P1399 Honda - Prelude only goes on when passing 3500 rpm or 55 mph

Question

have a 1998 Prelude with a jdm H22a swap in it. Has around 55k miles and drives good. The car runs perfectly fine and strong. The light only comes on when I get to about 3500 rpms or over 55 mph. It starts blinking until I slow down. Even when the light is on i cant feel any misfire and the car runs the same. I have no loss in power, hesitation or any of that. This code has been haunting me and I nor 5 different mechanics can figure out what it is. I have changed the spark plugs/cables, cap and rotor, pcv valve, fuel filter, egr valve, iac valve, catalytic converter, cleaned the egr ports, throttle body, air filter and got a valve adjustment.

Answer 1

P1399 Random misfire

This is a induction condition dealing with the EGR system, When the EGR operates it uses inert gas or exhaust gases to control Nox. When you have a mifire you get a incomplete combustion , the EGR uses exhaust gases for nox control and now it is introduced into its chamber a incomplete combustion that can now support cumbustion and as a result you get P1399. The reason you have P1399 is the speed or the load where the misfire occurs. At 55 mph the engine is at its optimum. P1399 when this occurs on a Honda or Acura these are mostly system that have a VTEC.

 When dealing with P1399 mainly with Hondas  what has been found would be a partial plugged EGR port open for cylinder 4 and clyider 3 and a misfire code condition with cylinder 1 and clyinder 2, With the freeze frame saved it was always at load, and never at idle. This one is a Honda Accord, when it coded a P1399 with a P0171, what threw us off was the fuel pressure regulator leaked fuel into the intake from its vacuum hose connection. You would  expect to see a running rich code due to the fuel leakage into the intake, But this time the 02 saw it as a lean condition,