chevy blazer hesitation

I have a 2000 chevy blazer 4x4 4.3 liter with California emmisions. I am having a problem with a tip in hesitation. That is as soon as you step on the pedal to accelerate the engine hesitates or misfires and
then clears up. It seems to happen only when the engine is cold and once its warms up to operating temp it is gone. The truck also seems to be hesitating at highway speeds around 60-65mph. Scan data shows that long term fuel trim is 10% + so I think the PCM is trying to compensate for a lean condition. Recent work performed include new fuel pump and full tune-up all using factory gm parts. Can a small vac leak cause this problem. Has anyone ran into this before? Any ideas to repair? Thanks!
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on Tuesday 11 September 2007 07:15 pm, someone posing as pkup83 took a rock and etched into the cave:

You didn't say how many miles are on the truck. However, I'd suggest the following to check:
EGR Valve O2 Sensors (there are two, AFAIK) Fuel Injectors
This is what I remember being the weak parts of the 4.3L engine. There may be more but that's what I remember. The vacumn leak may or may not even be an issue.
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Thanks for the reply. The vehicle has 96k miles on it.
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on Saturday 15 September 2007 08:05 am, someone posing as pkup83 took a rock and etched into the cave:

And when is the last time you changed the FI units?
Is there a check engine light? Regardless you can take the truck into AutoZone and they should run codes for you - maybe there's a point to start from.
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As of right now there are no codes or pending codes. Do you think I should just keep driving it until I get a check engine light? I have not touched the fuel injectors at all but I would like to at least check them to see if they are ok.
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on Monday 17 September 2007 06:00 pm, someone posing as pkup83 took a rock and etched into the cave:

Well, my list still stands from above - EGR Valve, O2 Sensors or FI. Those three will cause teh problems listed.
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I had the same problem with my 2,001 S-10, 4.3. It turned out the fuel pump recently installed was faulty. And was working intermittently. It ran nice for a while, then the hesitation came back. So the spark plug wires were changed, the old ones were only a year old. It's been running fine ever since.
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On Sep 12, 9:20?pm, snipped-for-privacy@mydaja.com wrote:

Thanks for the info. I thought it might be to fuel pump too but I double checked the fuel pressure output of the pump and its within spec. So thats what has me scratching my head??
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