Coolant temp gauge issues and 'missfire'

First off my temp gauge is a bit ropey. Ill be driving along then the needle will slowly drop to cold then just drop like a stone off the bottom and the red warning light comes on. Turn the ignition off and back on and it comes alive again. Will it be more likely a faulty temp sender as opposed to the gauge?

Also during very wet weather i get a missfire type of problem. Now i know being a diesel its not a missfire but its those symptoms. Normally between 2500 and 3k it does it and regardless of gear, or speed. Its not all the time as i say, just normally in very wet weather. Ive searched and seen various issues causing similar symptoms, but theyre normally all the time, not just in wet weather like mine. Dry weather issues have included sticky actuator, oil in some loom somewhere and ECU fault. I dont get a EMM light either so cant get any codes.
Any thoughts would be appreciated. Im new to D2 ownership but familiar with D1s so i might need help on where to look on the truck for whatever people suggest.

Thanks all i hope you can help!
Daniel

first of all check the plug of the ECT(engine coolant temp) sensor, it used to slacken and corrode, if it's ok put a new genuine sensor, as it's not expensive and even if it works they used to loose qualities in time and mixes up the management, it might be the cause of both your issues together, this sensor is more important than it seem for some people:

ENGINE MANAGEMENT SYSTEM - TD5 DESCRIPTION AND OPERATION 18-1-15 said:

Engine Coolant Temperature (ECT) sensor
The ECT sensor is located in the coolant outlet elbow on the top of the engine. It provides the ECM with engine coolant
temperature information. The ECM uses this ECT information for the following functions:
l Fuelling calculations.
l Temperature gauge.
l To limit engine operation if coolant temperature is too high.
l Cooling fan operation.
l Glow plug operating time.
The ECT works as an NTC sensor. As temperature rises, the resistance in the sensor decreases, as temperature
decreases, the resistance in the sensor increases. The ECM compares the voltage signal to stored values and
compensates fuel delivery to ensure optimum driveability at all times.

In the event of an ECT sensor signal failure any of the following symptoms may be observed:
l Difficult cold start.
l Difficult hot start.
l Driveability concerns.
l Instrument pack temperature warning illuminated.
l Temperature gauge reads excessively hot.
l Temperature gauge reads excessively cold.
In the event of component failure the ECM calculates coolant temperature from the fuel temperature sensor signal. If
this occurs, the limit engine operation if coolant temperature is too high becomes inoperative.
The MIL will not illuminate in an ECT sensor failure.

Click to expand...

Cracking, ill pop into LR and order one then
Thanks

yes, you must rule out the sensor first...cos there is a small chance to be(pray not to be) a wiring issue too between the sensor and the ECU but that's hard to find and the sensor must be ruled out first anyway

Ordered one from LR this afternoon so ill see what happens!

Cheers guys!