7.4L backfire under load

Alright so here’s the deal. I’ve got a 94 k2500 burb with the 7.4 tbi. About a month and a half ago i started having misfiring issues. Did the distributor cap, rotor, wires, and plugs. Adjusted the timing as you should with a tbi (warming it up, unplugging the computer under the glove box, starting it again, setting to 4* btdc, then cutting it and plugging the computer back in). However, when i’d set it to 4* btdc, the computer would advance it to about 10-12* btdc and i’d still have a misfire. Checked the fuel pressure and it was low 20s (I believe 26-32psi is spec), so i dropped the tank, did the fuel pump and filter, got the psi where it need to be and still misfiring. Flash forward and I’d done the map, ignition coil, and o2 sensor. Still not right and no CEL. Put in new fuel injectors and she was still misfiring, but whenever i went to test drive, it would stall out whenever given throttle. Decided to replace entire distributor as well as temp sensor. I also checked compression after doing this and everything seemed to check out. Got it all back together after cleaning the IAC with some carb cleaner and set the timing. Computer advancing it to about 8* btdc now after setting it to 4* with computer unplugged. It runs smooth with no misfire that i can tell and revs fine in park but as soon as you go to drive it it accelerates very poorly and backfires when you let off the gas. Really stumped on this one. Appreciate any and all input. Thanks.

compression test

1- 150
3- 140
5- 149
7- 140
2- 143
4- 145
6- 145
8- 140



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Backfire??? Into the exhaust, or into the intake?

Restricted exhaust?

Worn cam lobe, exhaust valve doesn't open properly?

Connect scan tool, look at data stream.

Hartwell said:

Adjusted the timing as you should with a tbi (warming it up, unplugging the computer under the glove box, starting it again, setting to 4* btdc, then cutting it and plugging the computer back in).

Click to expand...

Huh? There isn't a one-wire connector in the engine compartment near the distributor?

Schurkey said:

Backfire??? Into the exhaust, or into the intake?

Restricted exhaust?

Worn cam lobe, exhaust valve doesn't open properly?

Connect scan tool, look at data stream.

Huh? There isn't a one-wire connector in the engine compartment near the distributor?

Click to expand...

Backfire into exhaust.

Cut off the cat- didn’t change anything.

Was thinking about pulling valves to look for carbon build up.

No, the one wire connector is under the dash on the passenger side on my truck.

Most likely going to take it to a shop first thing monday and have them run diagnostics.


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