I have a 92 civic CX with the d15b8 and I’m having an idle issue where it jumps up and down from somewhere around 1k to 1.5k rpm. Here is all the work I have done to it so far. NOTE: THIS IDLE ISSUE IS ONLY PRESENT WHEN CAR IS WARMED UP, WHEN COLD IT IDLES FINE.

Replace Map (OEM)
Replace TPS (OEM)
Replace IACV (1a Auto) x3 times
Replace Throttle Body & Gasket
Replace Distributor
Replace Ignition Wireset (All 4 NGK)
Replace Spark Plugs (NGK Iridium)
Replace Fuel Pressure Reg
Has perfect distributor timing
Bleed Cooling System


Performed a smoke leak test: very small leak on IACV bolt but no where else.

The only CEL code I have is code 41, which is for a o2 sensor I believe, and it’s probably cuz I have the wrong exhaust system on it and a universal catalytic converter.

I’m thinking the idle issue could be the injectors or something with the IAT (intake air temp sensor) or possibly the ECT sensor. And maybe just maybe the IACV.

 

adjustment procedure was followed for installing the new TPS?

set the base idle properly?

you have slack on the throttle cable? or in other words the throttle stop hasn't been messed with and the plate closes fully?

any other drivability issues once up to temp or just the idle surge?

 

i think you should trust your gut on it being an ECT/coolant problem.

have you tried bleeding the coolant system yet?

 

so something in the warm idle circuit is amiss. by chance does this engine have the FITV?

 

This engine is not equipped with the FITV.

 

A long time ago yes, I’m gonna have a go at it soon tho, there could be air in the cooling system.

 

The idle just surges, when I accelerate its fine, it does NOT sputter or feel like loss of power. Could it possibly be my spark plugs because there iridium ngk’s and that engine calls for copper ngk’s, the last time I check them they were black with carbon deposits, but that’s because the car runs rich. Any other possible suggestions?

 

in my experience the heat range and brand/quality matters more that the material used in the spark plug- when they're new/un-worn. Platinum and iridium will have longer life but otherwise will work exactly the same as copper. make sure you're using the same heat range as the recommended copper plug. this should be pretty straight forward since you're using the same brand. they probably even have the reference chart on their website. I know I've seen it before.

it's a pretty easy thing to try though. I wouldn't get my hopes up, but put a fresh set of plugs in and see if there's any change.

In the name of SCIENCE!

 

Update:

I recently did a “test” on the IACV. I ran the car with the IACV plugged in until it warmed up and got to operating temp, then the idle started bouncing up and down, then I shut off the car and un-plugged it, and now when I go ahead and start my car up, it idles perfect. So what could this possibly mean, could it be the IACV itself, the connector, or another input sensor that is failing, giving false info to the ecm, therefore telling the IACV to constantly open and close, please let me know your thoughts!

 

If you don't unplug the IACV, do you get a code (CEL) for it ? If not, all that false info, other input, etc. thinking that you are doing is nonsense. If the IACV is unplugged, the engine (when warm) should BARELY stay running at 450-500 RPM (this is the first line above ZERO on your tach). If it "idles perfect" as you say... this means that there is some other air leak, improper T/B setting, FITV leak, vacuum leak, etc. that still needs to be discovered. Your IACV cannot "compensate" for other problems, so you will have to do some more fixing before you get a handle on this.

 

I have no CEL for the IACV when connected. Btw when I meant perfect, I meant like it wasn’t going up and down, and I checked the tach, it was below the 1k mark, probably around 400-650rpm. I have checked for vacuum leaks with a smoke machine, and had no leaks. Btw this does not have a FITV. I’ve checked all of my TB settings everything is fine.

 

If you disconnected the iacv and the idle drops but doesnt stall, that means you have a vac leak somewhere.OOORRR air in the coolant system OOORRRRR the idle adjustment screw in the throttle body MAY have worked its way out (doubtfull but a possibilty)
Leave the iac connected, and plug the hole in the throttle body (at about 11 oclock looking at it) with your finger. Does the idle drop or stay the same ISH? If it stays the same you have a vac leak somewhere, Check the PCV valve

 

UPDATE: I bleed my cooling system & checked for vacuum leaks and then gave the service port a good old try and I just figured out that I have a code 14, which is the IACV. But here is the confusing part, 1st could it have popped up when I ran the car without the IACV? And if so what does that mean. 2nd there’s a possibility I’ve been have this code for a while and I never bothered to check.

Anyways, I think I’m gonna buy myself a replacement IACV. I’m gonna go for OEM unless any of you peeps know a good aftermarket brand?

Anyone in the SoCal Area Selling a OEM IACV for my 1992 Honda Civic CX (Engine code: D15B8) (ECU Code: P05) (Chassis: EH2) (Transmission: Manual)

Thanks a bunch!

 

code 14 means that either your IAC is not seeing enough vacuum, or it is physically damaged. What are your vacuum numbers? Cheap Harbor freight vacuum gauge is priceless when looking for vacuum leaks.
Serrvice port? do you mean the obd check engine connectors in the foot panel on the passenger side or the vacuum port in the throttle body (not enough coffee in me yet)
Check your iac electrical connections

 

First, clear the engine codes by pulling the ECU and "Back Up" memory fuses in the under hood fuse box (back row, closest to the right fender and the one next to it), wait 30 seconds, and then re-install them. Make sure that your IACV plug is connected to the IACV (grey plug - wire colors are black/blue and yellow/black). Crank the car. If the CEL comes on within a few seconds, check the code as you have previously using the service connector and make sure that the code stored is 14. If so, you either have a shorted IACV, you have a wiring problem (break or short in either/both wires), or you have a bad ECU. If you get no code within about a minute, unplug the IACV plug. You should immediately hear a change in the idle and the RPMs should drop, and the CEL should illuminate almost immediately after. If there is no audible change in the engine sound and the RPMs stay the same, your IACV is not functioning. Check to make sure you have 12v on the yellow/black wire. If so, check continuity between the IACV plug and the ECU to make sure there is no break/short in the black/blue (it could be blue/black too) wire. If the wiring is good, replace the IACV. I agree with you that only a genuine Honda product should be purchased from your local Honda Parts Department or approved online store... NOT EBay or Amazon.

The IACV cannot measure manifold pressure, so not enough vacuum is NOT a thing. What is relevant, and this may be what youstolemybeer was trying to explain, is if you already have a really high idle due to some other vacuum leak/leaks, and the IACV duty cycle is reduced to nearly zero to try and lower the idle RPM yet the target engine speed is still not met... you will get a code 14.

As a reminder, at warm engine temperature (and all accessories off) the engine should idle at 750-800 RPM... assuming no vacuum leaks and a properly functioning T/B and IACV. If you unplug the IACV, the engine speed should drop to nearly stalling, which is about 450-500 RPM (this is the first line above the 0 RPM line). If the cooling system is properly bled, you should have a stable (not "hunting" up and down rhythmically) idle... even if it is higher than 800 RPM... like 900-1000 RPM. If you can reach this... I would call it good and drive.

 

Seal that bolt with the known leak just to illiminate that. Check grounds, check ecu, i had a couple pins back out on me and caused it. Age old honda problem. Is your ecu chipped/modified?