Hi, and thanks for having a look,
I have a 94 Import ‘S’ Special 1.8 Mk1, which has been absolutely awesome fun upto the last month…the short version is I was driving home on the motorway when a huge cloud of steam appeared, temperature almost immediately went into the red and I pulled over to find the top of the radiator had blown off !
Cue a new radiator fitted by me and a satisfactory test drive with temp gauge normal, so I drive home from work and the car overheated after approx 30 mins at 70mph and only cooled down with heating on full and speed approx 60…also the fan didn’t appear to be working. As it is my only car and I had to get to work the next morning I drove back with heating on full etc, quite happily until I got about 40mins into my journey and the car suddenly overheated, clouds of steam etc and total coolant loss from the water pump. Cue tow off the motorway and a lengthy sojourn in a garage. New water pump and thermostat fitted and car appeared to work again. Then drove home and again approx 40mins into the journey, the car very suddenly overheated…but kept the coolant this time…again with no fan working.
I took it into an auto electrician who measured the temp of the radiator/engine block, managed to get the fan working by wiggling a few wires and described it as perfectly normal just a loose connection somewhere on the fan. Subsequently appeared to be ok again and drove to work with minor overheating which rapidly turned into terminal overheating as I drove up a hill at 70…lots of coolant lost and steam etc.
So…the sum is new radiator, water pump and thermostat…apparently the fuse for the fan is working so my only port of call is perhaps the fan relay ? or the supposedly dodgy wiring…although I believe the fan is only really for lower speeds, so why it would overheat at 70 on the motorway is beyond me. The radiator is a pattern one, with a pattern cap, although it does appear to hold the pressure so I am not neccessarily sure a Mazda cap could be the answer.
The mechanic who looked at the car said he thought the head gasket could be on its way out with the overheating…so I am very worried about that aswell, but having had a similar experience with a Porsche I had a good shufty in the engine oil and theres no mayo anywhere, it appears normalish and the car does seem to run perfectly normal (although I’m totally paranoid about it now!).
Having spent approx £600 on it this month and having 3 mouths to feed (plus my own!) I can’t really afford to leave it laying around garages various and I have exhausted my limited experience of the Haynes manual…any help gratefully received.
theres a temperature sensor / switch ( thermoswitch) at the back of the head under the coilpack
these heat up and tell the fan when to turn on
if this isnt working then the car will overheat. - get it checked out
I thought the fan was triggered by the wire on the thermostat housing.
I fitted one of these pattern radiators; the radiator cap that comes with these is junk; is the expansion tank filling with water? I fitted a new genuine Mazda cap (£15!).
I had a problem with a 1.6 with the fan running at speed; the issue was a blocked thermostat housing bypass pipe. Of course, after fixing this, I also had to fit a new fan, as the old one had managed to wear itself out.
If you’ve got a new radiator, water pump and thermostat then the problem almost certainly has to be the head gasket.
There’s no way the fan needs to be running at 70mph - in fact if it was hotwired to be running at that sped then there’s a good chance that it would actually stall the airflow through the radiator.
You don’t always get oil mayo - it only takes a small exhaust gas leak across one of the gasket rings into the coolant to create overheating like that. It also creates pressure in the cooling system which could be why the rad blew in the first place.
If the engine still runs ok, get the cooling system pressure checked and “sniffed” for hydrocarbons - that will help diagnose the problem, but in all honesty if you had a number of bad overheats then you really ought to have the head off - and checked for distortion - before reassembly with all new gaskets.
on a 1.6 , yes the switch is ontop of the thermostat housing
on a 1.8 the switch is at the back of the head and is a 2 wire switch which sends info to the ecu and then tells the fan to kick in @203 F
Thanks for all the replies guys, yes the thermoswitch thing is under the coil pack at the back with no electrical connections on the Thermostat. I did a little investigation this afternoon and managed to get the fan working by shorting the switch so it would appear the thermoswitch or wiring is at fault.
Obviously that won’t solve the overheating, however I had a little butchers at the heater pipes at the rear and the LH one (as you look at the engine) appears extremely soft compared to the RH (same circumference) and it also appears to be leaking from somewhere ! Given the fact that the rear of the engine is about the most inaccessible place in the entire world I couldn’t dig any further without my tools and probably some trained help…is there any way of removing the thermoswitch without taking the engine out ?
Finally…I couldn’t see it anywhere, but I felt all over the rear of the coil pack and couldn’t find the little ‘plug’ thing…maybe it is there and maybe it blew off when the system overpressured…again I can’t really see how I can investigate it any further without taking the engine out ??
Thanks for the reply about the head gasket…there is a small oil leak from the rear third of the head…but it is a small one and has been there since I’ve owned the car. I am in the RAC so may call them out to try the ‘sniffer’ test but I don’t think I’ll be getting the head skimmed etc, I’ve spent a small fortune on it already and I could go crazy when it’s still working well, basically I’m just looking to get it working properly again as cheap as possible.
Now fixed !
I replaced the coolant temperature sensor (an absolute b*gger of a job to do…but removing the coil pack made it possible) and the two heater pipes. As described previously the LH one had deteriorated and had a split near to the clip (where it attaches onto the engine) which was impossible to see and only the softness of the hose compared to the RH heater hose gave away the game. I replaced both hoses and the sensor in one evening, which took me about 2 hours of reasonably slow labour and have just driven it for 2 hours with no problems.
I can only assume that as the coolant heated up (always about 30mins into a journey and around 70mph/up hills) the pipe expanded and the split allowed the coolant to leak out until I slowed down to 60 and the pipe gradually contracted, sealing the leak. There was nothing leaking at standstill hence the difficulty in fixing it in the first place…just a shame it took me a month to finally find (and was also missed by the garage that ‘repaired’ it in the first place!)
Anyway, thanks for the help and I hope this explanation helps anyone else with a similar situation…I appreciate it is a fairly obvious solution, but it was extremely difficult to find !
I know this thread is old, but to just add my 2 penneth worth…
My MX 1.6 91 Eunos suffered a total coolant loss on the motorway the other day. I noticed a misfire, with a cloud of smoke to the rear, as I tanked it Southbound. Didnt think to look at the temp gauge as I’d noticed the ignition leads were starting to break down previously, and had suffered power loss from breaking down leads before. So, looking for the exit I glanced at the temp and it was OFF THE SCALE !!! Time to panic and head for an exit, before motorway towing charges entered my mind…
Pulled over, after I nursed the car off the motorway… and as I stopped, a whoosh of stinking steam, thought it was smoke and the car was on fire, bellowed from the edges of the bonnet. I jumped out and lifted the bonnet to find rusty watter all over the engine bay. The L/H heater hose had split, gushing the coolant over the engine, shorting out the leads, giving it the missfire, a
d the overheating.
Called MX5 Parts and ordered a full red hose set… well, if its got to be done, do it bling… The chap said the L/H heater hose is the one that goes the most. So if you have an old MX5, replace these two heater hoses if they are the slightest bit iffy…
Only worry now is that the head gasket might be shafted, so its a hose change, a new set of leads, and fingers crossed the engine isnt shafted…
My red silicone hoses from MX5Parts arrived, and were fitted… they look lovely, and the car seems to be ok… 
After fitting new rear arb bushes, links and rear shocks on Saturday morning, I went for a short test drive, only to see the temp gauge begin to move above the normal central positon. I quickly switched off and coasted to park at the side of the street. It needed almost 2 litres of water to fill the radiator again! No obvious leaks anywhere or steam escaping.
So I limped home and parked in the garage - a short time later, I found a small puddle of water beneath the very rear of the engine, in line with the bulkhead.
Had a good feel around the rear heater hoses and discovered that the one that goes right behind the engine was as spongy as an extremely spongy thing and there was evidence of some water by its clip.
I’m kicking myself that I didnt just replace these when I had the CAS out and cam cover off to do the ‘o’ ring and gasket. On visual inspection, they look OK, but the section of hose that has evidentl;y been exposed to oil fron a leaky cam cover/CAS over the years is completely trashed and bears no resemblance to the other half of the same hose that feels as firm as new!
So don’t delay, change those heater hoses today!!
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