Trying to isolate the main problem on my 2001 Nissan Altima. Has major hesitation on acceleration and can hear/feel it during idling. The engine sounds good other than this and it doesn't stall. I am a firm believer in the issue is most likely related to what you did last (wires and plugs). But when you have 235k miles coincidences can occur. Here is the breakdown of events.
1. Engine running fine but I changed wires and plugs as standard maintence
2. About 1-2 days later I began noticing a little hesitation/sluggishness when accelerating. I thought maybe I needed some fuel injector
cleaner and added it. Nothing major.
3. Hesitation go worse as the days went by and about a week in a half since changing plugs wires it completely stopped. Stalled and didn't start back
4. I found that my oxygen sensor connection was melted on the exhaust manifold and thought that was the issue and changed it. It started but not able to keep running.
5. Called mechanic and he came over and eventually adjusted my timing via distributor and it ran but not that great. He contributed it to the timing chain (original).
6. He changed the chain and distributor and cleaned the catalytic converters in front and back (front was replace 2 yrs ago and rear is original)
7. He ran the diagnostic test and now the EGR is what is showing up as faulty. It was replaced two years ago but the mechanic said that if the converters were bad it could have clogged the EGR valve.
Sorry for the long details but thought it might help.