04 NISSAN SENTRA 1.8L HARD START

could be a throttle sensor, but I would get it looked at. Your car needs to be scanned to see if there is any fault codes

same here my 2004 sentra does the same it starts - then it starts then shakes and die immediately or sometimes it just cranks for a long time then finally starts ..it wierd...most of the time its fine.

Check the IACV, your fuel filter, and especially all your vacuum lines. It really sounds like you have unmetered air coming in somewhere. You can also try re-grounding the MAF (Mass Airflow Sensor) and/or cleaning the MAF - just remember not to use anything other than MAF cleaner to do so. If none of that does the trick, then check your TPS (Throttle Position Sensor).

I have a 2004 sentra too and it just started doing it. It starts in the morning in about 5 seconds. I can drive it and it great. I can kill it and restart it in a matter of minutes and starts up better. If it sits for 20 minutes or longer it will take 5 minutes to start. Just turns over. I've replaced the cam sensor, crank sensor, head gasket, the idle and tps is one piece with the throttle body and replaced that as well. New plugs and 2 coils, coolant sensor, checked for vacuum leaks which was none, reprogram my computer and relearned my throttle body. Out of ideas now.

its most likely the upstream o2 sensor, replace it, clear the code and it should be fine, happened to mine, put it on a scanner, brought up the DTC and put the 02 sensor on, car started up great and haven't had a problem since