Local chevy dealer contacted us as they had a customer with a 1996 C4 that was ill running and they could not figure out the problem

Right side of engine was running much richer then left and rear fascia on right side was real black.

Owner has recently bought the car and knew none of the cars history.
We replaced the front O2s due to age of car, then decided to replace the orginal injectors

When we popped off that cover over the injectors on right side to find a real mess as I assume mice were living in there and chewed the vaccum hoses.
This was the main source of problem causing fuel regulator to not be getting full vaccum needed

Image shows the mice nest and chewed up vaccum valve so you never knows what causes are without looking for even the odd things that can happen

Interesting: over here (central Europe) martens use to visit cars in the night, make nests with the hood insulation and chew every piece or rubber, especially new ones:
ignition cables
cooling hoses
brake hoses
...

It's usual to include a "marten policy" in the car insurance, this gives you an idea of what those cute animals can do

Unlike their American cusins our mices don't like cars!