Coolant temp running from mid to low at times. defi meter water temp show 80 plus 90deg but on the speedo shows 1/4.
Usually it reads slightly below half. Have check grounding all good.

the thing is, it never goes above half mark. just happened few weeks ago.
what could it be? sensors? burping? bad rad cap?

 

Try a new OEM thermostat and a new pressure cap.

 

Where is your Defi sensor located at vs where is the stock one at? That should help drive the solution.

Personally doubtful that it's the thermostat as if your engine runs for an extended amount of time without getting to proper operating temp the ECU will throw a code that can't be cleared with a reset on a reader and will go into limp mode. Found that out the hard way by swapping in a lower temp thermostat when I flushed out my system at last major service interval. Only way to clear the code was to put in a normal thermostat and after warming up to normal temp the code disappeared.

 

Not sure where are the senosrs at. i dont think its the thermostat as i just changed it and my rad cap is new. My defi is working well so I think the prob is the stock sensor. I want to have confirmation before i buy the sensor dont wish to have a wrong diagnose since the sensor is pricey.

 

There are two coolant temp sensors on the factory setup, one for the gauge cluster and one for the ecu. If the cluster is reading low confirm that the reading is off by checking what the ECU is reading. It sounds like the sensor is faulty to the cluster, but would be good to confirm prior and then you could try cleaning the contacts prior to ordering a replacement to see if that's all is needed.