I recently purchased a 1990 Chev.Truck with the V-6 engine. The truck has always started and has always idled a little rough but ran decent. Just recently it has gotten worse. It still starts and idles a little rough but when you push down on the gas it hesitates and if you push down real hard it backfires. It acts like it is starving for gas. I recently replaced the fuel filter and the old one looked as though it was original and was somewhat plugged compared to the new fuel filter, but no difference. I'm now thinking its the Idle air control valve. Is this IAC located on the side of throttle body and does it have a 3 pin connection, if not what is this 3 pin electrical connection on the side of the Throttle body?When I take this connection off while engine is running the engine sometimes, not always dies. What would suddenly make the engine start too hesitate and backfire.

 

the iac is beside that one it has a 4 pin sqaure connector that 3 pin should be your tps.

 

When was the last time it had a good old fashioned tune-up? Compression check, plugs, wires, cap, rotor, etc.