99 gmc suburban running rich

[mncowboy2001]

I have a 99 gmc suburban 5.7 running rich fouling plugs to the point it wont start. New fuel pump filter and poppet injector set. True duel exhaust with no muffler but still has cats under it. No code coming up and not sure what else to do. once running I can get it to go down the road it runs great. off the line falls on its face and can smell gas from the tailpipe  and black sooty smoke at idol runs like crap. Popping out the exhaust. what else can it be. This thing has me boggled. 

[davester]

From googling, it looks like your burb is prior to the gmt-800 platform, but with injectors/cats it likely has a similar setup, so air leaks can cause fueling problems.  I know for my 6.0L, it had a bunch of broken off exhaust manifold bolts, that allowed clean air into the exhaust, so the O2 sensors would read lean all the time, so the ECM was just dumping as much fuel in as possible.  Maybe you have a similar leak somewhere upstream of the O2 sensors?  Maybe an intake manifold leak results in something similar?

 

You might also try reading engine codes.

[mncowboy2001]

Once I put the upper plenum back on clean the plugs and try to start it and check for any vacuum leaks and check the exhaust manifold gaskets thank you for the advice