suonds like a blown exhust manifold gasket.look for carbon traces around the intake manifold.Ya check the dounut gasket on the exhust pipe and the heat riser on the exhust manifold if it free.sometimes the rod that holds the butterfly is worn if free an ya get a blow by leak.if this is the problem,take off the manifold,torch,cut out butterfly,weld up holes.the truck will run fine.The butterfly just helps the engine warm up a little quicker.Now I'll hear some hell from that remark.good luck.engine idle is 500rpm.changing the cap &plug wires will help,also plugs,points,cond,just give it a good tuneup.show your truck you care.It will care for you.Its the old saying pay me now or pay me later twice+++++
