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Hello all,

After searching through some other posts about overheating issues I still cant out my finger on my specific problem.

I have an overheating problem when driving for over 25 to 30 minutes. I drive it everyday and the commute is about 20 minutes in duration with no issues but if I have to take a different route because of a crash then the car overheats anything over approximately 25 minutes.

It does not move past 3/4 point on the gauge and it doesn't not matter if I am sitting on 100-110 or 60 through back streets it will still get warm. However if I pull over and let it idle down the fans cool it down over a 1- 2 minute period. If I stop at traffic lights and take off with a heavy foot then the temp falls right down to just below half (where it usually runs) or if I pull over and rev it to about 3500rpm the temp will drop almost instantly back to normal operating temp.

Everything has been replaced in the last 500kms. I bought the car only recently after it had been sitting in a backyard for 2 years so as precaution I replaced all hoses, radiator, water pump and thermostat. The cooling system has been flushed about 5 times until i have got nothing but nice clear water and it is now running premix coolant and I am confident all the air is out of the system. It doesn't use any coolant.

Things I have done to troubleshoot so far:
- Run until it reads warm, pull over and check engine is actually getting warm. I have not measured the temp but it certainly feels warmer than what it should be. (perhaps a way to measure actual temps from current temp senders?)
- Used a coolant pressure tester to check for potential leaks. No leaks and it held pressure for over an hour and did not budge.

Before I go any further with pulling things apart again I thought I would ask and see if anyone could tell me where the best places are to look.

Thanks for your time.

If it were me, I’d be investigating the new thermostat. I’m not suggesting it’s not opening at all, or opening at the wrong temp, but rather that it’s not opening fully, restricting flow more than it should. I’ve read they can be faulty out of the box - did you happen to test it before installation?

water pump

I didn't check the thermostat function before fitting as I have never had a problem with them in the past. I will start with removing the thermostat and checking it (possibly run without it as i do live in QLD).

The water pump is brand new so the fins wouldn't be warn enough to not flow the coolant through. How else would a water pump fail to cause overheating after a half hour drive?

Thanks for the advice I will check out the thermostat and post back the results.

Is the OEM cooling tray installed?

Yes. OEM cooling tray is installed. I removed the thermostat and tested it. It seemed to open up but not what i would consider to be fully open. I read somewhere in the past about gutting the internals to just leave the cage so I am giving that a go. initial drive after fitting it back in seemed to go well with temp rising quickly and holding nice. (was only 10 min drive) yet to do a longer drive to see how it goes.

I came across this thread just now and can see it's a little outdated; how did the next drive go? Did you end up finishing the diagnostic?

project.r.racing wrote:water pump

This.

All the stuff about undertrays etc wont make that much difference unless your cooling system already has issues.

While the cooling system in a stock MX5 isnt exactly the most beefy thing in the world, it's certainly not the fragile delicate thing that people seem to make it out to be.



But yeah, my method would be:

1 - Coolant level, is there plenty? (check radiator, not just the overflow bottle)
2 - Thermostat, dont even bother checking it, just buy a new one, they're cheap.
3 - water pump, probably ok since you said it's new-ish
4 - radiator, might have a blockage or something restricting flow through parts, which would make it less able to remove heat, regardless of airflow.


Try a cooling system flush, usually it wont actually remove a rad blockage, but it's worth doing anyway.
The proper way is to remove the radiator and have a shop remove the tanks and blast the inside clean. But frankly, these days MX5 rads are so cheap you may as well replace it.
You can use a heat gun to check temps across the rad and see if there's a hot area where it's not flowing.

If its an alloy radiator, just spit on the top tank.

If its genuinely overheating the spit will start to simmer in just a few seconds.

Generally instant boil=125
2-3 seconds=110
10 seconds plus= 100-105c.

Serious.

Dann

You're probably aware of this already: Under normal operation, the dummy CLT gauge hovers around the 1/3 mark. If it gets too hot and goes critical it stops at the 2/3 mark, at which time you're already past the danger point and should try and STOP driving before getting to this point.

Your primary fan should already be going flat out if your gauge is at the 2/3 point - if it's not then you have a problem with the fan (electrical or the ECU is not activating when it hits the trigger temperature).
ie. Check if your fan is running when it gets hot.

If you have an A/C, crank that up to activate the A/C fan for extra cooling.
If that still doesn't do anything then it's either the water pump or you have some blockage in your coolant system because coolant flow is the issue.

Lokiel wrote:You're probably aware of this already: Under normal operation, the dummy CLT gauge hovers around the 1/3 mark. If it gets too hot and goes critical it stops at the 2/3 mark, at which time you're already past the danger point and should try and STOP driving before getting to this point..

Can confirm that an NB8A gauge wont stop at 2/3
it will happily peg the needle against the stopper.

Research system burping including the heater core.
Probably not the problem but you never know.

Sorry to leave this dead for so long but I never got to the bottom of it. Things that I have been through so far:
- thermostat has been gutted
- new radiator
- new water pump
- new coolant hoses
- it has now been flushed 6 times, twice with a radiator flush and it is now coming out very clean

So I took it for drive a couple days ago over nebo/glorious and back, about 3 to 3.5 hour round trip by the time I was back home and the needle did not move at all. But driving around today just commuting and within 30 to 40 minutes it started to rise. I pull over to let it cool (I can hear the water overflowing into the expansion tank) and then I can keep driving without it moving. This is mostly highway driving at 100kph. Other times I can pull over and let it settle for 10 minutes and it will come back up within 5 minutes of driving. This is not all the time though. i have a 25 minute commute to work everyday and it does not move at all.

Could it be something more serious, cracked head possibly? and any more ideas to troubleshoot it? the only thing I can think of next is to take the head off and make sure everything is clear. Work has a thermal image gun so I will use that tomorrow to try and find hot spots.

Thanks or the help so far, much appreciated

I think you need to get an actual read out on the temperature, not just the OEM gauge which can be vague. It could be operating at normal temperature and you're thinking it's overheating. Have you tried Dann's spit test? While not exactly conclusive it might give you some guidance.

I don't think its overheating. You've dríven 3.5 hours without problems, so there is no major issues with the engine or the cooling system.

3/4 point on the gauge is warmer than normal, but usually the gauge has to reach H before its too hot (check the owners manual).
The temperature on the gauge seems to fluctuate, so it may be worthwhile testing the temperature gauge sender unit, & replace if required. Have the engine coolant temperature sensor checked at the same time (different to the gauge sender).

I wouldn't leave the gutted thermostat in. Most engine wear occurs when an engine is cold, & it takes a lot longer to get to operating temp without a closed thermostat.