i replaced the blown motor in my 98 ZR2 Blazer with a 4.3 from a 2000 Astro Van. It runs good but surges occasionally. Usually when it is in a cruising state, not accelerating. I had 1/4 tank of gas that sat for 4 mo. and i added 4 fresh gallons b4 i started it. I added some heet but so far no help there. I m still using old plugs, new cap, plugs and wires tho. I blew the cat trying to get it timed but at the moment i do not have 450.00 to do that. i don't think that is the problem tho, i am just mentioning it so we have the big picture. Thanks, i appreciate any help.