Timing off?

[jt24]

So I am at a loss with my sons pickup. It is a 94 GMC 1/2 ton with a 350 that is not orignal and a auto trans with 4x4. The pickup runs fine except under load like going up a hill or when its windy the pickup starts to run weird and there's a shake coming from the engine. It doesn't do it all the time. It has back fired a few times when this has happened. I have replaced the rotor,distributor cap,wires,plugs,and the fuel filter. I believe it is timed by hooking it to a computer. Could the timing be off without the engine light coming on???

[Eric Wettschreck]

My 90 chev, 1/2 ton, 5.7, with 140-ish miles recently started doing the same thing. I would run just fine by itself but once under load, (towing, up hill), or if I really put my foot into it, it would spit and sputter and backfire out the throttle body. I've seen a couple similar posts in here recently with the same issue.

So, here's my story.

I went in to replace cap, rotor, plugs, and wires. I couldn't get the old rotor off without a bar. Once off, imagine my horror when I saw this.

Yuk. I'm really surprised it even ran.

Anyhoo, I replaced the distributer, cap, rotor, plugs, and wires.....all OEM. Broke the oil pressure sensor nipple in the process so now that's also new. (That thing really spits oil) Bypassed the electronic timing advance and timed to 0 degrees TDC as the book says. Re-connected electronic timing advance and down the road I went. Ran like a champ.

Then I hooked up the fish house and down the road I went. Ran much better until I really put my foot into it. Then came the spits, sputters, and backfires......and what sounded like a ping. I'm thinking the electronic timing advance isn't working properly, but honestly not sure how to troubleshoot.

I should add, check engine light does not come on and no codes.

After some interweb research I found others with this similar problem had faulty idle air control valve.