I bought a 2001 Honda Civic EX. 1.7l Vtec / Automatic.

I've had surging idle issues since I bought it. When It warms up fully, the throttle is jerky. It's either on, or off. Ended up fouling the plugs. There is an accident and head rebuild in the car's history.

So far:
- When either the TPS/IACV/IAC is unplugged, it maintains a 1500 rpm idle with fast surging.
- We tried starter fluid/Brake Fluid to find vacuum leaks, and there is none that we could find. (Done twice)
- Drives well when cold, though idles high, once it's warm it bucks.

I've gone through:
- Ignition Coil (cyl 4, just in case)
- New Plugs (cylinder 4 misfire, the plug was fouled. The rest were fine.)
- New IACV (I cleaned the old one initially, no change)
- New IAT Sensor
- PCV
- Throttle Body cleaned, (I made sure not to destroy the ring on the bottom of the butterfly valve)
- New vacuum hoses.

It's throwing a code for the downstream O2 sensor. The catalytic converter has been removed, and the sensor cut. I believe this is the source of the idling issue. Has anyone else had this specific issue? I am debating whether to just buy a new catalytic converter and downstream sensor and see if that is the issue, or if there is a "flash" or "tune" that could correct this. That leads me to ask, are there any specific remedies for this issue that doesn't involve buying a cat?

I got the non-fouler and O2 sensor mounted into the downpipe. We did the idle re-learning process. We drove it around for about 20-30 miles, and aside from some transmission shifting issues, it drove fine. The stuttering and bucking were gone. (It was also cold this morning, but the car got to operating temperature -- Also a mix of regular and heavy-footed driving to see if the car would act up) It got rid of the CEL, but the idling surge remains. It doesn't activate the CEL, but it's still P0505. -- I parked it for a few hours, and now the p0505 is throwing an engine code, and the bucking/stuttering is back with a vengeance. I also wonder if the shifting issues are related to the engine surge, or if that's just another thing that I will have to repair.

Any thoughts?


Update 4/2/23 : I replaced the throttle body with an OEM one, fully dressed. It had Denso sensors, and it also had the bottom of the butterfly valve cleaned. Usually, there is what looks like paint at the bottom of these, this one had it entirely cleaned. I don't necessarily know if that is an issue or not. But It completely fixed the issue. ALSO, with the p0505. the transmission would not shift into first gear, now that It's idling normally, first gear is back.

tl;dr OEM fully dressed throttlebody.

 

Anyone?

 

Have you actually put a new oxygen sensor in ? You should only be running Denso oxygen sensors. With the new IACV installed, you can disconnect the connector and check base idle. It should be 550ish. If not then it's possible someone messed with the base idle screw.

 

I have both. The old one didn't throw any codes, the new one was attached to the downpipe via the non-fouler trick. I don't believe that my model has a base idle screw. I believe it's electronically controlled by the ecu.

 

2001 LX
I had this code and it went away after cleaning IACV with some throttle body cleaner and Q-tips. Did you replace it with a Denso?

 

I cleaned the original thoroughly, then bought a replacement from O'rileys. It was $180. I don't remember what brand the O2 sensor was(Bosch?). It was from O'Rileys as well.