I spent the past month trying to figure out what exactly is wrong with the engine and I am officially stumped. There are no codes and all the data looks ok based on the service manual and some additional online reading. The main issues are the sluggish acceleration and a rumbling in the cabin when the engine is under load.
At cold start there is also a puffing sound coming from the bottom of the engine - sounds like it's purging some air every 15 seconds or so.


Here's what I've done/checked so far:

1. replaced valve cover gasket (done by a shop)
2. Checked cylinders
   1. compression is around 250psi. much higher than what it should be according to manual
   2. some oil on the spark plug tips, replaced them
   3. scraped the bottom with a small screwdriver and got some black stuff (carbon?). Need a borescope to inspect
3. Replaced PS pump due to wobbly pulley
4. fixed intake hose leak
5. replaced purge solenoid and PCV
6. Replaced all mounts except the rear
7. Ran a bottle of seafoam through the gas tank
8. Vacuum test passed

I'm hoping the rear mount fixes the rumbling issue, but I also noticed the idle drops to about 680 rpm when I put it in drive. In park it's around 750 +/- 20. The throttle position is 9.8% fully closed. B1S1 O2 sensor reads about 0.6v at idle which I thought was pretty high. ST Fuel trim also jumps to about 12% frequently. All measured with an OBD scan tool btw.

So any thoughts or suggestions on what to do next?

 

A few notes on the video: you can hear the puffing early on when camera is on the driver side. it's like a pff-pff sound. Also can hear the air being sucked into the engine, it seemed unusually high to me but it does die down once it's up to temperature.

 

Maybe check the cat. converter next?

 

Hmm...I expected a CEL if the cat was bad.

according to this article though it's worth looking into. I do get a blast of unusually hot air from under the passenger door when I'm down there - not sure about the smoke or smell but I'll check again when I get home.

thanks!

 

I don't remember if I had a CEL when my cat went bad, it's been awhile. You would hear it vibrating every time you would start the car, and a little bit under acceleration.

I didn't have the same symptoms as you though. The honey comb had completely crumbled, but I could still hit VTEC perfectly fine.

 

Test pipe's are relavtively cheap. Although wouldn't recommend it, the raw exhaust smell can be pretty bad.
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In fact that's why the one on the cls was quickly removed. Like to be able to pull up to a drive thru without gassing people out. lol

 

You're saying you like to be considerate of others and not be that civic at 2AM? Psssshhh, poser.

 

lol... a 3" cat is still a cat right?

 

It is unless you gut it.

Reading your post makes me not want to to do a straight pipe now.. but the flames! The flames! But I will, maybe I won't. Gotta figure out the fully welded exhaust issue first.