89 suburban back fires through the throttle body mainly when it's cold. Also hear a little knocking noise through trottle boby. Seems to lack power. I've changed several parts: fuel pump, oil sensor switch, throttle switch, sparkplug wires, timed it. running out of ideas???

You may have a valve sticking. Just a guess.

Take the airbox cover off spray some starting flood into the throttle body see if it fires if it does week fuel pump

is this even in summer time, or has this just started occuring since winter got here... if its since winter got here, its perfectly normal to hear the valves clatter when the engine is cold.... if it knocks in summer time also, you have something a little more serious goin on, could be rods, could be the mains, could be both rod and main bearings, lifters, scarred cam... but being this new of a vehicle, id say its just a clatter from the lifters being in cold weather.. hope this eases your mind a little

try the fuel pressure regualtor

If your sure that the pump is good, change the fuel filter and get the fuel injectors cleaned by a quick lube place. It's cheap and will make a world of difference.

Check your timing. With 5.7's from 1989 I have encountered slipping harmonic balancers, so make certain tdc is actually where your "mark" is, otherwise you'll get an incorrect setting and it'll run like ****. Depending on your mileage you may have a toasted timing chain.

it may not be a vacumn leak from a line,check the throttle shaft for movement,for example,i had a 89 1 ton 350 and my throttle shaft was moving around a lot and in order to keep it from idling high,i had to kick the throttle pedal.But also check the idle air control valve,which is the round sensor on the side of the throttle body.

Sounds like the knock sensor has failed. It's on the passenger side at the bottom of the engine block, in front of the starter. One wire going to it. Have you checked the timing lately, maybe it has moved?

Chk. your egr valve, that it is not sticking and you got vacume to it. could be carboned up and is not working.

That does not sound good. It seems that you may have broken or worn rings or pistons. You could try timing the engine and see if that is not a problem but I think it is a bad piston and rings.