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MG MGF Technical - Schizophrenic temperature reading

Hi all,

The water temperature gauge over the past few weeks has alternated between realistic (dead on 50%) and off-the-scale. Scared me the first time, now it just adds an amusing unpredictability, but doing a few track days soon & thought it wise to get it back to reality/reliability.

Checked the archives (and I have to say it's very quick now - new server highly commended) and the cylinder head thermister seems most likely to be the culprit. But, electrical gremlins being slippery b*ggers at the best of times, wanted to ask whether anyone else has found the cause of this to be anything other than the thermister?

Thanks guys and gals

Mike Hankin

Mike, usually the temperature goes 'cold' rather than hot when the thermistor goes (high resistance usually means cold) - unless something is shorting?

Maybe worth cleaning the contacts anyway, and replacing the thermocouple if that doesn't work. Failing that, you may be looking at a damaged wire to the sensor.
Rob Bell

Thanks Rob, been pondering some more and IIRC the times the gauge has gone off the scale have been after there's been a reasonable amount of rain; you may have hit the nail with the shorting. Hadn't spotted that the failed thermistor symptom is downwards not upwards. The gauge has always behaved on longer journeys, it's only been wayward on the short errands, where it's possible the underbonnet temperature isn't getting high enough to dry everything off completely. Armed with WD40, I shall go investigate further...

Did you crack the wireless problem in the end?
Mike Hankin

Take care *ironic post*:

I'd recommend a 35mm diameter sticker adhered to the glass with hand made scale and a painted arrow showing 9 hours.

IMO this gauge is sowhat of useless.

My 2pence.
Dieter K.

Mike, I have to be honest that I haven't even had a chance to look at the wireless! Two and half month old daughter is responsible for that... ROFL :o)

Good luck with the WD40.

Following on from Dieter's comment regarding the temperature gauge being as useful as a chocolate teapot, you could use a multimeter to monitor resistance across the sensor - but I do what you suggest first, and then only if the problem persists do some further checking in this way.
Rob Bell

Out of intrest is it the gauge or the sensor that is very heavily damped?
Steve Ratledge

The gauge. The Elise uses Stack instrumentation, but uses the exact same temperature sender.

No reason why you couldn't do the same on an MGF - but would it serve more to fright than reassure and therefore be more of a hinderence than a help?
Rob Bell

Liberal dose of WD40 applied, connectors cleaned with proper contact cleaning spray, gauge behaving normally for now. Checked the weather forecast and all I can expect for the next week appears to be snow...

Atmosphere in the cabin is now petrol fumes mixed with WD40, will report on any hallucinogenic properties I discover!
Mike Hankin

LOL! :o)
Rob Bell

Uh oh, happened again tonight while bone dry, thoroughly warm engine, 5 miles into journey. Gauge leaped up to beyond maximum and stayed there. Stopped to check for any other symptoms, nothing else seems to be backing it up. Started again, gauge still high but then slid down to normal, took it easy for a bit and then tried a short burst of wider throttle, gauge started climbing again, dropped again when I backed off. Now not so convinced this is electrical, only plausible thermomechanical theory I can come up with is air/steam pockets in the coolant, producing the higher reading when it surrounds the sensor, but eventually being 'moved on' by the flow of coolant - does that make sense?

Perfect timing - 500 mile round trip and 2 track days in the next 3 days :-( So, any suggestions of further diagnostics welcome.




Mike Hankin

Mike,

No useful suggestions, except to say that mine (96 VVC) did this a few months ago. It went on for 2-3 days and was also apparently related to driving style etc. The only thing I could think of was to clean the connectors and have not see it since.

It is still a worry.

Peter.
Peter Ambrose

Mike, I had a similar problem sometime back, my fault was traced to a short in the wiring loom just behind the facia panel, when the guage went off the scale I could press the facia and it would sometimes drop to normal other times when on normal I pressed the facia and it went off the scale. Also as mine is later model VVC the temp warning light also came on.

Mike
Mike (Mersea)

>> took it easy for a bit and then tried a short burst of wider throttle, gauge started climbing again, dropped again when I backed off. <<

Uh oh - this doesn't sound like the shorting that we thought it could be Mike :o(

I agree with your idea - that there could be an airlock in the system somewhere, either that, or the radiator is well past its sell-by date.

Bleeding the coolant system is pretty straightforward, and this is what I'd do as a next step. If you can, inspect the condition of the radiator too, but this is usually best achieved after removing the bumper...

Fingers crossed that you can get this nailed before the hard fun driving starts!
Rob Bell

Thanks guys, I'm slightly encouraged that no-one (yet) reports 'I had this and then the steam started appearing'! Maybe the root is still electrical, which I can live with at least until after the weekend. Coolant is crystal clear, level still unchanged, oil looks fine too, fingers crossed. Might join the AA just in case though...
Mike Hankin

Mike, I'd definitely bleed the system - even if you suspect that the problem is electrical: leaving a potential air lock is a recipe for diaster of a head gasket flavoured variety.... :o(
Rob Bell

>took it easy for a bit and then tried a short burst of wider throttle, gauge started climbing again, dropped again when I backed off.

Hummm,...
That's _funny_ ?
Would underline Rob's advice on a bleed session.

However, something totally different.
Do you see other strange indications ? Like interieur dimmed or brighter, analogue to engine speed ?
I think that would be alternator or battery related.
Dieter K.

It gets more puzzling. During the 5 minute drive from the B&B to the Anglesey track on Friday, the gauge went off the scale, and stayed there. But, behaved normally for the sighting laps, and then continued to behave normally for the rest of the sessions, the 240 mile drive through the mountains in 'orrible conditions to Llandow circuit, and still didn't misbehave for another full-on track day.

Now convinced it is electrical, the variance with driving style must have been coincidental. No other electrical symptoms, so by a process of elimination it must be local to the instrument panel.

Thanks for all your input guys, if I find anything obvious I'll submit an entry for Rob's troubleshooting section.
Mike Hankin

Hopefully you'll be able to track this particular gremilin down Mike! How were the track days? :o)
Rob Bell

Try driving with the thermistor disconnected. The gauge should read zero, it it suddenly reads high again then the problem is not related to the sensor or engine.

Brian

Good tip Brian, thanks.

Rob, you'd have enjoyed Anglesey lots - well worth the drive. I'll not mention the small indiscretions, discovered that the mods to the suspension & bigger wheels gained a lot of corner speed but lost some of the 'forgiving' handling characteristics ;-)

Llandow circuit was open pit lane with some very track-oriented machinery taking part, shorter lap & not as technical as Anglesey, but good fun nonetheless. Began to wish I'd gone for a VVC, but was pleased to discover that I could almost match the Caterham brigade in one or two sections. My 2nd driver was on his first track day, had a baptism of fire when the pressure of a SuperLight hounding him became too much and he attacked an apex bollard with the passenger door :-(
Mike Hankin

Good to hear that you enjoyed Anglessey Mike - been there once, and my favourite 'bit' of the circuit is that section into Radar :oD

Oops - sorry to hear about the door... 8o(
Rob Bell

Yup, exiting Abbotts and going uphill into Radar was where I came unstuck a couple of times ;-) Was finally consistent in the very last session (about usual for me!), so I'll be back next year for another go, hopefully you'll be there too to set the pace?

Door damage fairly superficial, the bollard never reappeared though so seems Dave did a thorough job on it...
Mike Hankin

It's usually me who wipes out the cones, so thanks for taking up that mantle for me! ROFL

Glad to hear that the damage is only superficial - phew!

Yes, it would be great to return to Anglesey... maybe next year?
Rob Bell

This thread was discussed between 02/03/2005 and 15/03/2005

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