engine temp reading up and down

[Drax]

this is not my lude, but the wife's old BB4 (now owned by my mate Will) - nipped to bri's in it yesterday, and on the way home after a 10 min 'enthusiastic' drive to a nearby village with Bri i noticed the engine temp reading was up and down between cold and normal, never went above normal but kept going up and down then stayed "cold" all the way home after that. i could still vtec ok, and the engine seemed its normal self.

all i have done to it recently is clean the IACV, throttle body and egr valve, then topped up the coolant, bled it briefly to make sure of no air in system, then at Bri's he kindly checked and adjusted the timing for me.

seems a bit of an odd one, im going to disconnect the battery in a min for an hour to reset the ecu, then take it for a drive an see if it does it again, but any suggestions on what i may have done/not done are welcome

[FrontBackSide2Side]

sounds like the thermostat is stuck open.

[Drax]

sounds like the thermostat is stuck open.

hmm ok - this was replaced about a year ago im pretty sure, i can always test it in a jug of hot water.

[FrontBackSide2Side]

its worth checking, I've had cars doing this in the past, gone to check the stat and its completely missing. Of course I've put one in and it starts overheating then lol.

[Supermarine Blues]

Does it lose a lot of water? It is often a sign of head gasket failure. Although it often reads hot when you VTEC it and cold when you don't.

[Drax]

Does it lose a lot of water? It is often a sign of head gasket failure. Although it often reads hot when you VTEC it and cold when you don't.

not as far as i know but ill tell him to keep an eye on it. the thermo i swapped myself a year ago after the old one looked dodgy.

[Bri]

I am wondering if it is the electrical sender thingy or the display itself, it is an odd one that Dan

[Space1999]

Thermo or temp sender. If the hot heater air gets cool along with the low reading, it's more likely to be the 'stat. Just because the stat is a year old doesn't guarantee it's working now; they often fail, sometimes with disastrous results!