Dodgy thermostat???

  • Ok so a few weeks ago my little mx was getting very hot very quick.
    On investigation i found the coolant was not circulating and suspected a dodgy thermostat. thermostat removed and was completely knackered.
    I ordered a new one from the net and in the meantime have been running the car with no problems.
    finally got round to putting the replacement in today and the old problem is back with a vengeance!! new stat, new coolant mix all topped up with no
    leaks but within a couple of minutes the temp guage was showing above normal and quickly went up from there before i lost my bottle waiting for the fan and switched her off.
    Is it possible I've got a duff stat or could it be an indicator of something more serious??
    theres no sign of the dreaded mayo around the oil filler or dipstick and no visible loss of fluids.
    help please :?

  • Has the sytem been bled? as an air lock could cause that, could you rule out a broken water pump..rad cap holding pressure ok? or you could be really unlucky and have another duff Tstat

  • I agree with dirtyfox, check the system is bled properly and all air pockets purged. when you do this, ensure the heater in the car is set to max, and on, to allow the water coolant to circulate fully. I always take the rad cap off, do the coolant, start the engine, and only put the cap back on as it levels with the top of the filler, then let it run ….

  • hmm sounds like an airlock for sure, start the car from cold with the filler off on top of the engine

    leave it running and the waterpump will circulate the water and air and keep topping it up until enough heat has opened the thermostat

    If there is air in the top of the engine you will get a false reading as both the heat sensors are right next to the filler cap on the top and air gets hotter much faster than water

    Just keep it topped up and when it opens the water level will suddenly drop, you will see bubles coming out for ages, if its starting to over heat it will boil the water and will expand and force its way out, thats when to turn it off and recheck thermostat

    It is perfectly safe to run the engine with the filler cap off on the water system to bleed the air out the system, the reason its pressurised is that water boils and turns to steam at a much higher temperature when pressurised, thats why the car will boil over when a pipe bursts or the cap is not fitted correctly or defective

    to test the thermostat indoors, just boil kettle pour in cup like you would a coffee and drop it in, it will take about 20 seconds for the wax to melt and the thermostat to open = working fine
    Also there are sometimes 2 types of thermostat one runs the engine slightly hotter than the other, dont think there are variations for the MX but you never know you may have a hotter one fitted than previously

  • Thanks for the replies guys. :D
    The car is currently laid up on the drive until I get a chance to work my way through your suggestions hopefully this weekend.
    I will post any results, findings and possibly more pleas for help after :?

  • Ok hands up which numpty didn't bleed the system properly!!!
    I thought i had, honest :oops:
    ran her today with filler cap off as suggested by the posts from the guys and lo and behold some nasty air bubbles did appear :evil:
    once the air stopped coming i topped her up, replaced the cap and ran for about 25-30 mins with temp going no further than just above the middle which is normal for spike. decided not to let it run any more as it will probably run out of petrol!!
    So thanks again for the assist guys, greatly appreciated as ever :D


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