Chevrolet Repair: 93 S-10 Blazer bucks and surges, intake leaks, propane bottle

Question
Hi Robert...
I've got a '93 s-10 blazer with a 4.3 vortec V-6 engine.  The problem is that the motor has recently started to "buck" and surge. (Almost rapid fire like).  It started out doing this only in gear and under load, but now it does it all the time.  At first, I thought it felt like a fuel delivery problem due to the fact it seemed to have a "dying" feeling to the surging.  However, it would rev up to about 4500 RPM without a problem, and seemed to be most prevalent around 1500 to 2400 RPM.  The local parts store and some co-workers feel that this problem is ignition related so here is what I've done so far.
    I've changed the fuel filter, plugs, wires, cap, rotor, ignition module and coil.  I've cleaned every sensor that I could find. (Some were quite dirty)  I don't have a pressure gage, but I've disconnected the downstream end of the fuel filter and covered the end of the line with my thumb and couldn't hold back the pressure.  I just about at my whits end!  Can you help me please?

Thanks in advance!

Chris

Answer
Hi Chris... check for a vacum leak... check all hoses and around the intake... a propane bottle will help...turn on propane without lighting it and pass it around hases and intake area...if engine picks up move propane away to see if it dies down...pass it over area again to be sure that is the leak...these engines are known for intake leaks... hope this helps