I just put a new motor in my 91 4.3 s10. I got it started and it ran like a bucket of crap. I couldnt figure what was going on with it so i took it to a shop and they said I had an intake gasket leak on both sides. It sounded weird to me but I took it home and changed the gaskets out again and it runs worse now than it did before. Im debating if I could have a warped or cracked intake but then again it also ran perfect on the old motor. Its popping smoking and running for about 2 seconds everytime. Any suggestions?

Even with a serious vacuum leak, an engine can be massaged to run longer than a few seconds. While a vacuum leak(s) may exist, I'd suspect some other things going on as well. Is this a TBI installation of CPFI?

1991 should be TBI.

I would check to make sure that the timing is right, You might have set the distributor a little off.

Is this a TBI installation of CPFI?

Careful asking that question....heh heh

I've done a few intake gasket jobs where it ran like crap for a few minutes then smoothed out after keeping it running. What color/odor is the smoke? Was the "new" engine really new, or a salvage yard pull?

yea it was a rebuilt motor. The smoke is a dark gray or black. Does anyone have any tips on sealing an intake? I've done it before on several trucks with no problem until I got this one. The shop also said I needed a new throttle body because my old throttle shaft was sticking? I found a few on ebay but they have different casting #'s. Are these all the same? I found one off the same year but an astro van. Looks the same to me.