Okay, as said in the title, it's not on a Blazer so sorry if this isn't the correct spot.

Anyways, a coworker has a 1992 Cadillac STS 4.9, we recently did a whole engine swap because the previous motor blew.
It was a junk yard motor with 50k on it, and runs excellent when hooked up to an external charger. But once you disconnect or shut the charger off, The engine runs Very rough, and won't respond to any throttle input. It basically sputter like crazy when you push the accelerator, and the tach starts to dance like crazy.

Could it be a battery issue? Alternator issue?
He did the spark plugs before putting the new motor in, and I JUST did wires, cap, and coil today. New starter, too.

 

*Moved to 'The Lounge'*

 

My guess is alternator is toast.

battery is dead so it's using the power from the charger to run the car, when you shut it off the volts in the battery drop way below 12 not alowing it to get a good spark or for all the accessories to turn on.

 

sounds like a charging issue to me. I'd pull the alternator off and swap it off the junk motor, check wiring and fuses also

 

have you metered voltage with it running off of the charger to verify that it's putting out anywhere from 13.3-14.4 volts?
that'd be the first thing to check....

 

Definitely sounds like a charging issue, did he keep his old ALT, or is that the ALT from the "new" engine which was probably exposed to the elements... if he still has his old ALT swap it on and see what happens.

 

The new engine didn't come with an alternator.
Anyways, he replaced the Alternator, and he says it's still having the issue after a few minutes.
Bad ground?

 

bad ground or fuse, does it have a megafuse? check all the fuses in the fuse panel, too.. Oddly enough, when I thought the alt on my SVT died, it turned out the fog light fuse had blown. Why that kept the alternator from charging is beyond me.. after I replaced that fuse it started charging.

 

I think he's checked all the fuses, but I'll do a double check tomorrow.

 

might be stupid, but check the connections at the alternator, and ground cables. did you remember to ground the block to the frame/battery?

make sure you are getting power to the wire at the alternator (all the time for charging wire as well as your ignition turn on) make sure the alternator is grounded good as well!|

I have tested an alternator and it read at 14.2 at the alternator but only 12.57 at the battery, spend about an hour tracing wiring untill i realized I was grounding my multimeter to the alternator case, when grounded to the frame I only read 12.2 or something, had to run a ground wire to the alternator and it worked great again.