halfords)
  2. started car
  3. check thermostat worked (it did)
  4. once thermostat opened I topped off with more coolant
  5. kept squeezing pipes etc to get air out
  6. Turned heating on full blast (not sure if this does anything coolant wise as its always flowing through the heater matrix
  7. checked fans cut in etc
  8. left it overnight and checked for leaks/coolant level in morning
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