Bogged down and backfiring until "warm"

[Scaper]

I have a 96 c1500 5 speed 4.3. The issue I am having is at start it fires up and idles fine. When I start driving it, it feels a little bogged down and under heavy load(hills) will back fire. But at a point in time it will all of sudden clear up and run great. Until I shut it off and even if it is only five minutes, at restart will need to " clear itself out again" and then run great.

Plugs, wires, cap and rotor, ignition coil and module have been replaced. I do not have a scan tool, but the local garage guy as drove it with a scan tool hooked up to it and at first does show miss fire in cyl one, but when it " clears up" shows no issues.

Any ideas would be helpful, thanks.

 

[Schurkey]

I have a 96 c1500 5 speed 4.3.

it fires up and idles fine. When I start driving it, it feels a little bogged down and under heavy load(hills) will back fire. But at a point in time it will all of sudden clear up and run great. Until I shut it off and even if it is only five minutes, at restart will need to " clear itself out again" and then run great.

Plugs, wires, cap and rotor, ignition coil and module have been replaced.

Seems like you have ignition covered. Possible you have "new defective" parts, but unlikely.

Also seems like this is an open-loop issue that goes away when the thing goes into closed-loop. But that's not guaranteed either unless you confirm it with a scan tool.

I do not have a scan tool, but the local garage guy as drove it with a scan tool hooked up to it and at first does show miss fire in cyl one, but when it " clears up" shows no issues.

That vehicle needs more time with the scan tool. Confirming open/closed loop when it "clears up", for example. Fuel trims when it's misfiring. Verifying TPS, MAP, MAF, etc. sensors when it's running good AND when it's misfiring.

What is the fuel pressure when it misfires? At prime? When it's running good?

What is the cranking compression pressure of #1 compared to a good-running cylinder?

 

[Scaper]

Thank you for the info. It is going back to the mechanic with the scan tool this week. I will share this with him.

 

[Scaper]

Just posting an update. Scan tool testing showed the MAF and 2 O2 sensors out of range/spec. I replaced the MAF and all 4 O2 sensors and EGR valve. The truck is running great for a 96 with 180,000+ miles.
Thanks Schurkey for the info.