Chevrolet Repair: chevy s 10 skipping problem, snap on modis, air fuel mixture

Question
i have a sputter/ skip. i am not rally sure what it is. i have a 1998 chevy s10 vin x v 6. 4x4. standard 5 speed. when i start it up, it idle's fine. but if i give it a little gas, it hesitates until i get above 3000 rpm. same as when i drive down the road. if under a load going up hill, etc do not feel it.  but driving down expressway, or through town ( low idle ) it jerks like a cylinder misfire.   my check engine light has given me p300. multiple random. so i changed plugs, wires, cap and rotor. code went away but skip didn't. i took it a step farther and replaced ignition coil. no difference. had snap on mobius, said mass airflow sensor. changed that. no difference. light came on. said p420 ( bank 1 catalyst efficiency). changed oxygen sensor. light off, still skip.  please help me before i use my insurance coverage.... i love the truck. getting sick of this skip.

Answer
Hey,

  Disconnect the EGR valve and see if the hesitation goes away. There is a ECM reflash for those years that eliminates incorrect EGR positions and EGR codes when they come up. Sometimes when a 4.3 gets older the EGR ports can get clogged adn full EGR is directed to the front cylinders causing the densified air fuel mixture to not have adequate oxygen for ignition on those cylinders. Its tough to diagnose a vehicle with a problem like that without feeling it myself but you could try that.  It sounds like you have replaced everything igntion related.  If you have a SNAP-On MODIS then there is secondary scoping capabilities on it if you have the inductive ignition pickup.  Depending on what software version your MODIS has it has a very nice Raster display that makes ignition missfire diagnosis a snap.  I have a MODIS myself and can show you how to set it up if you like.



Wayne