Hi! I suffer from the same problem for a long time now. The car is a 2004 UK AP1. A vacuum leak seems most likely, but after a year of searching i can't find one anywhere.

Until now I already changed:
- the fuel pump w/ sock
- the fuel hanger (with the filter inside)
- the fuel tank breather valve on top of the hanger
- both fuel hardlines
- the fpr and fuel damper on the rail

The car has an EGR delete, but i checked all lines to hell and back and can't find any leaks.

The car hasitates under full load and is kinda sluggish until nearly 8000 rpm where it feels like you fire a rocket. There is a very noticeable power dip at VTEC too. Next step for me is to change the O2 sensors since i already got them and do a valve adjustment. The car is around 80.000 miles.

 

The code is gone. My mechanic replaced the fuel pump and regulator as a final hail mary. This did the trick. It was the last thing left to try even though the fuel pressure checked out per the Honda manual specifications. I cannot fully explain why the issue was intermittent but the code is finally gone. I cannot say that this is the same issue that others face but this was my final problem.

 

I changed the O2 sensors but nothing is different.

The car does idle MUCH smoother after the change of the fuel hanger. I broke it open and the filter inside was really nasty.

The problem still persists though. After the sensor change i reset the ECU and let it relearn idle.

After that my STFT is between 10 and 15 and and LTFT over 10.

If i pull the return line from the fuel pump it's under quite high pressure and i hear and feel it when i remove the line. Is that normal? I did hear the fuel pump more and more while i did the idle relearning process. If i release the pressure of the return line it goes back to being relatively quiet.

Does my fuel circulation create back pressure? Is the pressure in the tank too high? If so, how could that happen/what could cause it?

 

I am not an expert on STFT and LTFT but I would recommend getting the Honda S2000 fuel pressure gauge and hose to verify the pressure that you are seeing rather than "quite high pressure." Any fuel injected car will have what is perceived as high pressure. This check is called out it the Honda service manual. Part numbers 07VAJ-0040100 and 07406-004000A.

I would imagine that the recirculation line should not see an elevated pressure since the downstream tank should be at atmospheric pressure but once again, I am not a mechanic. The regulator should keep the high pressure fuel line able to supply the injectors, but then spill back at atmospheric pressure back to the tank. The low pressure fuel return line will see flow, but not a high pressure.

Have you changed the fuel pressure regulator? I have not seen that in your list above in post #31.

 

I had a lean fuel trim issue a couple of months ago, I replaced my fuel pump and it cured that condition.

 

That is the summary of what happened to me as well. The fuel pressure was fine according to the procedure in the Honda service manual with a brand new Honda gauge. I guess once more flow is needed and not just pressure when the vehicle is in motion, the problem occurs.