Coolant Sensor ???? Need some advice
 

Hi I'm new to the forum, and was looking for some advice or direction to go with the problem I'm experiencing. Its a 2001 lb7 with 315kms. Injectors done 80k ago. Issue I'm having is after a 70 mile hwy ride other day the truck was idling at 800-850 rpm at idle... about 200 more than normal. no engine light. Its really loud as if it was -30 and it just fired up. Shifting is harsh on the way down thru the gears coing to a stop. Also the glow plugs are cycling every start for 6-8 seconds again, as if it was -30. Had it scoped today with a snap on scanner, that said the coolant temp was cold -50 I think but the truck was about 80 Celsius on the functioning gauge, but said to change the thermostats. mechanic said that its the temp sensor causimg thr truck to think its cold. I bought a GM sensor and put it in but still have the issue, except now the engine light is on, but I haven't brought it back to check yet. Anyone have any suggestions or ideas?? Thanks in advance... all help appreciated Cory