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Discussion Starter #1 (Edited)
I run across this SO often I am just going to make a DIY article.

Honda ECUs hate to idle fast, they do some crazy stuff to try to get the idle within the correct range, and it can cause serious driveability issues and CELs and just make the car VERY annoying. Just remember, the car doesnt hate you, no matter what the hunting/surging/ craziness may seem like, your computer is trying to slow the engine down but cant.

First off, no matter what code it gives you: IT IS NOT A BAD IACV. IT IS NEVER A BAD IACV. Sometimes (rarely) the issue is a dirty IACV, sometimes IACV wiring is cracked or bad, but unless your IACV is visibly damaged, dont replace it until ALL other possibilities are exhausted. If you are sure that your IACV is bad, please PM me and I will give you my address to send your "bad" one to, because I have NEVER seen one go bad, and I am always up for a learning experience.

Reset your computer.

Then check to make sure your throttle cable isnt to tight.

Next close your idle screw in your throttlebody all the way.

Check if your engine's idle airflow is correct by unplugging or blocking the IACV: it should now idle 400-600 rpm. This is with a warm engine & the idle screw adjusted properly. Anything over 600 rpm means you have an air leak somewhere.
Next, check for vacuum leaks, use whatever method you like. I like the smoke method if you have no clue where the leak is, but flamable brake fluid or starter fluid work too if you are careful and have a general idea where the leak is. There is TONs of youtube videos on how to find vacuum leaks.

If you think you have a leak but cant find it, look up pictures of your intake manifold and locate all the ports, most are pretty obvious, but there are some ninjas, the y8 intake in particular has one low on the driver side that is easy to miss. At this point I start plugging the ports, something external like a leaking brake booster diaphram or bad PCV valve can cause a leak that can escape notice pretty easily, but if you disconnect it and plug the port, the problem should change if not go away completely.

Next take off your IACV and spray it out, I like to use carb cleaner or brake cleaner, just spray it out until the screen and liquid flowing out look relatively clean.

Next, check your base timing, advanced timing can speed up the idle enough to cause an issue.

After that it is time to adjust your throttlebody. I HIGHLY suggest you take the TB off for this. Take the throttle cable off the TB and clean out the inside of the TB, take a toothbrush or something similar and make sure it is all clean, especially near the throttle plate. Once it is all clean, back off the throttle stop set screw and open and close the throttle, once the throttle starts sticking slightly, slowly close the set screw until the throttle no longer sticks, close the throttle quickly a few times to make sure it isnt sticking. Your goal is to have it as closed as possible without the throttle plate sticking in the bore. Tighten the lock screw and recheck to make sure the throttle doesnt stick even when snapped shut.

Before you reattach the throttlebody to the intake, attach the TPS sensor wire to the TPS, and turn the key to the ON position (do NOT start the car) and then check the TPS voltage when the throttle is closed to make sure it is within spec (0.45-0.5v for most older hondas, dont know about newer ones), Adjust as needed to get within closed spec, dont concern yourself about the wide open voltage to much.

And if it won't idle like this, check valve lash
Once you are definately vacuum leak free, your IACV is clean your base timing is correct, and your throttle plate is definately closing all the way, and your throttle cable is not to tight, and your TPS is within spec for the closed position, and your valve lash is within spec, you should be good.

If none of that fixes your issue, it is time to check your mechanical timing. Look up how online or in the FSM for your vehicle, I am not going to cover every honda here.

If NONE of that works, check it all again. If you are still having an issue, PM me.
 

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Discussion Starter #7
warm car and unplug the iacv. The iacv should close and the idle should go to the normal range. If it doesnt then the cars idle needs adjusting, vac leak, bad iacv, or bad wiring
simple
Except you missed at least half the potential problems.

With how often this issue comes up, some folks can likely benifit from a more complete answer.
 

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the artist formerly known as drexelstudent11
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Great article. I'd only add 1 thing:

FIRST: Check if your engine's idle airflow is correct by unplugging or blocking the IACV: it should now idle 400-600 rpm. This is with a warm engine & the idle screw adjusted properly. Anything over 600 rpm means you have an air leak somewhere.
And if it won't idle like this, check valve lash and then mechanical timing.
 

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94 Integra
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In all honesty 99% of the bouncing idle is from air leaks. Valve lash & timing doesn't cause bouncing idle (correlation does not imply causation), those problems just prompt quick "fixes" that actually disrupt normal idle/IACV funtion.

One thing I've noticed: if you take a car with bouncing idle & drop the RPM with load (slip the clutch while holding the brakes), the idle stops bouncing sometimes when the RPM's are forced below 800rpm for a short time. A massive leak won't allow that, of course. It also cranks the IACV open exptentially when there's a blockage, so it basically overcompensates.
 

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95 2dr coupe
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i had a minor surge issue with the mini conversion I completed yesterday. Embarrassing in traffic when people look at u like your trying to be all hard and revving motor. Swapped throttle bodies and prob eliminated. TPS or idle screw as mentioned in thread, though I have seen low coolant surge idle out too- it does flow through and have an effect on tb sensor
 

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94 Integra
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What about the FATV? Couldn't this be an idle issue as well? Especially at start up?

Or should I just block it?
If it's not adjusted properly, yes it will be an issue. If you have a STOCK ecu, then simply adjust it properly. There are a million write-ups on how to do this with 2 basic hand tools. If you have an advanced EMS like NepTune, eCtune, or S300 then you can block it. Those 3 have the right code to let the IACV do the work of the FITV.

Blocking it has virtually no down sides if your engine is in tip-top shape, but most 20+ yr old cars rarely have a motor in showroom-new condition. If you have Crome or a stock ecu, it may run poorly til warmed up on very cold days even if your motor is in good condition.


though I have seen low coolant surge idle out too- it does flow through and have an effect on tb sensor
It's the same as an open FITV, since the ECU thinks it should be closed but it's creating a big leak instead. Helpful but not relavant IMO since all basic maintenance & fluid levels should be checked before trying to diagnose any motor issue. Otherwise that's like checking everything besides your rubber & rim to figure out why you got a flat tire.
 

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90 crx si/96 accord
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Is this directed more towards the older honda's ...I had to replace the iacv on a 03 si with the k20 in it a couple weeks ago.
 

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90 crx si/96 accord
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From what identifix is reporting, yes there some failures of the iac's.

I could type out my experience, but long story short, after completing the troubleshooting procedures, as well as the iac monitor test sequence with a verus pro (failed 2 of the tests) I did end up replacing it. Mind you the ENTIRE time in the back of head I was thinking no way, honda iac's don't fail. Replaced iac, symptoms were gone and returned to customer nearly 3 weeks ago .

The only reason I'm bringing this up is because in your first post you state "it is never a bad iacv" ...that's a pretty bold statement lol
 

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Discussion Starter #18
I wrote that because there were like 5 threads in fairly rapid succession where people had some idle issue and the first thing they did was buy a new IACV and then wonder why the issue was still there. Even with me saying something like: It is VERY unlikely to be the IACV... People were still going out and blowing cash on new IACVs when what they had was a simple hunting idle from a vacuum leak.

And to be fair I did follow it up with this:

If you are sure that your IACV is bad, please PM me and I will give you my address to send your "bad" one to, because I have NEVER seen one go bad, and I am always up for a learning experience.
I swear to god that if honda had just named that the Air Control Valve and left out Idle they probably would have sold like 1% as many replacements as they have.
 

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warm car makes idle pull down to the point where its puttering and sounds like im missing on cyl 1 either low spark or bad compression ? exhuast valve or somthing not sealing /?



my iacv was cleaned and looks great
nice guy at junkyard gave me one and both have no change effect
map sensor changed
this wasnt issue in winter months ago when i bought car but it was hard to start and got valves adjusted they were tight


was starting pretty spot on and now starting is a jerk its like a old person taking a low breath and then it turns over not even loudly either its all quit and im like did it start other times it cranks and cant start not even sputter but it always starts on 1st 2nd or 3rd try but never 3 trys in row hardly ever .... that bad cly idea?

but seems to have good power other wise shaky at very low rpm too

so now i think its the dizzy maybe hoping low spark to cly 1 when its 115degrees outside and i drive around yes no lie vegas heat comming to a stop idle drops to unacceptable rpm i really dont know how the engine doesnt die instead it violently shakes and sputters fans kick up the idle and other times the fans running dont affect idle things is fans for ac and rad work sensors checked obd1 d15b7 engine no egr system org intake with pcv to intake

the o2 sensor looks new and the cap and rotor are clean

idea now is maybe change the thermostate maybe its stuck open cause the dash temp never reads half way or hotter ?? but if i do have a bad cly and i get one and it closes and raises coolent temp and give me a head gasket issue lol that would suck ...

since right now no smoking
no burning oil
no coolent mixing
134k but untrustworthy cause engine was swapped before
as far as it seems i have id the intake mani the head and the block as all matching and dizzy to be correct for setup

thinking also maybe change the coolent sensor 2 wire one that feeds info to the ecu ?

or get a timing light ND MAYBE DO SOMTHING I REALLY SHOULDNT BE DOING??
idle relearn process doesnt fix this it still eventually pulls down the idle while at stop lights in the heat of the day even tho no fans or lights are being used too

ive come to the idea
the engine bad cly 1
bad cly 1 from intake or exhuast valve leak or damage from previous owner

bad valve timing out of spec again within 10k ??
bad ignition timing?
bad o2 altho looks new
bad coolent 2 wire sensor altho no other seeming issue
change thermostate since dash never reads past halfway mark ?

but also this car has a 8200rpm si dash and this is a d15b7 but would that matter? verifyed p06 ecu doesnt look molested

have 4 wire o2 sensor
and 2 wire iavc sensor fitv doesnt seem bad i have no issue at cold start

i dont have hunting idle issues either

i cant use this car now cause while im at any stop light it like the car is having a heart attack seizure rpms seriously dip to like 400ish and the car sounds like its burping out the backend misfiring no loud poping but chugging really badly and removing spark plug 1 i can see spark but engine has hardly a change while other plugs cause more violent shaking when being removed then cly 1 did ---

weird thing is when its cold or maybe in limp mode not using o2 while warming up sounds pretty healthy and roms are steady at 750-800 super sweet and nice


when the rpms are low take off into 1st is somewhat of a personal art and is tricky thing to pull off without a super slow take off or a jerk back and then a slow to fast acel so timing at that part seems off or somthing ??


im no expert but i have def have more then i care to deal with on this car its making me super angry .... i mean if i need to get the head off and send it out for porting or somthing to fix and change the valves whatever but thats extreme im hoping theres a better solution
 

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BATSLOMAN GIVES NO FUCKS.
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thanks for the story in a dead-ish thread. have you tried a compression test and leak down test? pulling plugs while its running is a sure fire way to kill the dist. have you pulled the plugs and looked at them? fuel in the oil? checked timing? read this thread an attempted to fix the idle? tried to turn on the a/c to see if it brings the idle up at all? taken it to a shop to have it looked at?
 
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