Installed IACV, still dies while idling

My car has been having idling issues for about a year now. While idling the RPMs oscillate between 1 krpm and almost 0, with a frequency of about one oscillation per second. That got worse over time, and it started dying while idling too. I ignored the problem because it wasn’t that bad, until a few weeks ago it died about 7 times on a 2 mile trip. (I developed a technique of using my right foot to press the gas and the brake simultaneously so I could come to a stop while keeping the engine going. But that only gets you so far.) I took it to a garage, they said I probably need a new IACV. Instead of having them do it, I just bought one and put it in myself. I’m not an experienced mechanic, but this seemed fairly easy; I’m pretty sure I didn’t screw it up.

So, now it’s displaying almost the exact same symptoms. It’s not oscillating like it was before, but it’s still dying pretty frequently, only on idle. I’d like to know if there’s anything else I can try before I take it back to the shop.

A little more information about my car:

  • Replaced the motor about a year and half ago, put a few thousand miles since then. I don’t know what other parts were replaced as part of the motor, but probably not many considering what I’ve had to replace.
  • Since the motor replacement, I’ve had the following replaced: rear main seal, clutch, distributor, distributor again, and water pump
  • The odometer reads 223k miles, but I don’t know how many miles the new motor has.
  • 1990 Honda Civic LX 1.5L

Have you cleaned the throttle body?

Sometimes they’re so dirty, you should clean them on the bench

In regards to that idle air control valve and throttle body . . . is there a coolant line going to the throttle body and/or idle air control valve?

Is the idle air control valve getting power and ground

I could think of a number of things that could cause this kind of problem. The ignition timing not being correct after the distributor replacement or a vacuum leak would be the first things that come to my mind.

Don’t take this the wrong way, but there are some things involving the shop that do not sound good.
They installed a used engine without a new rear main seal before the installation.
Reusing a clutch on an engine install.
A distributor that has been replaced twice.
A water pump that was replaced and hopefully the timing belt at the same time. If not, ugh.

Maybe there’s some details that would swing my opinion the other way but as presented it kind of stinks.

Believe it or not, there’s a base idle adjustment that must be performed on your ODI, 1990 Civic depending if it’s throttle body injected or port injected.

If it’s throttle body injected:

1.Start the engine and allow it to idle until the radiator cooling fan comes on at least once.

2.Turn the engine off and connect a tachometer.

3.Disconnect the two pin connector from the IAC valve.

4.Set the front tires straight ahead with all accessories turned off.

5.Start the engine.

6.The idle adjustment screw is on the throttle body where the throttle cable attaches.

7.Turn the adjusting screw until 575-675 RPM’s is seen on the tach.

  1. Turn the engine off, remove the tach, plug the IAC valve connector back in, and remove the HAZARD fuse in the main fuse box for ten seconds to reset the ECU.

If it’s port injected:

1.Follow the steps above.

2.The only difference is, the idle adjustment screw is located above the air intake hose at the throttle body.


Concur w/Tester’s advice, many cars of that era used an air bleed screw to set the base idle. On my Corolla I have to disable closed loop ECM operation first, as idle rpm must be set in open-loop mode. But it is the same idea as Tester describes above for the Civic. If that base idle is not set correctly, or if it was set ok, but then somebody attempted to fix an idle problem by re-adjusting it, this is the exact symptom expected. See if you can find where the nominal position of that screw is supposed to be, as it came from the factory, and start from there. Usually somewhere around two full turns out.

Edit: If after setting the base idle to specs, it still does it, most likely it is the throttle body is clogged with gunk, or there’s unmetered air leaking into the engine somehow.


BEEN THERE DONE THAT…A MILLION TIMES. The new valve WILL NOT FUNCTION until the Cooling system is BURPED OF ALL AIR POCKETS… ITS TEXTBOOK “HONDA IACV IDLE HUNT” you are seeing. You introduced the air pocket when you changed that IACV

DO NOTHING until that Air Pocket is GONE. TRUST ME


I recall that the surging idle was sometimes the result of a failing throttle position sensor.

It can be Rod…but that’s a little bit down on the list of suspects and CERTAINLY AFTER the IACV. On Hondas we must go down the suspect list IN ORDER…and Numero Uno on that list is positively the IACV


When the IACV is replaced, does some coolant come out during the switch? If so – and this is the case any time the cooling system is opened and even a tiny bit of coolant escapes – the cooling system has to be bled of air pockets. Otherwise there is no telling what weird symptoms will appear. On both my vehicles , Corolla and Ford truck, this is easy enough to do. Just open the radiator cap and idle the car until it reaches normal operating temperature, the thermostat has opened, and coolant is pouring into the top of the radiator, then close the radiator cap. Done.

On other cars – like yours according to HBB – it’s a little more complicated, involving opening and closing various air bleed screws, but as long as you know where they all are, it is still pretty simple to do.

There is only ONE Bleed screw on this vehicle. IT MUST BE USED… When you install an IACV on most Hondas…you ABSOLUTELY Create an Air pocket HIGH UP in the cooling system…right where that IACV Resides. The IACV is now sitting in a pocket of AIR when it should be submerged.

I cant recall how many times Ive watched people replace the IACV and NOT burp the cooling system afterwards…The valve simply does NOT function…the Idle just continues to Surge… ALMOST immediately upon using that bleed nipple the idle will come back down and even out.

This process MUST BE DONE…There is no IF ANDS OR BUTTS ABOUT IT…Except MY BUTT in your business telling you to bleed the cooling system so that the IACV has the coolant running thru it that it needs in order to know how Hot or Cold the engine is presently.

Once you bleed the Air out of the system and make sure the radiator is completely full…This Honda SHOULD Idle nice n smooth. Its just Textbook Honda Stuff. Makes sense if you fully understand what is going on in this system…Not too difficult to grasp the concept really.

Bleed that thing and let us know how you make out. IF for some reason you don’t have the nipple to bleed the system…there are other ways “To Cheat” and fill the system up but it involves removing some items and then pouring the coolant right down its throat so to speak… Like pulling the Thermostat Housing and filling the system thru there…and other methods… Also keep the Overflow filled up during this time of Burping…as it will pull a lot of that coolant into the engine as well After Hot/Cold Cycles. BE CAREFUL NOT TO OVERHEAT THE VEHICLE DURING THIS TIME…You are in danger of overheating Until that Air Pocket is gone.

Just use the Cute Little Nipple for what it was meant for… It wants you to touch its Nipple…Believe me. You should make out A-OK after that. Let us knowski… There are other candidates to play with AFTER the Bleed Nipple process has been completed.


@Blackbird, as I was reading your last post it occurred to me that for many years burping the cooling system on vehicles had become such a habit to me that it was done without thinking. At first I thought you were going overboard with your insistance on thoroughly getting the air out but it’s been years since I closed the hood on a car after doing any work on the engine without spending the 60 seconds +/- to open the bleeder or loosen a hose clamp to see liquid appear. And in particular opening the IAC valve on several cars would throw a significant bubble in the head because I would remove the coolant bypass at the IAC and let it drain to prevent excess water from running into the intake. On those cars introducing 2 psi pressure at the radiator overflow before tightening the bypass hose clamp would totally burp the air out.

So many things that must be done can be taken for granted and I often find myself overlooking some of the seemingly unimportant steps that really are critical when giving advice here. It’s rare that any of the OPs acknowledge that the advice given here resulted in a happy ending and that’s a shame. I sometimes visit gear head advice forums and find them totally lacking. People with automobile problems that drop in here get their money’s worth.

Are you saying that I was going overboard when I first mentioned this YEARS AGO when I joined up here? Did I introduce the Honda Cooling System Burp to your repertoire ? I’m not sure if that is what you just said or not… HAHAHA…Sorry if I took it the wrong way or if I got it Screwy. The first time I mentioned burping a Honda System using the Bleed nipple on this forum WAS INDEED MANY MANY YEARS AGO at this point in time. The practice STILL “HOLDS WATER” and is MIGHTY Necessary on most Honda vehicles of the 80 and 90’s vintage at least. I think the 00-10’s also still have the nipple. OTHER VEHICLES don’t seem to have them very often…its all up to the design of the Cooling System in reality.

" " " It’s rare that any of the OPs acknowledge that the advice given here resulted in a happy ending and that’s a shame " " " . LOL


Most of the prior comments in this thread probably assumed I burped the system…they would be wrong, I did not. I’m a total novice, I probably should have mentioned that. I will try to burp the system today, and I will certainly let you all know if it resulted in a happy ending.

In the meantime, I’m curious to know how the IACV can tell if it has coolant or not? Has this bubble been travelling through my entire system this whole time?

A little more information about my engine swap: yes, it was shady, BUT it was dirt cheap, $1300 total. I didn’t know to ask about timing belts and water pumps and things like that at the time, because I didn’t know anything about cars.

I want you all to know I appreciate your advice. You all are saving me the trouble and money of another diagnostic fee, and I’m learning stuff at the same time, so thanks.

One more thing, I was going to post this question on, but hardly anybody offers advice there. Here, I got a bunch of posts in two days, all of them quality and well thought out.

Re the burp, @imuk, the IACV is located so that it is a trap for gas in that if air or exhaust gas is there it cannot escape other than going up and unless you purposely allow it to escape it will remain and the temperature in that bubble will exceed the temperature of the coolant. And in the overall situation a bubble of air at the top of the engine will result in an area of extremely hot, dry surface at the exhaust ports adjacent to wet metal at regulated coolant temperature and that will eventually result in severe damage. An air bubble can and often will find a high point in the system and remain there. The water pump will not move a bubble out of the high point. Until recently engines were located with the radiator top tank above the thermostat and the upper hose would carry a bubble out. Modern, compact engines stuffed tightly into a crowded engine compartment with the hood aerodynamically sloped to the front makes it necessary to bleed off air whenever it has been allowed to enter the system.

Thank you for the explanation, that gives me a lot more information. So my IACV was so hot it couldn’t even function properly?

I tried it today, I hope this is what we’re talking about:

I’m not sure if I saw a lot of bubbles, it was hard to see through the tube I was using. By the time I could really see the tube, there weren’t any bubbles. I let it run for a good six or seven minutes, and I goosed the throttle a few times for good measure. Then I drove it to the grocery store, about a mile away, and it had no problems. I will drive it much further tomorrow and give it a real test, and get back to you all.

Your link does show the bleeder valve and hopefully your efforts took care of the problem.

I’ve been driving it around town for the past several days. It died ONCE while idling. So the problem is way better, but maybe not totally resolved? Should I bleed it again? I guess I may as well.

The way air bubbles in the cooling system behave isn’t very intuitive. You’d think they’d just flow around the coolant path just like the rest of the coolant. That would be the case if the coolant path were perfectly level along the entire course. The problem is the coolant path goes up and down along the way. Air bubbles travel the up hill portions ok, but they seriously balk at going downhill. There’s a large buoyant force preventing them from moving downward. If you’ve ever tried to push a basketball under water in a swimming pool, that’s the kind of force the water pump would need to overcome to force the air bubble down hill. And it just doesn’t have the ummmpfff to do it. So the air bubble goes to the top of the cooling system and stays there. And in your car the top of the cooling system and the IACV must be near the same place.

What happens is that the IACV and the air pocket are many times in the same location. The IACV needs to have its finger in solid liquid coolant…Not Air…when it is literally in the air pocket you will see the temp needle drop because the temp of the coolant is not being telegraphed to the sensor in the IACV…since it is taking the temp of the air in the pocket…then sometimes the temp sensor will encounter liquid coolant and then you see the temp needle spike upward…then drop back down…etc. This continues until this air pocket is bled out so that the entire system is full of liquid coolant…and peace and happiness is restored in the Under Hood Kingdom. So…what happened after the cooling system burp?


Good explanation @“Honda Blackbird” !