Temp gauge says Hot, engine says Not!?

My 3.5 V8 Efi 90 conversion scared the jeebies out of me by suddenly 'overheating' this morning...

I had only driven a couple of miles in smooth flowing traffic max 30 when the VDU temp gauge shot up from 80 to 100 and continued to climb View attachment 178249 I pulled over and switched off immediately.

Opened the bonnet and noticed that the fan electric wasn't on.. it usually comes on at about 90 and isn't ignition-switched so stays on if hot; the top hose was pliable and cold, as expected after only a couple of miles from an overnight cold start, and the rad and bottom hose were slightly warm. All wiring to fan, thermoswitch etc ok and the engine bay felt cool, hardly any heat radiating off.

I squeezed the top hose a few times, heard some gurgling, waited 5 mins or so and carefully loosened the expansion tank, lot of hissing at first as it was still rather hot but the level was correct. Finally pulled off each connector between the fan thermoswitch and its relays, cleaned and refitted.

Started her up and all seemed ok, temp gauge reading the usual 80 and stayed there the rest of the journey and subsequent drive home tonight, apart from one traffic jam where it rose to 90 after a few minutes idling and the fan came on taking it back to 80 in under 30 secs.

Any ideas chaps? Some air trapped in the system? A dodgy sendor/gauge/wiring? The fact that it suddenly shot up and didn't switch the fan on via the thermoswitch makes we wonder if it lost its earth so a possible loose wire or poor connection?

Just a blip or something serious developing?

Cheers in advance.

Air bubble if you made it gurgle when squeezing pipe,

freelance said:

Air bubble if you made it gurgle when squeezing pipe,

Click to expand...

Thanks @freelance
That's what I thought and hoped but then makes we wonder where did this air come from? I'm praying it's not a head gasket.
The rubber hoses are soft on startup and stiffen up as it warms up and the pressure rises; when I had a failed hg in the past the hoses were hard on start up because the air being sucked in causes the pressure to rise quicker when there's a head gasket problem.
The thermostat could have got stuck closed but the fans would have come on if the engine was too hot but they didn't, the only overheating signal was the sudden swing on the gauge and the engine itself felt cool.
The fan thermoswitch is in the bottom outlet hose so if stat was stuck shut it wouldn't register the excess heat?
I've had a look this morning, water level in expansion tank, rad and filler tube all ok, no gurgling when squeezing the hoses and running fine. No fumey odours from the expansion tank and no bubbles when running. Once warmed up gauge reading ~80 and after 5min idling fan came on when gauge read 90 and fell back to 80.
Oh well, I'll keep a beady eye on things.

Perhaps get a new pressure cap