So, after I changed the head gasket and cleaned out the egr passages, I am getting a P0420, go figure, right?... So I followed the ECU reset procedure posted here https://honda-tech.com/forums/honda-civic-2001-2005-78/02-honda-civic-p0420-code-3014147/ by It Wasnt Me. During the revving of the engine part I noticed that the rpm would jump around a bit approx. every second. Here is the video of it:


I was noticing the same pulse driving it on the highway right after the rebuild.

Currently the CEL is not on, and I have yet to drive it around town to see if it comes back on.

there is a slight hesitation in any gear with maintenance throttle, and going from decel to accel slightly produces a noticeable driveline shudder. Has anyone experienced this before?

 

With an inconsistent idle my first thought is the Idle Air Control Valve (IACV) but, the surging idle in the vid isn't really consistent with an IACV problem. It would jump a lot more and not be as rythmic.

I think with the P0420 after the blown headgasket it's much more likely the catalytic convertor is bad and restricting your exhaust. blown headgasket= engine burning a lot of crap= bad cat

 

idling it purrs like a kitten. only with foot on the gas it does this

 

Sorry, sometimes my reading comprehension isn't the best. I saw "maintenance throttle" and thought "idle".

I still think you might have an exhaust restriction (bad cat). It's not uncommon to have the cat go bad (P0420) after the headgasket. Might also be a slight mis-fire, which could be due to any number of things. See if you can get another code out of it.

Here's some good video's about exhaust restrictions and the cat:

 

Thanks a lot for those videos, i will head out to harbor freight tomorrow, i always wanted a reason to buy a thermometer lol