I just put a cheap aftermarket EX header on my HX. I have an OEM cat and custom exhaust. All the o2 sensors are installed and I'm not throwing a CEL.

But when I cruise around near the VTEC engagement point, (which after some searched, I realized it varies in RPM for VTEC-E) the car hesitates like I'm tapping the bakes while giving it gas. It seems to do this in every gear if I cruise at 2500 to 3000 RPMs.

I've checked the brakes the best I know how. Checked the fluid and visually inspected the rotors and calipers for clearance issues. The car runs fine the rest of the time. And now I drive at around 2000 RPMs. But I know the car shouldn't do that, and it didn't do it with the OEM manifold.

What could be causing this?

CLIFFS: Car hesitates after header install at certain RPMs.

 

How cheap of an exhaust? How well do you know your engine? My guess is your exhaust caused the problem. See, the HX at low RPMs only opens one intake valve, so swirls the air. When you hit VTEC, it changes. The backpressure in the exhaust changes. The HX was designed with a wideband O2 so it runs lean a lot for fuel economy. It might want more backpressure for better emissions and MPG. Even on a EX motor, when you add power via intake and exhaust, the VTEC engagement is more prominent. Dyno test it, with VTEC on all the time and off all the time. Check the tq and hp curves, and see if there close around 2500rpm with your setup.