When i remove the "cold start air assist" hose the idle surge vanishes but the car takes a second to start its cold start and slowly climbs up so i know this isn't normal. Ive been searching for this idle surge for years... My head says "p2j-1 98" WHAT CAR does this go to? I've looked and there's no info on this head stamped with 98 at the end. My car is a 99 civic ex 2 door coupe auto. If this isn't the original head to my car, it explains the idle surging through the air injection ports.... Someone please clarify..

Ive narrowed it down to this and im 100% sure this is the cause of the idle surge when warm. ALl gaskets have been replaced, checked, sensors swapped. ecu swapped. Vacuum lines all replaced. Its none of that..

 

That year stamp is almost always a year behind the year of the car so a casting stamp of 98 is typically what is found in 99 car's. If you take a moment and look at the dealerships, the 2015 cars are sold in 2014, 2016 cars are sold in 2015 etc etc. Thus the manufacturer stamps are almost always the year before the year of the car.

And the P2J is the D16Y8 head code as can be seen here:

https://honda-tech.com/forums/tech-misc-15/d-series-head-codes-786093/

 

What manifold gasket did you use, 96-98 or 99-00

 

Ok so the head is the correct head for the manifold and was never changed. That's good to know. Thank you. So this tells me that that "cold air assist" valve that goes int he thermostat housing is bad. Since coolant runs below where its screwed in i assume its some type of valve that opens and closes with temperature? how can this thing go bad? Car only surges when warm with hose hooked to this thing. All other gaskets are new and yes i made sure i used a 99-00 gasket which has the squarish top extra piece to the gasket. So i'm assuming this valve is bad? Is this possible?

Edited for constant typos