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Low idle with an occasional stall

I have 2000 Civic EX that has an extremely low idle and will stall coming and or while at a stop light/sign.

I’ve tried the recommended “clean the IACV” that is so prevalent on various forums with no such luck. I have even swapped the IACV with a known working one. The idle is so low at times that the batter light will flicker for a second. I think it is a vacuum leak. I have sprayed in various areas check for such a leak but nothing.

One day I noticed that while pumping my brakes at a slow and constant rate my idle is great. Then when I left off the brakes the tachometer needle will fall to the first mark. I have a feeling it is the brake booster. Is there a sure fire way to confirm that I have a bad brake booster?



For diagnostics only - you don’t want to drive around like this - find the vacuum hose that feeds the booster. Pull it off at the intake and cap that port on the intake. If the idle problem goes away then that’s pretty much it. If so, then it may just be your check valve rather than the booster itself.

I get the same thing with my 1998 Civic DX, but the only time it ever stalls is when the air conditioner is on. It might be the mass airflow sensor, but the problem usually goes away for a year or so when I spray some carburetor cleaner on the throttle assembly. Your throttle (and mine) might just need a professional cleaning.

Is your “check engine” light on?

I don’t think that it’s the brake booster. I think that your engine is not getting enough air (pumping the brakes gives it more air, via the brake booster). Maybe the electrical connection to the IAC isn’t working. If you unplug the electrical connector to the IAC does the idle go down or the engine stall? If not, then the circuit that activates the IAC is not working.

I will do this and let you know what happens. Thanks for the info.

I’ve cleaned out the TB numerous times and even replaced the gasket and it still does it. I have no “check engine” light.

I’ve attempted to reset the idle which does require me to disconnect the IAC connector. If I recall the engine all ready idles low when I attempt adjust the idle and disconnecting the IAC connector doesn’t really make the idle go down lower but it does want to stall. I have to keep on the gas for a bit to make it not stall.

I should have mentioned this earlier. When I first start it it idles normal but after it gets warmed up, that’s when the idle get low. Also, when it does stall it starts up just fine and will idle fine for a few miles. Then, the cycle will repeat.

You haven’t messed with the throttle plate hard-stop screw have you? After that, I’m out of guesses.

Since it idles normally at first start I would also check the coolant temp sensor.

I would check out the brake booster just to be sure - but, even though I told you how you could find out I’m struggling with matching up the symptoms.

The other things to always deal with when you have any kind of running problems are the basics - how old are the plugs & wires? Air filter? Fuel filter? Get a fuel pressure gauge on it and make sure the pressure is in spec…even though this kind of problem would be more likely to show up while driving.

Why are you pumping your brakes?

After a 45 minutes drive I pulled into the garage and unplugged the connector to the IAC and the idle did go down but it did not stall.

I have replaced the coolant temp sensor too.

I’ve replaced the fuel filter with an OEM one a couple of years ago. OEM plugs and wires were installed two weeks ago.

One day I noticed that the idle went up when depressing my brakes. I then decided to pump them to see what would happen to my idle and pumping my brakes made my idle normal. I don’t do this normally though.

Hey, I lied - one more guess.
Check and see if the PCV system and see if it is clogged/plugged. Maybe the engine was counting on some air from the PCV and it isn’t getting it. (long shot)

Does the idle respond to hitting the brakes even if the car is idling in neutral? If so I would certainly check out the brake booster.

Correct, when I pump the brakes the car is in neutral. I should have explained it better. I apologize for the confusion.

I have a YouTube video of it. At the 55 seconds mark is when I am pumping the brakes.

I think you probably need a brake booster. And it seems likely that the problem is in the rear chamber of the booster (closest to the pedal). There must be a leak there which is a vacuum leak with your foot off of the pedal. When you push the brake the idle smooths out b/c then the vacuum port is sealed off from the intake vacuum - the rear chamber does fill with atmospheric air b/c it is supposed to.

That’s my theory anyway, and I’m sticking with it - at least for now.

I would get it to do the low idle thing and seal off the booster from the intake as I noted above. It you have access to a vacuum gauge - which are incredibly handy things and not very expensive - you can just hook one up and find out more.

I have an alternate theory. I think that the booster is fine. When you pump the brakes, you use vacuum, so more air is leaked into the manifold. The engine is starved for air and this additional air is allowing it to run faster.
Certainly, before I spent big money on replacing the booster, pull and plug the hose to the booster. If the issue stays the same, then it’s not the booster. If the idle is corrected, then I’m wrong and it is the booster.
You can also try unplugging the hose and letting some air in. If that lets the engine idle correctly, then I’m right and your engine needs more air at idle. If so, see if the PCV system is plugged. If it isn’t, maybe it is time to look at the throttle stop adjustment.

Yesterday, I disconnected the hose from the manifold to the booster and caped the manifold end. The verdict was that it did not make the idle better.