Try creating an exhaust leak- - - - -loosen the down pipe at the manifold, or remove the muffler, or drill a hole in the pipe ahead of the muffler, etc. One thing most people don't look for is a clogged exhaust system. I've found mufflers packed solid with rodent nests, collapsed double-wall pipes, and on newer vehicles, catalytic converters clogged solid with carbonized oil from a worn out engine that consumes a lot of oil. A sure sign of a clogged exhaust is intake manifold vacuum that drops to virtually nothing as soon as any load is put on the engine.
Jerry
