Chevrolet Repair: 1998 GMC 350 fuel mixture problem, mass air flow sensor, mass air flow

Question
QUESTION: Robert,
My 1998 GMC truck with the 350 engine is running poorly.  It is not sending any codes.  It idles rough and at lower RPM's than normal.  It accelerates pretty good, but not as good as it used to.  When I stop accelerating and just try to maintain speed it starts jerking and popping like crazy.  I have replaced the "spider injector" with the improved design and this did not help, although for about 10 miles it did run a lot better.  I suspected that the fuel mixture was too rich so I thought maybe the mass air flow sensor was causing the mixture to be bad so I unplugged it and drove the truck without it.  The truck ran better, it didn't jerk and pop, but it was still a little rough.  So, I bought a new sensor and installed it and nothing changed.  I need help.  I have been chasing this problem for some time now ( I do not drive this truck much).  In the process I have tried a new ECM, new plugs,new wires, and new distributor.  Thanks for your help.

-Jody Allen NC USA

ANSWER: Hi Jody. try your EGR system as well as a possiable plugged converter. hope this helps

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QUESTION: Thanks for your fast response.  By converter do you mean the catalytic
converter?

ANSWER: Hi Jody... yes .. check to see if the exhaust is restricted...

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QUESTION: Hello Again,

OK I changed te EGR valve and the catalytic converters.  The EGR valve was
squeecky and seemed worn and the converters were some what blocked.
However the truck did not quit popping and hopping (I guess rich), although
the high flow cats have freed up the flow considerably.    I install new up
stream O2 sensors and this has helped, but it is still doing the same thing,
but less constantly.  Could it be the rear 02sensors?  I'm really getting
frustrated with this truck.  What else could it be?  Again thanks for your help.  
I really thought the cats were going to fix it, but they almost intensefied it.
-Jody

Answer
Hi Jody.... just a thought... have you done a compression test.. does the pop sound like it's comming from exhaust or up thru intake...could be a bad valve...or bad cam... Gm had bad cams for a number of years...more so in the 305s, but they did get some bad ones in the 350s...cam lobes wear out...you might want to look at that...hope this helps