Yo! I got a 1992 Honda Civic CX equipped with a Non-VTEC D15B8, and lately I have been having an idle issue. It goes up and down, and I also have a issue when starting the car in the morning, it starts but dies, and I have to keep giving it throttle until it can run, and when it does run, the rough idle starts. Also when it idles rough, the brightness on the dash lights go down and up with the idle. And when I try to get codes from the Service connector with a paper clip, the check engine light stays solid. Things I have already replaced, IACV, MAP sensor, TPS, Distributor, Spark plug wires (NGK), and ECU. The car runs rich and is equipped with a eBay intake and eBay headers, and a universal catalytic converter. The car also has a misfire. I wanna say it’s the Wiring Harness, or some sort of electrical problem. Please let me know

 

Sounds like all symptoms of a low idle based on lack of air getting passed the throttle at idle.

Try the idle set procedure? Start car with iacv disconnected and adjust idle screw till you get a steady ~750rpm idle. Then plug the iacv back in, and it should be within range for the ecu to put the idle where it should be, a little higher.

The service connector is an odd one, cant say why it would just be solid. Maybe double check the wiring on that, one leg goes to the service check pin on the ecu the other is ground.

 

Page 11-42 of 94 Civic shop manual

 

I may have found the issue, but not sure in what I need to replace. This whole time I have had the IACV and the purge control solenoid valve connectors switched up, BUT here is the problem, when I switched them back, the car would start up rather quick, but the idle would be thorough the roof, I’m talking like 1,500 -2,000rpm. BUT as soon as I un-plug the IACV connector, the rpm drop but still goes up and down. When the car was idling very high I put my fingers on the hole (located in the throttle body) that the IACV sucks in air, and it was like the valve was stuck open when connected. Then I disconnected the iACV connector and did the same thing, put my fingers to block the IACV port in the throttle body, and it was still sucking in air even when disconnected (not sure if that’s bad thing, pretty sure it is) . So I’m not sure what to replace/inspect. Do I get a new IACV or do I inspect the wires of the connector. BTW I checked the voltage to the IACV connector, and one is 12V and the other is ground (sent from the ecu) but when I checked the ground, the ground was flashing, meaning the ground appeared then disappeared, then reappeared. So that’s why im not sure what to replace/inspect.

 

With engine hot....

We'll first, test to make sure that the engine stalls if you cover idle hole with your finger. I believe this one hole feeds both the IACV and the idle bypass (set screw) which is why you are still pulling air when you un-plug the IACV. If the engine stalls, then your good. If it doesn't stall, then you most likely have a manifold vacuum leak somewhere that needs to fixed.

Next, make sure to try base-idle set procedure: Fully warm engine, unplug the IACV, start engine (may require a little throttle help).....if out of spec, adjust your idle screw per the manual (480.+/- 50). This sets the base amount of air bypassing the IACV. CX doesn't have a tachometer so this may be tricky. It's also tricky even if you have an OEM tach cause the first notch on the tach is actually 500rpm right next to zero.



If all that checks out, then simply reset your ECU by pulling the memory fuse for 30 seconds. Then just drive the car. If the idle is still bobbing up and down, it could be due to the ECU trying to learn a steady idle. My 94 takes about 2 days of driving to work before it settles in. Always has since I bought it in 99. It did that every time I reset the stock CX ECU the chipped P28, and it still does it with the stock P73 JDM ECU.


​​​​​​BTW the manual also says to re verify your base ignition timing after performing an idle screw adjustment.

 

UPDATE: I bought a used OEM Honda IACV and I had the same problem, were whenever I plug in the IACV it revs the engine high (not ideal when cold starting) I’m really starting to suspect that it’s something with the wiring harness. some things that do confirm my suspicions of it being a wiring/electrical problem is that my radio keeps resetting itself into demo mode (yes I deactivate demo mode but once I turn off the car and turn it back on it informs me that I’m in demo mode) and another confirmation that it’s a wiring/electrical problem is that when I try and pull codes (with the paper clip method) the CEL doesn’t blink at all, and yes I do have a CEL present when driving, and I know that if it doesn’t blink, it means no codes are stored, but clearly I have a CEL. Also the dash lights like to dim and light up again when the hunting idle is present. So if anyone who is experienced with wiring/electrical issues please tell me how to diagnose this issue I’m having with my civic 🙏