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Discussion Starter #1 (Edited)
So this is my first time posting, sorry if there are any mistakes.

I have a 1999 Honda Prelude with 131,000 miles. I have replaced the idle air control valve 3 times this week. The problem starts with the cars idle jumping, usually from 1000 to 1200 or 1500 to 2000. This idle jumping soon sends me a check engine code for the IACV. Whenever I clear the check engine code the car goes back to driving like a fun enjoyable sports car with no problems. The check engine light usually goes away for 5 - 10 miles before coming back. I have replaced the part with 3 times, each time was a brand new part. If anyone knows why it's doing that or a remedy for that, please tell me.

Sincerely,
Ali


FIXED

Check your idle set screw and adjust accordingly to the diagram below, I also found another fix. Replace the MAP sensor if replacing the Idle Air Control Valve doesn’t fix it. Also check for loose grounds and vacuum leaks. BEFORE YOU BEGIN TO REPLACE PARTS, unplug the throttle position sensor, it should be on the throttle body towards the fire wall. The vehicles idle should go up, but the bouncing idle will stop, if your bouncing idle stops, you need to replace the throttle body.

-Check Idle Set Screw
-Check Idle Air Control Valve
-Check for Vacuum Leaks
-Unplug MAP sensor (on top of throttle body) (engine should drop idle if part is good)
-Unplug Throttle Position Sensor (if idle stops bouncing, replace throttle body and all sensors connected to it)
 

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vacuum leak somewhere. check all the hoses connecting to your throttle body. Just fyi, I deleted my IACV and ive never had CEL come up. start with evaluating your vacuum lines. the idle jumping back and forth is usually caused by a leak somewhere. make sure your oil cap is sealed and your dip stick is as well.

*oh and check your coolant levels. that can cause the idle jumps as well
 

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upon reply, I can make a quick video for you to guide you on what to look at. But as I said, focus on the vacuum lines. They run throughout the engine connecting to odd places such as your windshield wiper tank.
 

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Discussion Starter #4
Can you please make a video or give me instructions on how to check my vacuum lines, and which ones to check. I already checked the coolant and topped it off, which didn’t change the circumstances. I also didn’t know that the windshield wiper tank had vacuum line running to it, can you tell me what it does?
 

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Discussion Starter #5
I also forgot to tell you, under heavy acceleration it doesn’t do that idle bounce thing
 

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Can you please make a video or give me instructions on how to check my vacuum lines, and which ones to check. I already checked the coolant and topped it off, which didn’t change the circumstances. I also didn’t know that the windshield wiper tank had vacuum line running to it, can you tell me what it does?
sure. but before i do. can you take a picture of the throttle body area for me. before the symptoms began, did you do anything at all to your car?
as for the windshield reservoir tank, i havent had any issues in that area and not too familiar with what it does. I just found that out while I was sorting out a different problem, thats why i mentioned it cuz its such an odd ball that no one really knew that could affect the vacuum lines. whether that could or not affect or be the issue I cant say for certain though lol
*in addition to that, there are vacuum lines that run down to your rack and pinion boots to relieve pressure. you can check that by turning your wheel to one side and look under. its a very small rubber tubing fastened by a metal clip
 

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Discussion Starter #7
Sure thing, I’ll take the picture tomorrow in the morning, and I have not done any work before the symptoms began. The only thing I’ve done was replace the IACV 3 times.
 

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Sure thing, I’ll take the picture tomorrow in the morning, and I have not done any work before the symptoms began. The only thing I’ve done was replace the IACV 3 times.
yeah stop that lol especially after the first time. those things dont go bad lol just remember this: CEL indicates abnormal parameters, and does not mean that part needs to be replaced. Its often that something else that causes the reading to go off and triggers the CEL with the area/part that needs attention.
 

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Discussion Starter #9
Got it, well I’ll have the pictures upload in the morning and hopefully we can get this situation sorted out.
 

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tomorrow, have the engine running and follow the vacuum lines with your hand (no gloves) to feel and listen for leak.
Theres a possibility that someone would suggest its your EGR valve. dont replace that right off the bat, im pretty damn sure its not that. Moving forward if its not a vacuum leak, I would start looking at the throttle body as it is most likely gummed up. a partially open/closed butterfly can confuse the crap outta your ecu and can send wrong air/fuel signals to compensate. Sometimes people find their engines to spike up and down right after they cleaned out their throttle bodies. Youd have to reset the ecu and run the engine as is so it can get familiar with the correct position of the butterfly.

*also no harm in checking your pcv (L shaped thingy installed on top of your valve cover where the line runs to the throttle body), i mean just replace that if you like as its only a few bucks. careful with the rubber plug.Replace if cracking or shows any sign of deterioration
 

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Discussion Starter #11
I checked all the hoses and the pcv valve, they were all fine. And I was wondering to check the butterfly flap, I would just take the air intake off and look into the throttle body right? (Does car need to be on or off and if it is dirty how do I clean it). Because today the check engine came back and after clearing it, the car drive normally until 8 miles later when it came back). When the check engine comes back it’s a very fast bouncing idle and it starts to smell like gas (my fuel also runs out faster when check engine is on). Also, when I clear the check engine the car isles at 1500.
 

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1500rpm idle is too high assuming engine is up to temperature. Once engine is up to operating temperature, these engines idle at about 750-850rpms depending on transmission type. High idle or bouncy, surging idle is classic symptom of vacuum leak or throttle plate stuck partially open from carbon deposits. Fast idle thermo valve could also be out of adjustment. H series are well known for idle issues and there are plenty of great step by step write ups in this forum.
 

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Discussion Starter #13
Could you please provide me with the links on how I could test each and every possible symptom, as well as how to remedy it.
 

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Discussion Starter #14
I solved the problem, the idle adjust screw was incorrectly set (probably unscrewed itself over the years). The car now idles like new, and no more bouncing idle!

I will have picture of how to set it posted. Hint: Turning the screw to the right lowers the idle speed, turning the screw to the left raises the idle speed. (essentially a custom vacuum leak)

27067
 

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Discussion Starter #15 (Edited)
I also found another fix. Replace the MAP sensor if replacing the Idle Air Control Valve doesn’t fix it. Also check for loose grounds and vacuum leaks. BEFORE YOU BEGIN TO REPLACE PARTS, unplug the throttle position sensor, it should be on the throttle body towards the fire wall. The vehicles idle should go up, but the bouncing idle will stop, if your bouncing idle stops, you need to replace the throttle body.
 
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