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Discussion Starter · #1 ·
My 97 prelude has been idle surging for months now and I have tried every single fix possible that I could find and i’m completely stuck. It happens the moment the car starts, it will jump to almost 4000RPM, and then cut down to 1000RPM, and then jump back up over and over again and it does not stop even when the car is warm. The surging happened randomly when I started the car one day, and it has worsened over time
Here’s what I know so far:
-I have no vacuum leak
-MAP and TPS are working properly
-No air in coolant
-FITV is brand new and working properly
-IACV is cleaned out
-All IM gaskets have been replaced
-All vacuum lines have been replaced
-EGR valve and ports are cleaned
-The car has not overheated (at least since i’ve owned it)

The idle surge will lower when the IACV is unplugged, but it still surges to 2000 instead of 4000.

It has an H22a4 with full exhaust, stock intake manifold with an aftermarket intake tube, and a VTEC controller. Spark plugs, wires, and distributor are brand new. Injectors have been cleaned and have new seals, also have an AEM fuel rail

If anyone has any tips or things I should try, please let me know, i’m open to anything at this point.

I’ve also considered bypassing the IACV to see if that works, and if anyone has any info on the details of doing that, it would be appreciated.

Thanks!
 

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Have you removed the intake and looked to see if the throttle body is opened too much?
Are you using a factory throttle body?
Is your throttle cable hanging up?
Have you or anyone messed with the idle set screw?

Positive you have no vacuum leak? Have you checked the canister beneath the intake manifold for the butterfly valves?
 

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JDM 4th Gen Prelude 2.2 Vtec Turbo
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With the intake pipe removed, cover one of the two little holes on the throttle body on the left hand side.

The upper one feeds the IACV and the lower one the FITV.

If covering one of those affects the idle, you are narrowing it down.

Failing that, systematically squeeze each vacuum hose that comes out of the intake to see if it makes a difference.
 

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Discussion Starter · #4 ·
With the intake pipe removed, cover one of the two little holes on the throttle body on the left hand side.

The upper one feeds the IACV and the lower one the FITV.

If covering one of those affects the idle, you are narrowing it down.

Failing that, systematically squeeze each vacuum hose that comes out of the intake to see if it makes a difference.
If the idle changes while covering one of the holes, does that mean the part is bad or is working properly?
 

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Discussion Starter · #5 ·
Have you removed the intake and looked to see if the throttle body is opened too much?
Are you using a factory throttle body?
Is your throttle cable hanging up?
Have you or anyone messed with the idle set screw?

Positive you have no vacuum leak? Have you checked the canister beneath the intake manifold for the butterfly valves?
It’s a factory throttle body and I did check to make sure the cable wasn’t hanging and it wasn’t opened too much, I also relaxed the TPS.
I haven’t touched the idle set screw, should I be moving it at all to try and fix it?

I’m almost certain there is no vacuum leak, the manifold has all new gaskets and lines, and I checked each area for a leak, and the vacuum levels are normal
 

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JDM 4th Gen Prelude 2.2 Vtec Turbo
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If the idle changes while covering one of the holes, does that mean the part is bad or is working properly?
If the surging stops with covering the upper hole, its the IACV doing its thing. Or unplug it to see if there is any effect.

If its the lower hole, you have a leak on the FITV.

Have you tried bleeding the coolant? An air lock is known to cause a surging idle too.
 

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Discussion Starter · #7 ·
If the surging stops with covering the upper hole, its the IACV doing its thing. Or unplug it to see if there is any effect.

If its the lower hole, you have a leak on the FITV.

Have you tried bleeding the coolant? An air lock is known to cause a surging idle too.
I have bled the coolant

I plugged the holes one at a time in the TB and neither one effected the idle
When I unplugged the IACV the idle would drop to surging from 1k-1.5k and the check engine light came on
 

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JDM 4th Gen Prelude 2.2 Vtec Turbo
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So the surging is a by product of the IACV trying to do its job, it is effectively fighting something that affecting air intake.

Weird how covering the top hole inside the throttle body did not affect the surging though, as it is where the IACV gets its air from.

This does feel like a vacuum leak on the intake somewhere.
 

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Discussion Starter · #9 ·
So the surging is a by product of the IACV trying to do its job, it is effectively fighting something that affecting air intake.

Weird how covering the top hole inside the throttle body did not affect the surging though, as it is where the IACV gets its air from.

This does feel like a vacuum leak on the intake somewhere.
Would you say the IACV is working properly then?

I will have to go through the entire intake again and check for leaks. It has a brand new gasket set, but I may just throw another new set at is and see if that’s the issue
 
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