Need help. Idles on 3 cylinders

[bgoodwill]

I'm having problems with my '92 240sx. Had to have the engine replaced last winter, used block installed supposedly has less than 40,000 miles on it. Many components switched over from my original engine and the car just rolled over 221,000 miles.

This week the car started shaking when idling. Smooths out and car is running fine at 1000-1200 and better RPMs. Appears that cyclinder #4 is dead when idling. Is getting a spark, spark plug is good, thought it was the injector but have changed it out twice and still the same problem. Overall power doesn't seem to be compromised at any speed. Any suggestions appreciated.

[vancouverbc]

how do you know cylinder 4 is dead when idling? That would be hard to explain electronically. Maybe something else is causing the rough idle?maybe cyl 4 never works

[bgoodwill]

I'm a novice here, so please bear with me.

I believe #4 is dead when idling because if you pull that wire, there is no change in how engine runs. If you pulls any of the other wires, the engine bogs down (even more). Have tested the wire and it's getting current. Have looked at and even changed plug and that's not the problem, either.

It could be dead all the time, but the engine smooths out and seems to be running as normal when you increase the rpms. I'm not feeling any loss of power or performance change when driving, so I assume that cyclinder is kicking in somewhere along the line, but maybe it just runs okay on 3 cyclinders, although I'm not sure how that could be right.

I'm thinking maybe I should clean out/change out the fuel rail but somebody told me that's probably not going to help.

[180fan]

if #4 is sparking but nothin's happening there, it'll most likely be something with your injector. Have you tried pulling the injectors one at a time to see if it changes your idle?

[nisstek]

Do you hear any hissing, AKA a vacuum leak, at idle? Sounds like your intake gasket might be leaking if it smoothes out off idle. I haven't seen this too much on 240's but D21's and Altimas with the KA are notorious for this problem. Spray some carb cleaner around where the intake manifold meets the head and see if Idle changes.