If the ECM doesn't "hear" from the knock sensor when it first fires up, it runs in limp mode. That also means that your engine won't be getting much timing at all, as it'll be a static value set in the limp chip set.
Since retarded timing does not = backfiring through the intake, I wouldn't be concerned with the EFI at this point, I'd be more concerned with making sure my cap / rotor is good, that the distributer is actually installed properly (not a tooth off), and that your wires are put on in the correct firing order (18436572).
If you don't find a problem there, make sure your valves are adjusted correctly, and that the timing chain hasn't slipped a tooth.
All that said, get your knock sensor figured out ASAP, its an essential peice of equipment that will make your truck run like crap... The wire that runs to it runs from the flat square module mounted just to the passenger side of the throttle body. It'll be blue in color. Make sure you get the correct knock sensor, and do yourself a favour and buy it from GM. The aftermarket units are crap more often than not, right out of the box.
i just installed a reman 350 in a 1990 suburban i bought the burb with a bad motor now that i have the new one in it's backfiring in the intake
timing is right i sure its the knock sensor but when i got the burb the knock sensor was not hook up and the wire is gone so now what. i think this is why it backfires when i give it gas so can i run a new wire and if so where does it go
