bygddy

Need a little help guys, 85 TA, stock guages, temp guage was working fine until I swapped heads, the heads that were on it were an older GM casting, and they had a 1/2 sized sender in the drivers side head. Single wire, I believe green, running into the factory harness. When I switched heads the new port was only 3/8 so I just ordered a stock replacement sender that was application specific to my car. Plugged in the wire and nothing. When the car is turned off guage sits under 40 in a "rest" position the same as all the other guages, when the car is running it moves slightly to 40* and sits there, it will not come off 40*.....any suggestions to why?