Hi there, I recently bought an Eunos Roadster NA 1.8 and its got an overheating issue.
The coolant system/heater core has been flushed out multiple times until the water runs clear
The waterpump was replaced 8000km ago and i have checked to confirm it was actually replaced
I bought a new genuine thermostat that has the main and secondary values plus the little flapping mental thing ( I tested it in boiling kettle water - it didn’t open, I then tested it in actually bubbling boiling water in a pan and it opened)
I’m not sure if the radiator is OK but it looks fine and the plastic tops are still black (which means it cant be the original because they turn brown after a while?)
The wire to the temp gauge sensor at the back of the engine is intake and not worn through
The cooling fan works but currently it doesn’t work automatically, one of the PO’s has ridged up a manual interior switch which works
I changed the oil shortly after getting the car and the oil was just oil no signs of coolant mixing so the head gasket should be OK
With the new thermostat installed in the correct orientation, the temp quickly rises passed the half way mark from a cold start in around 5mins until I have to turn it off when it reaches around the 3/4 mark on the gauge, running the cooling fan constantly doesn’t stop the temps from rising either. The coolant just doesn’t circulate at all by the time the gauge is at 3/4 for some reason. The top and bottom hoses and water in the radiator are cool even passed the 1/2 mark on the temp gauge?! You can feel the top hose is empty with no flow coming through it, looking in the radiator cap you don’t see any flow either. I had the car jacked up at the front and was squeezing the top hose to try and get any air out also. Its a new genuine thermostat though how can it be that faulty?
After doing the above I thought I would remove the thermostat completely and run it without one, this was to test the water-pump as it should then be able to see movement through the radiator cap and to in a way check if the gauge was just getting a wrong reading. After starting it up with no thermostat and the cooling fan manually switched off the temp rose to the 1/2 mark but it took longer I then turn the cooling fan on manually (I am going to remove the switch and getting the fan working automatically soon) the temps then quickly dropped below the 1/2 by a good amount and stayed there fine. Looking in the radiator cap now you could clearly see the coolant flowing from the top hose so the water-pumps fine then?
Whats going on here? Its seems like the new thermostat I bought is faulty but that seems very unlikely. Am I missing something here?
Buy a Mazda thermostat there are reports of non Mazda ones sometimes being the wrong specification.
If that does not fix it the next culprit is the radiator on the MX5 as they have very narrow waterways.
Your external inspection cannot cover the possibility of blocked internal tubes.
One final one is that some water pumps come without a gasket and users use gasket cement instead. This does not always lead to a happy life ever after.
I already have a genuine thermostat installed that I bought from mx5parts, I have read that the passageways in the radiators are prone to getting blocked easily but water doesn’t even flow through the top hose to start circulating through the radiator. The thermostat just doesn’t open and the gauge can be reading 3/4.
I would check out the thermostat first. Do not put the thermostat in boiling water but put it in when the water is cold and heat it up. If it has a temperature stamp on it and you have a suitable thermometer you could see if it starts to open at the correct temperature. It should open before you get 100 degrees centigrade.
If it is good I would suspect an air lock. Try back filling the system by taking the top hose off the radiator and rotate upwards so it is above the highest point in the system, then very slowly fill the system through this. Leave the cap off so the air can escape. Once filled quickly put the hose back onto the radiator, a bit messy but worth a try. Much squeezing of the hoses will also help the air to be eased from the system.
Let’s start again I did not read you had a new Mazda Thermostat.
As advised above test it again in hot water with a thermometer that can read up to 100C.
It should start to open around 86C or so and be fully open at 100C.
Therefore you need to heat the water out the hot tap in a pan and watch the thermometer to check when the thermostat starts to open and is fully open.
If it does not start to open around 86C or there about, you have a faulty thermostat.
Now the next possible problem that Malc has identified.
With a working thermostat fitted.
The system is supposed to take 6.3 litres or so of coolant, it takes a bit less than that.
When drained but the heater still has water in it, the system will take about 4 litres of coolant with the coolant at the radiator neck.
Have at least 1.5 litres of coolant at hand not a distance away and start the engine with the radiator cap off, heater control to hot.
Do not assume anything just stand there and watch the level it will go down a bit and tweek both the top and bottom hose but watch for the fan and put in fluid, after a good while the thermostat opens and the level just drops like a stone, use the coolant to fill the system again.
You appear either to be not waiting that long to get the large drop in level or you have a faulty thermostat that you as yet have not checked with a thermometer or there is crap in the thermostat cover gasket that does not allow the plunger in the thermostat to lift or maybe the wrong thermostat gasket that is getting in the way.
Thanks for the help so far guys, the thermostat housing is clean enough no excessive build up and the gasket is oem as well. The thermostat has 85/88 stamped on it, ill go get a thermometer and put it in a pan of cold water and heat it on the stove. It should open around 85? If its OK I try back filling the system has suggested.
Right just finished testing the thermostat using a thermometer. It started to open at 92 degrees and was fully open at 100 degrees. Mad Malc i’m just wondering using your method to back fill the system how would the water get passed the closed cold thermostat when filling it from the top rad hose? The only hole would be the tiny giggly thing on the thermostat?
You have spotted the reason I said ‘slowly’. It takes ages and is a last resort if you have issues with air locks. All carried over from Caterham 7 ownership where air locks were sometimes an issue.
Have you tested the original thermostat? Your new one does seem a little late in starting to open.
‘D’s’ advice as always is on the money. Have another go and follow his advice and I think success beckons.
Its still acting up, I done the back fill method using the top hose to fill the system. After a while it wouldn’t take anymore water and I had only put a couple of liters in. So I just filled it to the top of the radiator using the radiator fill hole. I started it up and within a few minutes it was creeping past the 1/2 mark on the gauge, at this point I turned on the cooling fan and adjusted the fan settings to full speed and full heat to try and keep it cool. It then hovered around just very very slightly past the 3/4 mark on the gauge. Normally at this point Id just turn it off but I left it running after around 8mins (all this time the top hose and bottom hose it cold as well as the water in the radiator) the water level fell quickly so I topped it back up till I could see it in through the fill hole and let it run some more squeezing the hose etc etc. The temp gauge was still at 3/4?! A few minutes passed and then the water would rise up the fill hole neck and overflow then drop back down it done this like 5 times then I gave up and turned it off. The water coming out was cold and the hoses were still cold…
I have heard of radiators full of set sludge that wont flush. A small amount of water has a passage through but an ineffective radiator. Only solution being change radiator.
I gave it another shot, started it up the temp went up to 3/4 on the gauge after 5mins, the engine to the touch on the cam cover is warm at this point, its not where near burning the hand level weird when its reading 3/4?. The cooling fan and interior fans are on the whole time.
After another 5mins the thermostat opens and the coolant drops in the radiator, at which point I topped it back up let it run for another 30secs and put the radiator cap back on. And it will idle at 3/4 on the temp gauge and it doesn’t move. The top and bottom hose and radiator are hot to the touch so it was all circulating. The cam cover wasn’t extremely hot you could hold your hand on it for 5 seconds straight,.
Could the temp sensor that sends the reading to the temp gauge be faulty? It could also be a gummed up radiator because the car was sitting for a while with the previous owner, but that doesn’t explain why the temps at 3/4 very quickly before the thermostat opens up?
What happened - unsurprisingly we do like to hear back?
Clearly you are struggling with fundamentals here as well as a car inherited with existing problems and non standard modifications in the cooling system.
If you have not already done so. I would contact the previous owners of the car to find out why they altered the wiring, etc.
Replace the temperature sensor if you suspect it is faulty.
Check the radiator for flow.
Clearly cold coolant will not open the thermostat or activate the cooling fan. Check the heat at the rear of the engine where the temperature sensor is located.
Given your continuing issues probably best to pay an MX5 specialist to look at this for you.
If you have doubts about the various temperatures it might be an idea to get one of the infra red thermometers. It takes a while practising on different surfaces to learn how to use them for best results, ie where to aim them, but at least you don’t burn fingers and can achieve reasonably accurate results; for example the thermostat housing shell is likely to be the same temperature as the water within, but if polished to a high gloss might give a lower reading than a cruddy dark surface. You can also use it on other places like the brakes and wheel hubs and exhaust manifold pipes for diagnosing different problems.
I have this one, it is cheap and generic, lots of them much the same, and it works
Rhino - I have been fixing other things on the car and thought it would be best to get the car running at a normal temp before fiddling with the cooling fan wiring. Yes it doesn’t cut in and out automatically but it works so I can still in a way simulate the automatic system using the temp gauge and turning the fan on when the gauge reaches 11 o’clock. The cooling fan fails to stop the gauge passing 1/2 anyway so fixing the wiring doesn’t really matter atm, I will go back to it after it stops over heating. The last two owners were older woman so I don’t really see them installing the switch them self. They just took it to a garage if they had any issues going by the receipts I got with the car. It was most likely one of the earlier owners that perhaps used the car on track that installed the switch. I’m not struggling with the fundamentals haha its not the Saturn 5 rocket just an mx5, I have a good idea how it all works, hopefully I have just been mislead by a misreading gauge.
I checked the radiator for flow earlier today, with a running hose inserted into the inlet of the radiator and it ran straight out the outlet. There was no sign of the water backing up inside.
I haven’t ever tried to feel if the part where the temperature sensor and sender is located is hot, I could try that. Relating to the modified cooling wiring, I did unplug the two wires from the fuse box and put the stock cooling fan relay in, but the temp passed 1/2 on the gauge and the fan did not turn on, so I suspected it could be the temp sensor or something else. But if the gauge was reading wrong I could have turned the car off before the temp sensor read high enough to turn the fan on.
Richard - Thats looks useful, it would work great on the thermostat housing for seeing what temp the thermostat opens like you said. Thanks
Possibly nothing to do with it, but on another old Japanese car I have worked on, failure of the cooling fan made the temp gauge immediately go to HOT regardless of the actual engine temp. No idea if the MX5 has this feature.
I still think you have an air lock. Without being there it is difficult to judge but are you giving it long enough to heat up enough for the thermostat to open. There should be no need to put the fan on till way after the thermostat opens.
Now it has had chance to settle have another go, otherwise you need an experienced spanner man to help you.
To recap, if the water flows without a thermostat in then the system is good. Your new thermostat does open before 100c so is working. (Yours appears a little ‘lazy’) The only answer is either you are not getting it hot enough (Lower outside temperatures in winter does mean it will take some time) or you have an air lock.
Another last resort is to jack the front of the car as high as you can so you raise the filling point.
Please let us know how you resolve this, there is no ‘disgrace’ in resorting to the ‘professionals’, good luck.
I got a new thermoswitch and temp sender for the gauge. The cooling fan after the new thermoswitch now turns on and off automatically, the car was started from cold and in 3 minutes (I timed it) the gauge was at 3/4 as usual… so the temp sender didn’t change anything. The thermostat opened a while later and as the water fell in the rad and I was quickly topping it up the cooling fan turned on and then turned back off a minute later. I then left the car idling for a further 10mins and the cooling fan switched on and off a few times. How can the gauge read almost H when the automatic fan isn’t even on which should mean the temp is under 97 degrees. I’m going to have a look at the gauge tomorrow, maybe its been tampered with.
Ok the 1.8 cars have a thermosensor that switches on and off the fan via a relay and it also changes the mixture and ignition timing to suit the water temperature.
These cars also have a separate thermosensor below the coil packs to feed the temperature guage.
You have changed both and it is interesting that the fan is now cutting on and off that suggests there was a problem with that sensor in the past which may or may not have caused overheating.
You are now saying that the guage is still reading high.
There are three possible reasons for this either you have a faulty guage or there is a blockage in the cooling system in the cylinder head upper block area or there is still air in there.
You say with no thermormetestat the guage reads OK.
Now I still think there is some air in there but I also think you may have a blockage. Was the heater set to hot and do you get heat out the heater with the blower on.
Have you lifted the passenger side carpet to check for dampness rust on the ECU connections.