Mazda b2600i 93 transmission idle problem
 

Hello , i just swapped my engine of my other b2600i 92 truck inside my 93, now when i start de engine, the Rpm goes to 750, but if i press the cluth and i enter à speed, the Rpm CAN go up to 850-1000 that is correct but it also do it many Time, the idle go to 500!... Idk where the problem, i Will go test my Check engine today. I tryed with the Green pin but idk how to with à test light or whatever. Anyone know solution?