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Is my Idle Air Control Valve bad without a CEL?

Home Forums Stay Dirty Lounge Service and Repair Questions Answered Here Is my Idle Air Control Valve bad without a CEL?

This topic contains 18 replies, has 7 voices, and was last updated by EricTheCarGuy 1 EricTheCarGuy 9 years ago.

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  • #448210

    Eric, great write up in the Honda idle issues thread. I posted there but didnt get a response so decided it was probably best to copy paste here:

    I have some suspect issues with my vehicle and wanted to contribute to your ongoing ‘Honda idle’ thread. Here’s the scoop…

    Vehicle: 1992 Honda Civic VX (Canadian Built, Canadian VIN, if that matters…no fit valve on this engine; 1.5L D15Z1) …I live in the south-eastern US.

    The engine seems to be experiencing an occasional sticking of the IACV(as far as I can tell). For instance, when coming to a stop light, the engine RPMs will have the ‘raise/fall’ symptom happen, at times, occasionally. It was cleaned before, to fix the raise/fall, but symptoms seem to start back more so in colder weather; but progressively I’ve noticed another symptom.

    My question is, would a sticking IACV ‘also’ cause issues like a rough or low idle/RPM lug? This happens at full temp only, as the cold start seems to be just fine around 1,500 RPMs or so.

    Here’s a scenario, when waiting in line at a drive thru, traffic light; etc, night or day. The engine will start to ‘lug’ and cause some chatter/vibration (feels like the engine wants to shake or shimmy at times it gets so low? heh), the ‘lower’ tach mark is where the RPMs are when this is happening. I’ve heard that represents 500 RPM and others claim that is the stall mark(is it true the stall mark and 500 reading are very close together?). That sure helps when saying what it’s idling at, so I need clarification there. To me it appears to be just under 500 RPM when this is happening, sometimes appearing lower than that(if I read the tach correctly). When this happens, if I turn the A/C on, apply the brakes, those sort of things(cooling fan comes on will sometimes trigger the event), then usually the idle raises about 200-300 RPM or so, and feels much better no lug/vibration issues of obvious nature; when it’s then around 750-800 RPM.

    My idle ‘looks’ similar to this ‘newer’ VX(an HX model) Civic video on YouTube, and has similar symptoms as the description laid out, my tach would read lower than his, but my temp is higher on this gen’s current display.:

    Here is the original PCV valve:

    If this can cause an idle issue I may change it to check it off the ‘list’ anyway. The PCV valve on this car originally was located down at the ‘breather chamber’ underneath the runners nearest the distributor, seated into the chamber itself. So, my ‘elbow’ location at the Intake Manifold has the ‘hollow elbow’ instead of the PCV valve, makes replacing a pain if OE dealer was ‘upgraded’ to go into the runners on the manifold itself(sigh). Not sure what the dealer has, but wanted to clarify which PCV valve is in use currently.

    Spark plugs replaced last summer(I over dosed slightly on some Red Line SI-1 FSC, suspected reason for the red blotch):

    I’ve had the following jobs/work done on this vehicle(260k miles currently; owned since 230k):

    Clutch w/ an EXEDY ‘OE’ + Flywheel re-surfaced w/ rear main seal replaced (December 2011)

    All brakes(save for the rear wheel cylinders/hoses)…brake fluid ‘normal’ flush by repeated bleeding, same for clutch’s ‘brake’ fluid

    Timing belt + water pump and their relative seals; along with drive belts…at about 240k

    Radiator cap(Honda), the heat works great and it rests slightly under the ‘mid’ point on the temp gauge.

    CVs twice(one reman failed after only 1 year…lame, got some ‘new’ GPS brand CVs from AAP with lifetime warranty we’ll see…if problems I’ll go dealer $$$)
    replaced axle seals as well

    Inner & Outer tie-rods + bellows(last summer)

    Valve adjustment(by a honda tech, 1 year ago) + VCG

    EGR valve + port cleaning(2 years ago)

    IACV valve cleaning(2 years ago, gaskets were left alone?)

    replaced leaky vtec solenoid + housing gaskets(1 year ago)

    Distributor cap + rotor, OE NGK 4043 plugs( ZFR4F-11) + Sumitomo wires(OE brand new) 2 years ago

    CEL, code 48: Replaced ‘original’ 5-wire LAF(02) sensor with an SG335(SMP brand), works great no CEL currently, not for over a year now)

    Change the oil with a full synthetic every 5,000-7,00 miles with Used Oil Analysis to back it up(I post on BITOG site)
    Changed the transmission fluid several times now, I’ve used PEA based fuel system cleaners, replaced the fuel filter twice(for kicks, before I knew better, a little more OCD).

    Not sure if this is making the ‘feel’ worse, but I ‘do’ suspect the motor mounts need replacing(some at least?).

    When I’m driving in gear, holding steady for instance in any gear, and I let off of the throttle I get an either ‘delayed’ or ‘immediate’ rocking sensation. Like the engine is trying to rest but I can feel it doing this, not extreme necessarily, so IDK. Having my clutch’s master cylinder replaced this week so I’ll ask the shop to look at the mounts. Also, while this ‘same’ shop did the clutch job, they also had the car stall on them during test drives. I’ve also had the vehicle stall easily twice in the 2+ years(30k miles owned) trying to move from a stop while going uphill. ???

    Those are perhaps anecdotal, but may point to a sticking IACV or even perhaps engine mounts(EDIT: I noticed in your suspension vid maybe strut bar or control arm bushing instead of mounts)? That’s my suspicion as of now. If this can confirm the IACV sticks anyway. I’m thinking of removing it myself with the engine cold, replacing the screen and using some tb/intake cleaner on it. If the hot idle ‘also’ improves doesn’t this mean the IACV was/is the issue? If it returns promptly, would a new IACV be advised? Thank you.

    Any more info feel free to ask.

    The engine was compression tested 2 years ago, it passed. Coolant is full at the radiator cap when cold, though the level goes down from the reservoir over time; the water pump seems to weep a bit at the lower timing cover weep hole(no coolant visibly on the ground where parked nor any coolant smell but visible from the weep hole, manual says this is normal?) My UOAs don’t show any coolant either. 1 quart of oil is consumed every 4,000 miles, so the rings are still okay, etc.

    Thanks again for your work and the info already out. I’m a mechanic in training.


    Today, the idle lugged really bad at idle. It almost wanted to stall and was really low. No oil pressure light and the engine runs great otherwise. I didn’t have time to remove the IACV, so I sprayed TB cleaner at the port for the IACV external side of the butterfly while blipping the throttle to keep the car from stalling. It stalled 2 times anyway, heh.

    I didn’t think to plug the hole and see what happened to the idle at the time. Is that a good method to diagnose IACV function? If the idle lugs again really low as described previously, and I plug the port with my finger(with a glove as this is a full temp ‘check’) and the idle stabilizes/stops lugging, does that mean the IACV is plugged/bad?

Viewing 15 replies - 1 through 15 (of 18 total)
  • Author
  • #448211
    college man
    college man

    next time the idle goes low try tapping the IAC with the handle of a screw driver to see if the idle changes. If you try to plug the hole as you suggested
    you will probably stall the motor. that doesn’t prove the IAC is the problem. If theres no change with the tapping test. take the IAC out and clean it.
    reinstall it and see if things get better or worse. good luck and keep us posted. C8-) Btw Be sure theres no air in your cooling system and the
    system is full.


    There should be a fee good video links in that Idle Speech thread.

    How to solve Honda idle issues – … t3EVppbf88


    That’s probably the best thing I can do for now, the tapping w/ screwdriver method going forward. I have seen that vid. I also found out only the VX in this era has an EGR and Constant Vacuum Control unit. So, that’s another beast instead of just fast idle valve. Not sure how much different the VX is from other Civics, but those are the obvious differences I’ve noted.

    The cooling system could afford a check for air bleeding, I’ll give it a go with one of those handy spill-free funnels like in the vid eric has out.

    Thanks for the replies so far.

    EricTheCarGuy 1

    I didn’t see anywhere where you checked for air in the cooling system which is the FIRST thing to check with an idle issue on a Honda there is a link in the ‘idle speech’ you mentioned. It is possible that your IAC is bad with no CEL and to be honest I’ve never had any luck cleaning although everyone seems to try that, I think you would have better luck replacing it if you find that the IAC is faulty. Also don’t forget to check for vacuum leaks which is the #1 cause of idle issues in my experience.


    ^Thank you for your reply. I’ll be doing these things and reporting back, but for now I believe the question was answered. I’ll bump this up with any updates in the future.

    +1 guys.T)


    It’s funny, when the dash lights flicker on/off I can apply pressure to the switch location and get it to stay on whatever dimness it’s set at, then a bump comes along without me holding it steady after it decides it wants to behave and it goes out again. lol. It is a separate switch that turns on the headlights themselves along the steering wheel(headlights, turn signal switch), this one controlling the dimness setting is off to the side of the steering wheel; left-hand side, as a alone switch/control device but could still be wired together?

    Is that sort of wiring in the ‘cluster’ itself? Are there specific terms related to this area of wiring in an engine I should know? Totally new to electrical on a car aside from basics on the charging system and perhaps a few relays, etc.

    Also, in gear the RPMs aren’t an issue, and weren’t before today. It happened relative to neutral, like the tach for RPMs wanted to die. Speedometer worked if the car is moving, fuel gauge and temperature still reporting their readouts to the ECU.

    I’ll monitor for now as I’ll have to make time for researching this myself. Thanks for the links. Free Chilton has helped in the past. =]


    Wanted to mention something strange that happened today. I tried to whack the IACV last night at idle with the blunt end of a screwdriver, but the symptoms weren’t bad ironically and so there was no change to note.

    Today, cold start this evening(day off) and headed out for some fast food. During warmup, in neutral and coming from a stop 1st gear/takeoff(only split second until actually moving and not on the clutch pedal); mainly related to being in neutral…well the RPMs jumped instantly from 1,000 or so RPM down to near stall then back to 1,000. ???

    It was really strange. There was no audible change in the actual engine RPMs when this happened. Once the engine was at operating temp, it no longer happened. The switch which controls the dash lights whent he headlights are on is flaking out, too. Not sure if that switch flaking is coincidence or related, but could that mean the cluster is going out? Never had to deal with that sort of electrical issue.

    Any ideas?


    You’ll have to find a wiring diagram and see what circuits are related. If the tach/gage cluster power runs thru the dash light brightness switch, then I would start my diagnosis there.

    You can find free wiring diagrams a couple different spots on here. If you enter ‘chiltons manual’ in the search box at the top of this page, it will lead you to a free Chilton’s manual site. I have personally taken a liking to BBB’s free wiring diagrams, found here:

    Good luck man.

    Edit: To view the wiring diagrams on the Chilton’s manual site, you have to have Adobe Reader up to date.


    The problem with the IAC valve is that it’s not stick but rather the filter screen is dirty and blocked.
    I would also check the screw inside the FITV if you are having problem when the engine is cold and idle is low. Here’s a more complete link to all of the important idle issues.
    I am not sure you mentioned cleaning the throttle. I had the issue of a slightly low idle. When turning on the AC, it would shake the motor a little like the idle is low. My car is 90 Accord LX and normal idle for an auto should be dead at 750rpm. With AC on 800-850rpm. My idle is dead on and never even shake, unless I press the gas pedal. I fix that slight low throttle with a good throttle cleaner. NOT carb cleaner. For God’s sake do not drown the throttle. Use a rag, you can spray it a little more with the throttle close. Soak the rag with the throttle open to clean behind.
    If you remove that IAC valve, make sure you bleed the coolant system. Those valve got coolant running into them. My friend did all the stuffs and didn’t bleed the system, it some how made the idle worse. I bled his coolant just to eliminate the possibility and next day, his idle was steady as no tremors. I have done it for a few Accord 90-93, his car was the first with a problem by not bleeding the coolant.

    Good luck.


    ^The idle issue is probably either air in the cooling system or the IACV itself. I have done throttle body cleaning, but perhaps the gasket needs a manual cleaning or outright replacement. I’ll remove and inspect when possible. 2 year anniversary tonight, so I’ll check back in later.

    Thanks guys.

    PS: No FIT valve on the VX. It also has EGR and a constant vacuum control system.


    If you suspect an issue with the rheostat switch for the cluster, you may need to look at some wiring diagrams to rule out any wiring issues from loose connections to bad grounds before condemning the switch.

    If you use the search function, we do have a link to Chilton’s online that does have some wiring diagrams that may help you.

    EricTheCarGuy 1

    In my experience cleaning IAC’s is mostly a waste of time, if the symptom doesn’t’ clear up after the first cleaning replace the valve as a second cleaning never seems to make any difference especially if the IAC is having an electrical issue. I think you should still check for air in the cooling system, also recheck your cluster light issue after the idle problem has been addressed just to be sure they aren’t related.


    Not sure if it appys to your ride (Should). on a 90s ford pick. Start the engine let it idle – then unplug the the air idl control. If IT DOES NOT DIE it is bad. if it does die its good.

    if it trys to die -it may just need a good cleaning / even if it does not die give it a good cleaning and try again.

    Hope i helped.


    ^Thanks guys. Waiting on my spill free funnel to come in I ordered from Amazon to do the cooling system bleed.

    Other thoughs I’ve had:

    Could an engine coolant temp sensor be ‘slow’ like an 02 sensor and cause idle issues? I read the other thread when someone had a cold start issue, and I’ve had similar but relative to warm re-starts. I had the car do this twice, last summer most recently. I had moved the car in the morning, a short start to re-park for someone else’s vehicle in the driveway when I should have just put it in Neutral with key foward and steered, lol. Anyway, later that day I restarted and the engine seemed to struggle to find a starting RPM, if that makes sense. Almost like a choking sensation. When this happened a good rev to 5,000 RPM and the RPMs settled… its probably happened twice that way. The other symptom is the low idle deal.

    I do have a developing cluster/switch wiring issue so that’s probably the most difficult beast potentially for me. I could always buy an IACV from the dealer same price as aftermarket(ironically?) for $200 or so, but I’m willing to see what bleeding the cooling system and learning other checks for next time.

    My worst fear is if this was an oil pressure issue, it passed a compression test 2 years ago but you just never know if someone else let the engine get too low on oil(previous owner). I wish this thing had an actual Oil Pressure Sending Unit instead of a switch. Will report back post bleeding and cleaning.

    EricTheCarGuy 1

    Just about every idle issue is mixture related and to be honest a slow TW sensor or O2 sensor won’t effect it all that much unless they are dead or way out of spec which would set a check engine light. It’s mostly vacuum leaks or perhaps even a misadjusted idle screw (as stated in the idle speech this should never be adjusted). The problem you had with the restart after a short trip is common to Honda’s an I don’t believe related to your issue, it’s referred to as ‘short tripping’ which causes there to be too much fuel in the intake during the restart.

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