I just swapped a new HT 3.4 from GM performance and took out a 2.8, in my 1991 S10. The RPM goes up and then down every five seconds, or so. It seems to settle down a little when the engine warms up. I've only put about 20 miles on it since the swap. Does the computer take longer than that to recalibrate? I changed the injectors to larger ones and everything seems to being going right, but the change in RPMs. Any suggestions?

 

A hunting idle could be caused by a sticking IAC valve. Did you reuse the throttle body from the old motor? If so, did you rebuild it or clean it before reinstalling it?

 

I used the old throttle body. It does seems to do much better when it warms up to 190 or so. The throttle body was cleaned, but maybe not perfect. The injectors were used . Could they be crudded up?

 

How does it run while you are cruising down the road? Do you still feel a bit of a surge?

If it is just while at idle and it gets better once it warms up, it really sounds like it is a problem with the IAC valve being gummed up & sticking when it tries to make small adjustments to the idle speed.