Vortec problem

[skinmxz]

Hey,I have a 97 gmc with the 5.7 vortec motor,for the past 3 months i only drove it once a week.The other day i went to go into town and the truck started to pop,sputter and backfire.It starts fine and idles fine but when it reaches 2000 rpm the problem starts and seems to be worse when driving.I scanned it and the 2 post cat o2s came up bad,changed all 4 while i was down there and the problem is still there with no more codes.I also did the plugs,wires,cap and rotor with no positive outcome.Anyway any thoughts or help would be greatly appreciated.
Thanks,
Andrew

 

[SAATR]

Get a fuel pressure gauge and check pressure with key on, engine off and running. Should be 60 lbs with the key on and between 55 and 60 lbs running. If the hose is long enough, run it between the cowl and hood and tape it to the windshield and go for a drive. I will bet that it drops off as RPMs increase, caused by a faulty pump or clogged filter. The FPR is possible, but you usually get a rich condition when that occurs. The other likely options are plugged cats or a worn distributor gear, in that order. Check pressure first and let us know.

 

[firefighter_2319]

maybe the coil is trying to break down. i would see what color the spark is coming out of the coil. orange meaning weak and blue meaning hot.

 

[Aloicious]

Get a fuel pressure gauge and check pressure with key on, engine off and running. Should be 60 lbs with the key on and between 55 and 60 lbs running. If the hose is long enough, run it between the cowl and hood and tape it to the windshield and go for a drive. I will bet that it drops off as RPMs increase, caused by a faulty pump or clogged filter. The FPR is possible, but you usually get a rich condition when that occurs. The other likely options are plugged cats or a worn distributor gear, in that order. Check pressure first and let us know.

+1 on all this...

 

[skinmxz]

Thanks for the help,I just did a pressure test and all seemed normal,the cats are high flow magnaflows and are only 2 years old(shouldnt be clogged).When revving it up it seems like theres a lot of greyish exhaust,I checked the oil and its not milky.The distributor gear is something im unfamiliar with and will have a garage look at.Is it possible that it just has bad gas from sitting so much.
Thanks Again

 

[SAATR]

You did the fuel pressure test just with the truck off and at idle, or at speed and under load?

 

[skinmxz]

I apologize,I did it with the key on truck off,and with the truck at idle with revs.With the truck held at 2000-3000 rpm it spits and sputtered yet the pressure held.It fluctuated from 52-60 and averaged 55.I will attempt to go for a drive and monitor it sometime in the next couple of days.

 

[SAATR]

I apologize,I did it with the key on truck off,and with the truck at idle with revs.With the truck held at 2000-3000 rpm it spits and sputtered yet the pressure held.It fluctuated from 52-60 and averaged 55.I will attempt to go for a drive and monitor it sometime in the next couple of days.

That sounds like a good plan. Keep us updated!

 

[skylark]

Is it possible that it just has bad gas from sitting so much.

My Saturn sits for months at a time. With our typical 10% ethanol it has separation in anything more than 2 months and it runs like crap. I buy the non ethanol fuel and it can sit for up to a year with stabil and still be good.

Edit; Here is a link that explains better.
http://www.fuel-testers.com/expiration_of_ethanol_gas.html

 

[skinmxz]

Update,although brand new I decided to change the distributor cap believing it was a faulty part,swapped it out and the truck runs great.I want to thank everyone for the guidance and help.