1996 Suburban Misfire Under Load

Smokedoutz28 · January 9, 2008

This is my first post im a member of at FSC I came over here to get some more help on a misfire issue Im having with my Suburban.

It's a 96 1500 Suburban 4x4 with 137k miles has been acting goofy lately it developed a slight miss under load around cruising speeds 35-40mph and 1000-1300 rpms (1200-1500 on the tach) that isnt and wasent picked up by the pcm! The weather has been kinda goofy here in chicago so I figured it was the moisture in the air so i went ahead and changed the Cap, Plugs (ac delco), Wires & rotor all which appeared to be orginal and needed it but to no luck fixing the issue.

My Solus picked up a slight misfire in all the cylinders but not at once! (random misfire) but this isn't being picked up by the pcm which is odd because it's happening but only under load. WOT, 1/2 throttle, performance doesnt seem to be affected by this slight misfire. Theres a hint in Idle when parked but when in gear the truck idles smooth. The EGR, O2's & fuel pressure are all good and within spec! Im wondering if this is a sign that the "Spider Assembly" Fuel Unit is going bad Since the plugs, wires, cap & rotor all were orginal ac delco brand Im going to take a stab in the dark that the fuel unit is stock as well. Any opinions, Suggestions & Experiences?

-Fuel pressure at idle is 52psi and 57-60 at KOEO

-Timing Retard through the scan tool is -1 (Which I read is accpetable)

-Swap on a MSD Coil & Ignition module from my Camaro and no change was found

-No Check Engine Light, No Codes!

Thanks for the Help I really appreciate it!

GMTRUCKTECH · January 9, 2008

Being it's happenning under a load and your fuel pressure is good it's possible you may have excessive back pressure in your exhaust. Do a back pressure test on the cats. Anything at 5psi or over is cloged. If that tests good I would do a injector balance test on the injectors record the drops which should be 4 to 6psi any less the injector is cloged any more it's sticking causing to much fuel. If you find it to be a injector issue and flushing them doesn't help there is a new injector unit availble which is far better then your old unit. If you want the part numbers pm me. The only other thing that concerns me is your fuel pressure maybe good but your fuel volume may be a issue. good luck

Smokedoutz28 · January 9, 2008

Being it's happenning under a load and your fuel pressure is good it's possible you may have excessive back pressure in your exhaust. Do a back pressure test on the cats. Anything at 5psi or over is cloged. If that tests good I would do a injector balance test on the injectors record the drops which should be 4 to 6psi any less the injector is cloged any more it's sticking causing to much fuel. If you find it to be a injector issue and flushing them doesn't help there is a new injector unit availble which is far better then your old unit. If you want the part numbers pm me. The only other thing that concerns me is your fuel pressure maybe good but your fuel volume may be a issue. good luck

Thanks for the reply. Fuel pressure is good 56psi with key on engine running. I don't have anything to test of clogged cats is there anyway I could do that at home? Unfortunally my Scan tool can't perform a injector balance test on this vehicle like other vehicles and im not aware of a way to do that with the "Spider Unit" under the manifold, If you could help with that as well that would be great.

Im aware of the upgrade and I don't want to pull the trigger on it until I know for sure it's the cause. I do appreciate the help. Thanks again!!!!