Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Low Idle Mystery

Hello everyone:

I can’t get to the bottom of my '98 Honda Civic EX and it’s sporadic low idle issue. It typically idles at 600-700 rpms lately, but today, it dropped low at a red light and I don’t know what is causing it. It has 118k, and is a 4 - cylinder inline, v-tech engine.

In an attempt to solve this problem, I have taken it to Honda five times. I got a new Honda distributor, I got the throttle body cleaned, the idle air control valve cleaned, I put in fuel injector treatment, I got a full diagnostic run by Honda and they couldn’t find anything. While they had it for two days, they could not reproduce the low idle.

They ruled out the fuel pressure, the fuel pump, and the main relay. It has not stalled since it got it’s new distributor - it used to stall at traffic lights, and in parking lots after idling low. I also got the radiator replaced after it sprung a leak last week, so that should be fine.

It seems to do it more when the weather warms up. Like today, it’s 60 degrees here, when just yesterday the high was in the 40s.

Does anyone have any further insight on what it might be related to?

Thanks for your time!

You may have a vacuum leak.

Check all the vacuum hoses. (most are small and relate to the emissions systems)

Check for a vacuum leak at the intake manifold and air intake hose or sensor.

I’m with Roadrunner. Has anyone given it a smoke test? If you can’t find a leak, ask for one.

Tip: visual inspection is always a thing to do first, but these things can be really hard to find. Take a propane torch - DON’T light it - turn on the gas feed it around all over vacuum lines and the intake. If you get near it with the propane the idle should suddenly jump a bit.

Thank you for your advice Roadrunner, amateur. No one has given it a smoke test, I didn’t think to ask about it. I don’t know what to look for in terms of finding a vacuum hose - I will check the manual. Now that it’s gotten cold, it also rattles pretty good at idle. Now running the way it used to. Is the intake a large black tube that is angled upward pointing toward the underside of the hood near the air filter? I kind of think it is, but am not sure. Thanks!

I would think that a vacuum leak would make it idle higher, or trip a MAF related code. The IAC may still be suspect. I assume that you have no engine problems when you step on the gas? That would tend to rule out the fuel pump. Try replacing the IAC (after checking the wiring to it).

Thank you for the reply tardis. Sucks that the IAC might need to be replaced. Honda cleaned it for me for a price of $143. They did not mention it may need to be replaced - is that something they should have picked up on the diagnostic? When I step on the gas, there is nothing noticeably different in the way the car responds. The only other detail I can add, is that when I have to come to an abrupt stop after coasting downhill at a moderate speed, the engine seems to jump when the car downshifts (it’s an automatic - incase I did not mentioned that before). This little jump is more perceptable that when it shifts at other times. Could that symptom be related? It has done that for a while - before and after tranny fluid flush, and all those trips to the dealer. Still no code though. The dealer did check the fuel pump and pressure and ruled them out during the dignosis about two weeks ago. Did get a standard tune up when I first bought the car, timing belt, plugs, cap and rotor, etc. That was about 40k miles ago. Acc. to the manual, I am due to replace the spark plugs too, but I have experienced failing spark plugs before (on a prev. car) and do not recognize any of those symptoms.

Thanks again!

Scratch the TPS idea that I posted earlier. I’m on-board with the IAC group, now.

I don’t think that you can really fault the dealer for trying to clean the IAC before making you buy a new one. I imagine the new one is going to cost you a reasonable amount.

Thank you Mr. Josh:

I am speaking to other import shops about further serivce on the car. In looking at price quotes, one of the shops mentioned doinga compression test on the engine might uncover more about this issue. I am not sure what a compression test uncovers, but know it has something to do with the gas and air mixture within the engine. Have you or anyone else had found that

Sorry 'bout that…

Has anyone else found a compression test useful or neccessary? If so, what is their typical price range? I really can’t afford to keep throwing money at this issue and not having it resolved. Thanks, your insight is appreciated!