New member here! I've got a '90 isuzu pickup we just recently did a tuneup on. It ran worse after the tuneup! Double checked everything, new sparkplug wires AND new distributor cap were bad! Changed these items, NOW the truck runs like it's only running on three cylinders! All plugs are firing, firing order is correct, we just can't figure it out. Truck has 226,000 miles on it, hoping the engine isn't on it's way out the door! Thanks.

recheck your aftermarket parts.plug gap.dis cap,are you sure about the firing order?do a compression check all clyinders shoud be 150psi.you may need a valve adj,or you have a burned valve.the 2.2l are common for that if thats your engine,the 3.1 do also ,but not that common

If the plug wires on correct, and you made good connections on them, then it sounds like you may have knocked loose a vacuum hose?

If all the vacuum lines look good, walk backwards, replacing the new with the old and see what happens.