You could also try a Bluetooth OBD reader (I've got a small one í bought via eBay) with the torque all on an android phone, this will show you the actual temperature and will graph too, my reader is permanently in place so I can clear any faults that appear when driving!
Yeah I've had it on a bluetooth OBD reader borrowed from a friend, temp was bit lower than what the gauge said (though now I know the gauge is just a representation), but still went up to 118/120. Seemed to confirm that my stat was sticking "shut", coolant been kept from passing to the radiator until too late, letting the engine build up temp then suddenly releasing under vibration or a higher temperature. However, think it was a combination of problems, see below.
sounds like stat not opening early enough, or electrics as it seems to be very,very consistent in terms of the time it takes to happen, temp sensor is a good place to start, i'd replace that and the stat first...
Well I had a successful weekend, managed to borrow a garage and sod's law: it didn't rain! Changed the stat, new temp sensor and new fan resistor and relays. I've got a rock-steady gauge at 90, and now I know my low speed fan wasn't working at all, the old resistor was knackered when I removed it, it just fell apart in my hand! The low speed fan now cycles on and off as it should. Old stat looked clean inside with no gunk, but the temp sensor was very furry however, lots of oxidisation.
The thermostat went in fine, getting to it was a right pain in the arse though, trying to get the air intake out. My back isn't feeling too great this morning either, pulled something and its agony!
I used a little sealant on the block face after giving it a light clean with some 6000 grade micromesh wet and dry, no leaks, and I love how the system self bleeds, so easy compared to the older cars I'm used too. The hardest part was having the confidence to chip off those "clip-r" (?) jubilee clips, it doesn't feel right at all smashing them off. Nice to see lots of new shiny jubilee clips on there, very satisfying. Top hose is now nice and supple. I'm getting an extra 2 mpg which is good.
One door opens and another shuts though, I cleaned the MAF, throttle body and unplugged the HT leads and gave a squirt of contact cleaner in there. I didn't touch the butterfly just sprayed some Winn's cleaner through it whilst revving it, and since reconnecting the MAF, I'm getting the dreaded MCSF warning. I've reset the ECU and the throttle, I'll try and check the codes tomorrow and maybe clear it. I'm hoping its just excess cleaner on the sensor and it'll disappear soon.
My rocker cover gasket is leaking badly though, when parked nose down a hill, I can see it dripping out from the front of the cover. When I had the cam belt done a while back the garage said it had an oil leak, but couldn't tell me where from, seems fairly obvious to me now! Must be effecting the breathing system too as the coil pack area on top of the cover is soaked in oil, I'll take it all out and clean the plugs and HT leads when I do the gasket, can't be helping at all, might be contributing to the MCSF warning too.
Does anyone know the sizes for the rocker cover bolts? This is my main and only car so I could do with getting some in advance, just in case they shear off when I nip them back up.
Thanks for all the advice everyone, this forum is fantastic.