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Hello! My 2004 Honda Civic Si has had a bouncing idle ever since ive had it(a week). I cleaned the IACV and burped the coolant but neither changed the idle at all.

I reset the ECU by removing the battery negative for 15 minutes and then reconnecting and letting it idle for 10 minutes and restarting the car. After that it drives fine until I restart the car again, where it then returns to the exact same idle issue.

I have tried spraying carb cleaner literally everywhere in the engine tb area looking for vaccume leaks, but it doesnt affect idle at all.

The fact that it runs fine until the next restart after a ecu reset seems strange to me. What should I do next?

 
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should also mention that it only bounces the idle once its warmed up. Cold starts idle fine for around 10 minutes

 
Rep Power: 125

I'm guessing it may have a plugged cat or something else exhaust related. There should be a code for something. Even if the CEL isn't illuminating, get a code reader on it.

 
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also forgot to give yall the P codes. Codes currently are: P0507, P0453, P0420, P0134, P0137, P0325.

Is there any chance its that O2 sensor code that seems to be located on the exhaust? I believe i read it senses fuel/air mix?

 
Rep Power: 125

That would certainly help.

I can comfortably say that every code the car has will contribute to your issue. Your catalytic converter is probably the cause for half these codes, with the rest being a knock sensor plus some small things.

Were you aware of all this when you bought the car?

 
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i knew it had the hunting idle, the rest he didnt tell me. I forgot my code scanner at home and we were already an hour out when I realized it. That definetly isnt great as im assuming thats an extremely expensive repair.

 
Rep Power: 125

Hard to tell. I'd confirm first that it's actually the issue but it looks to be rather conclusive.

Typically, a rule of thumb is to determine the issue before purchasing the car. Hopefully this will be a cheap enough lesson.

 
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Hi, I ahev a 2002 honda civic which i.was driving and engine shut off car still have power key light keeps flashing not sure if its immobilizer or bad ecm, I open up.the ecm checking for water or burns on the board and there's nothing I need help and the fuel pump isn't coming on

 
Rep Power: 125

Start your own thread. Completely unrelated to what's going on here.

 
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Yea im not too upset. Was pretty cheap relative to other civics/cars in my area. Is there a way to make it drive better until I can get it to a shop in a couple weeks? Maybe unplug one of the sensors? I remember hearing to do it many years back when my parent had a clogged cat, dont remember which one to unplug though. Especially on a civic.

 
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and is there a way to 100% confirm that the catty is the issue before I try replacing it?

 
Rep Power: 125

Loosen off the exhaust manifold and see if the idle cleans up.

 
Rep Power: 367

https://www.google.com/search?q=clog...&bih=938&dpr=1

 
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so i just tried running it again(the next morning) and it DOES idle steadily now similar to that drive after resetting it. However, its idling steadily at 2,000 RPM. The battery reset last night cleared the codes out, however the "high-idle rpm" and the "evap purge" codes did come back.

Im not totally sure if its the catty though! The engine doesnt bog down during accelleration at all, and its code hasnt come back after 4 hours of driving. I did use a OBD2 scanner to check my O2 sensors and they do report different voltages as I drive(not sure if that means theyre functional).

 
Rep Power: 125

That's something that will return with time. P0420 specifically. That's the one I want to place emphasis on because it's almost always an issue with the catalytic converter.

 
Rep Power: 224

I don't care how well you cleaned the IACV, my money is still on it.

 
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think its worth replacing it entirely?

 
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Don't buy an aftermarket one, grab one out of a junkyard if you don't want to buy new oem

 
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why shouldnt I buy a aftermarket one? Id assume that have a history of not working with the civics?

 
Rep Power: 125

Aftermarket for civics have a tendency to fail. It’ll be more expensive to get OEM but it’s worth it.

 
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Just removed and re-cleaned the IACV again. I noticed a date on it that makes me think the previous owner replaced it with a aftermarket shortly before he sold it to me...? I have no clue what to think of that, idle still bounces though.

However, I did unplug my EVAP Purge Solianoid(the one on the oposet side of the TB to the IACV) and the car runs smothly and idles perfectly. Ive read when they fail they creat a vacuum leak in the tb. Since I wont have money to further investigate this issue for a few weeks, can I leave it unplugged for a week or two without it leaking gas or some ****? Im not entirely sure what the hose leading to it even carries. Google suggests its fuel vapors? Any info is appreciated!

 
Rep Power: 367

again, to clear the suspicion of the plugged cat, just loosen any part of the exhaust before the cat and let the exhaust gases escape to the atmosphere. If it is not plugged, should not cause much change

 
Rep Power: 224

It won't hurt anything to leave it unplugged, obviously it will throw a fault. But, if the purge solenoid were bad you'd have a code for it. None of the faults you listed have anything to do with EVAP. In my CRV (also K series) I've had the purge solenoid completely blocked (by mud) and stuck open (failed solenoid), neither case caused an idle issue.

When I first bought this CRV it had an idle issue. Idled okay until it got to a certain temperature, then the idle began surging. The cause......IACV was completely frozen in place. A new Honda genuine IACV is expensive, like $300. So keep testing to narrow it down.

 
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Alright I finally got a jack. I loosened the cat and still has a bouncing idle, although it does seem to go down like 250RPM lower when it drops before bouncing back up, so idk maybe that had something to do with it. Like I said, the IACV looks like it was already replaced with a aftermarket and I also tried cleaning it twice now, but there wasnt any carbon or anything in it either time I cleaned it(i still used a Q-Tip though and srubbed the it with MAF-Cleaner) Any other ideas on what it could be? At this point im starting to worry. Sprayed around the engine and intake area again with carb cleaner, still no vaccume leaks.

 
Rep Power: 367

if you still have it loosened, you can try to see if it looks like any of the ones below

 
Rep Power: 222

Need to try an OEM iac

 
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i just cracked it loose so that air could escape. I would have to completely remove it to check inside unfortunately. Unless I remove the outside end from the cat and look in that way? Could this be a vacuum leak that I havent found or a PCV valve? Is there somewhere outside the throttle body area that a vacuum leak commonly occurs?

 
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Id like to rule other stuff out hopefully before spendiing a couple hundred on another OEM IACV.

 
Rep Power: 367

cheaper than buying a new cat without knowing if it is plugged, in my opinion
Just keep checking items off the list

 
Rep Power: 367

Just to be sure, in US and Si model, so you have a hatchback?