I have had this problem ever since I installed my 3/8" powerlab plenum spacer about 3 months ago. From a cold start it starts just fine. Its after its been ran for a little while, shut off, and then started again. It will crank and start fine, but when the RPM's go up right after it starts it drops back down to around 50RPM and almost dies. The whole car shakes and finally after about 1-2 seconds the RPM's raise back to normal idle. It's almost like its not getting enough fuel after the start-up. The car runs fine while driving and idling. Did I mess something up with the spacer install? Please help because I would like to sell my Z this summer and its not a good selling point.

(Don't worry I will be getting another 350Z!)

 

Sounds like an air leak or you didn't re-connect the vacuum lines that connect to the upper plenum.

 

But if it's a air leak wouldn't I notice a problem during load? Also, it doesn't have a problem when I first start it in the morning (cold start) only when it's up to temperture.

 

A relatively small air leak will only affect idle or low RPM operation. There are certain enrichments in effect when the engine is cold. Once it warms up and is operating normally, any leaks would be more prevalent.

Check any connectors around the plenum as well. For example, on the back passenger side is where the purge solenoid switch is located. If you accidently disconnected the hose or wiring from it that could cause a problem. I believe there is also a hose from the air intake tube back into the head/PCV valve.