Temperature reading ?

[Big cheese]

oki have a guage on my truck and on a hot day the temp usually only gets up to 150  was wondering is there anyway to test the guage or the sender it has the wire that clips onto the block right buy the headers driver side how can i test this because i dont think my tuck runs that cld just want to be sure thanks.oh its a 350tbi with factory guage               
87 gmc by Big cheeser, on Flickr

[zieg85]

Open the petcock on the radiator and drain about a gallon out.  Take the upper radiator hose at the engine where the thermostat is.  My guess is it is either not in there or stuck open.  That would be a good place to start.  

If the truck has good heat your sending unit may be the culprit.  

[Big cheese]

I recently just changed the thermostat to a t stat that opens at 180 thanks still not to sure thinking the wire needs better contact on the sender going to take a wire brush to the both this weekend .I do know im getting some kind of reading because i di sconnect it and it goes to zero or all the way to the left