red herring on overheating

[lexi]

Voice activator on Vel comes on 20 miles into journey. "Water temp, stop the car and don't burn hand on engine"

Sure enough the stop sign is flashing and needle is up to top. On pulling in the fans are at high speed. Bonnet up and no sign of overheat. All seems cool around engine. I even got the overflow cap off, with an ordinary hiss but no drama. Fluid in bottle could be seen. No leaks and hoses all hot. Oil good, no water in oil or vice versa.
Drove to destination like that ( can't lose a fishing day guys). On way home the temp gauge went from normal towards high, 5 miles into journey. When it got to high, the voice activator came on and stop was flashing. It drove like that for 10 miles. On cruising the stop lamp went off and the needle dropped to normal. It stayed like that for a few miles and then resumed the shennanigans for last mile or 2. Fans were roaring as I pulled into drive. Turned engine off and the roaring speed stopped and the normal whisper of fans stayed on for about 30 secs after locking up. She will do that sometimes if she has had a long fast run.

Next morning had a more thorough check. Auto fluid was clean with no water. No leaks anywhere. I managed to top up expansion with about 500ml of coolant mix.
Checked coolant sensor wiring and cleaned connectors. Apart from the very slightly low coolant in exp bottle, the only thing I noticed was that during escapade the instrument panel would flicker as I put sidelights on. The panel illumination was not working at that setting, but worked at headlamp setting.
I took it out after all checks for a thrash. It behaved normally and the sidelight setting lit the panel perfectly during test.
Would a faulty coolant sensor or wiring send water needle soaring? I thought that scenario would see needle just sit limp? Would 500ml of coolant cause these stage 1 warning systems, turning fans full on? It could be the instrument or part of the wiring on instrument panel of course.

[lexi]

Hasn't come back, the water temp warnings. I know that on Cits, the stop light comes on if coolant is low. It doesn't send needle soaring though?
Could still have been a leccy fault. Ideas anyone?

[lexi]

Fault has returned. OBD test on instrument panel checks all needle gauges working.
I just wonder why the gauge fluctuates so quickly. On the journey with needle right at top, it can come down in 2 seconds to normal. Would a sticky Tstat do this? Like a gush of cooler water getting through suddenly?
It may be that I am overheating just enough to put alert systems and fans on, but not boiling over. Maybe new stat and new coolant with a flush anyway. I have to start somewhere?

[elma]

Id be thinking coolant temp sensor rather than thermostat. Doesn't sound like you've actually overheated the coolant.

[lexi]

Thanks Elma.
The CTS was my first thought. I may still check or replace it and check wiring resistance there. I have cleaned the connection plug on it. It seems that if CTS is faulty, then on the Vel, it will come up as an injection fault,
My injection computer covers that sensor and many others. It is only the fact that no injection fault is showing made me look elsewhere for the moment. It would be nice easy fix if it were that mind.

[elma]

Your faults in Renaults little book, they call it alp 5.
http://www.uas-bg.com/meganii/eng/3175A.pdf" onclick="window.open(this.href);return false;
Check out page 19a 14
I have no idea what an mot. 1700 tool is though, sounds handy.

I think if it is faulty, but reading within parameters then your ecu won't detect a fault and the temp will just display high or low.

[lexi]

That 19a section is stuck in my brain as I have read it so many times on my own manual
Thanks for the viable input and ideas. I think the MOT tool is measuring pressure and possibly flow.

[lexi]

ELMA: It is looking as if your diagnosis was correct. I replaced CTS and on two 60 mile trips now, the gauge has been normal with no drama.

[elma]

Great news.