97 Chev 4x4 P.U. 5.7 stumbles & misses

I'm going nutty with this vehicle. Intermitently the engine stumbles as I start to accelerate,doesn't do it all the time. Seems to happen more when I have the A/C on. as I slowly backed into my garage with A/C on it
started to stumble, I then turned the A/C off and it seemed to stop. Seems like under a load it starts to skip. I've already changed the fuel pump from another problem. seems like a spark problem to me, but why more so when A/C is on. I'd appreciate any suggestion you may have,thanks
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you new fuel pump may not be powerful. it might not give the load needed to power the truck or your timing may be alittle off or even you might have alittle miss fire
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thanks dunndunn, But the pump is brand new original equipment, and it solved my no start problem. It can't be the timing, cause it runs fine,but at intermittent times, more so when I use the A/C is when it skips and stumbles, after my speed is up it not as bad. Anyone else got any ideas
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It sounds like the same thing mine would do when the EGR valve packed it in. Stumbles at idle or slight load near idle speed. seemed to run fine at half to full throttle. was a pain in the butt because it wanted to stumble and stall at idle speed when backing a trailer, or looking for a parking spot etc. Changed the EGR fixed it right up.

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wrote:

If you can get your hands on a real time OBD2 scanner and watch timing. There is a remote chance that it could be a fuel octane issue. When A/C in on it loads engine it does increase octane needs and when ECM detects a knock, it retards spark quickly which can cause erratic engine operation. You could also have a flakey crank postion sensor that is not coding. Again though a real time scanner will track your problem down. ----------------- The SnoMan www.thesnoman.com
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Sounds to me like you have moisture in your distributor cap and when it looks for a heavier load it would be looking for that extra fire,, it misses cause of contact.. just some 2 cents to check out.
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