What does it indicate if raising an EGR valve diaphragm has no effect on idle speed?

Raising the EGR (Exhaust Gas Recirculation) valve diaphragm is intended to introduce exhaust gases back into the intake manifold, which can influence the idle speed of an engine. If there is no effect on idle speed when the diaphragm is raised, it indicates that exhaust gases are not entering the combustion process as they should. This can happen if the EGR passages are clogged with carbon deposits, preventing the proper flow of exhaust gases back into the intake manifold.

When carbon builds up, it can restrict or completely block the pathways needed for EGR operation, resulting in no change to the engine's idle speed when the valve is actuated. In this scenario, it emphasizes the need to inspect and possibly clean or repair the EGR system to restore proper functionality.

While a faulty EGR valve could potentially prevent the expected change in idle speed, the specific scenario described in the question points more towards a blockage in the passages rather than the valve itself being malfunctioning, especially if the valve operation is confirmed to be otherwise functional.