Wired start up

[Obs javi]

Hey guys I have a 1990 gmc sierra recently had the motor replaced with a lower mile engine runs great other than when it starts it runs weird trying to figure it’s self out then runs fine 15-20 sec later what could it be anything helps thank.

 

[Erik the Awful]

If you have a scan tool that can read it, what does your coolant temp sender (the one for the ECM, not for the gauges) say?