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I noticed this yesterday while on the motorway.
during normal driving the temp gauge sits just under halfway.

Since it was my first motorway speeds drive for an extended period I thought I'd occasionally glance at the temp gauge and I noticed that it would bounce around from just under half to way down to around the first 1/4 mark.
it's not a slow movement either.
It's like it was jolting around.

It was worst when I got over 110kmh.
revs around 4000rpm in 5th and it would be all over the place.

When I got off the motorway I checked again through the revs in lower gears and it does it slightly then too.

Where should I start investigating this?


Edit - Car is a 92 NA6

Possibly a sticking thermostat. Put it in a saucepan of water, alternately bring it to boil as quick as you can and add cool water to check that it opens and closes smoothly and progressively.

Or else just buy a decent new one. It's looking more and more like there are a lot of dodgy chinese ones being sold through the usual repco/supercheap/bursons sort of channels. Maybe it's best to do this through our sponsors: http://www.mx5mania.com.au/product1771.htm at http://www.mx5mania.com.au/oemparts.php#Cooling

In my opinion a 'bouncy/jolting' guage is more than likley going to be an electrical issue and not an actual temp issue. Pull the cowl off the instrument cluster and make sure the plugs are in tight and check the wiring in the engine bay for the temp sensor.

Yeah, if it persists, check earths & other connections.

Well it looks like a couple of people have or are currently experiencing this problem.

http://forum.miata.net/vb/showthread.ph ... temp+gauge

I'll pull the plug off the sensor and give it a clean this weekend and go for a test run.
If it's still the same then I'll test the thermostat.

I also noticed that the overflow tank was empty on my car but the radiator is properly filled.

bump.

So on the weekend I did the following work and I still get the jittery temp gauge.

Dump coolant.
Replace thermostat and gasket
Replace temp sensor
New coolant
Bleed properly.

Anything else to try before pulling the dash out?

Get a second gauge to confirm the master? Does it happen if you leave it idling or free revving?

Sent from my GT-I9100 using Tapatalk

it doesn't happen at idle.
it does happen at free rev and general driving under acceleration.

I should state that the fan works as expected.
Just the gauge moves around to the cold side with use of the loud pedal

never did get around to fixing this.
I'll tackle pulling the dash out this weekend.

Can anyone confirm if the cable from the back of the temp gauge goes directly to the top of the temp sensor on the thermostat housing or does it take a detour to somewhere else.. I tried tracing it from the thermostat back but it goes in to conduit and I don't want to start slicing those up.

I'm thinking that if it is straight from sensor to gauge I'll run a new cable.