I have an F22a1/H23a1 hybrid engine which I just finished putting together last week, so I'm trying to get the last of the kinks out. I've got everything worked out except this problem. For a while it was that the throttle body would get stuck open, but I got that fixed. I thought it was something wrong with the ECU, but I put the stock one back in and it still did it. A lot of people have said that it could have to do with the fast idle valve, but I have the FIV port blocked off, so that can't be it. What else could it be?

 

have you adjusted the tps yet? Did you change the intake manifold gasket or any other intake gaskets during the swap? Did you torque the intake manifold bolts in sequence?

Im going out on a limb here, but maybe when the intake manifold heats up after driving for a while, it distorts enough to create a vacuum leak. or maybe im on drugs