Re: 95 Crown Vic Rough Idle with code 402
Rough idle usually indicated a vacuum leak somewhere. It could also be your EGR Valve which is a very weak link in the Grand Marquis of the mid nineties. If you can reach it remove the EGR Valve (located in a difficult location low on the manifold on the passenger side near the Transmission dip stick.) If you can take it off, do so and clean it with some Trottle Body Cleaner. Also Clean the port where the EGR hooks into. A pipe cleaner brush is very helpful here. Soak the brush and run it through the opening.
They clog up and then then fail. Results, bad idle and stalling at a stop.
The EGR valve is about $50 bucks aftermarket. It's probably best just to replace it since it is in such a difficult location. And cleaning it may only help for a short time. Replacement would be my choice.
Give it a try, if that isn't it you could have another vaccum leak and those are difficult to trace unless you have someone with a smoke machine that can trace the leak. Another item you may change is you fuel filter. If it gets clogged it might be another cause of hesitation and rough idle.
Check your spark plugs and see how they look. If they are white in color it is most likely the EGR or another vaccum leak. You engine is running very lean and it can destroy the motor if not taken care off.