P0303 problem after a 1000 miles of no problem after new heads

1996 GMC JIMMY
144,000 MILES • 6 CYL • 4WD • AUTOMATIC
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96REALWE
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96 jimmy 4.3 4X4 144k miles, I rebuilt the upper end both heads machined a 1000 miles ago, bought ebay used injector spider was clean and listed as tested good, all new cap, rotor, wires, and plugs. Started and ran good with no codes for about a 1000 miles now throws P0303 every time you drive it a while 20 miles or so. I can't believe the compression has changed it test good after the rebuild and the electric parts are 2 months old hard to believe they are failing so it leaves the injector spider
anyone tell me how to test the injector without having to pay for dealer visit. Any help would be great!
Dec 8, 2010 at 7:27 AM
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MHPAUTOS
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This is a miss fire in No 3 Cly, check plug, lead, injector and comp on this Cly, also check for a vacuum leak near this cylinder as well, start here.

mark (mhpautos)
Dec 8, 2010 at 8:39 AM
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96REALWE
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ran a tank of gas with chevron injector cleaner and it seems to have straightened it out no MIL light no codes pending
Dec 9, 2010 at 4:12 AM
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MHPAUTOS
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My just have been a sticky injector, keep an eye on it and let me know if it acts up again.

mark (mhpautos)
Dec 9, 2010 at 5:11 AM
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96REALWE
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Hey Mark
Thanks very much for the first response that I forgot to THANK YOU for and this latest response, since I had done all the labor and installed all the parts. I was pretty sure it had to be fuel. Added a 2nd 20 gal chevron injector cleaner to another tank of gas for good measure drove it yesterday no light no current or pending codes.
Bill
Dec 10, 2010 at 3:29 PM