overheating engine...???

[MX5Ron]

Hello, the temp gauge on my mx5 is normally around the 1/2 way mark. Yesterday it went to 3/4 on the gauge and when i checked coolant there was nothing in expansion tank but rad was full. I topped up and drove for about 20 mins and checked again. gauge had moved a little up and coolant level was over the full mark and near top of tank. Today i drained the cooling system, replaced the thermostat and refilled system. Ran car at idle speed until all bubbles were gone from radiator and the thermostat opened. Let the engine cool and added small amount of coolant to expansion tank. Temp gauge was at 1/2 mark. Drove car for 20 mins at 80km/hour. Temperature nearly went to hot mark on gauge, stopped and coolant was coming from overflow on expansion tank.after 10 mins it returned to normal level and got car home before gauge went up again. Its parked for 2 hours and coolant is 1/2 way between high and low marks. don't want to drive it in case i damage it. Any ideas what problem could be?

thanks

[Steve]

Welcome to the forum!

Easy stuff :
could be blocked rad (disconnect upper and lower hoses, put a garden hose in and run it till the water coming out is clear, backflush if you can),
could be the fuse/wiring/motor on your fan has blown (if you have aircon, turn it and the 2nd fan on the rad will come on, see if temp comes down),
could be the temp sensor/wiring/connector for the fan is duff (it takes it from the temp sensor at the front of the engine, the rear one is for the ECU),
- see if your fan comes on if you ground the connector,
could be the fan relay, it should be sitting beside driver's side headlight (if there is two, one is for the aircon), give it a tap and see if the fan now works,
could be your rad cap isn't working properly, change it,
could be the thermostat housing to waterpump line is clogged (its the one with the righthand bend in it , clean it out).

Not so easy :
Could be the waterpump.. although it should weep a bit before it goes totally.
or .... could be head gasket but thats rare enough.

[leamyj]

My money is on the rad being gunked up, been there before myself!!

John