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IAC Valve??

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Ok, so i'm definitely not pro at this, so i'm gonna need some help from u guys. I have a 98 honda civic ex coupe. i just had my d16y8 engine swapped with a d15b. it seems like i had a busted rod, so replacing was the only option. so....about 3 days after the swap, my car started "hiccupp-ing" between 1500 and 2000 rpm. it wouldn't happen at a specific time of day or when the engine was hot/cold. It was random. And it didn't happen all the time either. When i had noticed it, i took it in and they said it was probably the IAC valve and i needed a new one. supposedly, the one that was installed was my old one from the d16y8 motor. idk. it would stop for a few days, then do it again. after about 2 weeks (which is the longest that its lasted w/out doing it), it just did it this morning. I stopped...turned off the car, then turned it back on. IT STOPPED! ugh...i just need to know what you think it might be. is it really my IAC Valve? Again, i don't know much about this stuff. Pleeeease give me ideas. Thanks <3

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I'm not a Civic expert, but I thought the Idle Air Control Valve (IACV) was there to steady the idle by allowing air to bypass the throttle valve. I don't understand why this has anything to do with "hic-uping" between 1500 and 2000 RPMs. 1500 and 2000 RPMs is well above idle speed which indicates this is happening while you apply throttle. As such, the IACV is not even in play when this problem happens.


I am curious if the check engine light (CEL) is on. If not, I would then be curious if your ECU is performing the initial diagnostic successfully: CEL comes on when you start it up and then goes out after a few seconds; make sure the bulb is not blown. If the ECU seems fine, then you may have a spark/fuel/timing issue since stuff like a dirty Exhaust Gas Recirculation Valve/Port, bad Heated Oxygen Sensor, etc. would all make the CEL come on if not for the specific system, then for a Random Misfire.


Spark/fuel/timing issues are basic, but dubious. Start simple:

1. fuel pressure: mid-thirties with regulator, mid-forties without

2. timing: all marks should line up at TDC, that's all you can do about timing*

3. spark: make sure your ignition coil, dist. cap, dist. Ignition Control Module (ICM), spark plugs, spark plug wires, and dist. rotor are good, ANY RESISTANCE OUT OF SPEC MEANS YOU NEED TO REPLACE WITH NEW.


Here are some questions for all the Civic-connoisseurs:

1. can her D16Y8 ECU run the D15B motor without changing the distributor or making wiring changes?

*2. I thought the D16Y8 was an OBD-II motor* and the D15B is OBD-I, wouldn't that mean her OBD-II ECU is trying to adjust timing using the CKP, CYP, and TDC sensors? Aren't some of those in the distributor on the civic?

3. I thought OBD-I motors lacked a CKP or Secondary Heated Oxygen Sensor (there are probably other major differences in the emission systems)?

Edited by James Matteu
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The imported d15b motors are predominantly OBD-II unless specifically requested to the contrary. If she had a professional install this motor I would hope they had the wherewithal to select the proper variant.

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well the check engine light is on and has been way before the engine swap. and they're insisting its the iac. i think they're tryin to pull one on me cuz i'm a chic. not cool. lol. and yes i was told to check the timing by someone else. so i'll do all that first to be on the safe side. cuz this part is like $200 and i just dished alot for the motor.

Edited by 98civichica
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Yes, pull the code before you or anyone else does anything to the car.


If she had a professional install this motor I would hope they had the wherewithal to select the proper variant.


They swapped a new motor in and let her drive away with a CEL on? I am starting to question their wherewithal.

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  • 4 weeks later...

ok, so i took it back to the shop and some other guy adjusted the throttle cables. seems to have fixed the problem. its been a little more than a month that it's been doing it. before it would do it every 4 days or so. so far, i'm pleased. we'll see.

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