91 sentra idle problems

A few possibilities I can think of would be O2 Sensor could be bad, Idle speed control unit... Have you checked around the engine to make sure there aren't any loose connectors or especially air hoses? Also you may want to change your fuel filter and take a look at the air filter, the air filter would have to be pretty clogged for that much trouble though... Has it been running poorly like this since you got it or did it just start happening. I'll look through the FSM and post again if I think of anything else. Let me know if any of the things I mentioned helps. Good luck

I just had another thought, it's quite possible that the cold start valve isn't working properly also. I don't have the specs for it but if you have the manual and a meter you could test that. Aside from loose hoses or connectors or fuel filter, that's probably what I would check first.

well, if it's idling at 400 rpms I would say that it's not only affecting at start-up?

Not the easiest to get ot or the cheapest to replace but it's between the plenum and the intake manifold. It'll have an air hose, gas line and electrical connector going to it. But yea, it's pretty much in the middle of the intake, I don't know if you can get it off without pulling the plenum.