rough idle

[Hoop]

Just bought an 04 with 35k miles on it, I'm the 3rd owner. The car appears to be completely stock. Fluids appear to have been recently changed, everything looks clean, but I don't know fluids they used.

It runs fine when on the gas, but has a rough idle. It doesn't idle rough all the time, but when sitting at a light, it will do a sudden shake like it's about to die, and RPM's drop a bit and then recover, shake some more, recover again, etc. Hasn't actually died yet, though. I thought at first that the AC compressor was dragging the RPM down at idle, but I've tried cutting off the compressor and it doesn't really seem to make any difference.

Any suggestions on what the cause is would be much appreciated. I'm not mechanically inclined enough to do any real work myself, but would like to have an idea where to point the mechanic when I take the car in. If some fuel injector cleaner might help, I'll definitely start there!

Thanks in advance.

[Goku]

You could start with your map sensor. Do a seach on that and I think you will find a DIY thread on how to clean it. Also make sure the conection is tight and clean. You may also want to do the Zip tie fix after you clean the sensor.

[Rocketman]

Take it to Curt at C&C's Northside Motors in Houston. He is the premier S mechanic in...well...just about anywhere. We're lucky to have him in SE Texas. :thumbup:

[repiv]

What is the rpm when the idle is stable? If it's around 850 to 950 rpm (at operating temp), then what you have experienced is not new and may not necessarily be deemed a problem. If your normal, stabilized hot idle is much below this, then you may actually have a mechanical issue that needs looking at.
I've written this many, many times in the past but since I haven't done it in a while, I'll do some typing once again.
First of all, if your engine is in a good state of tune and maintenance, then skip to the next paragraph. If not, make sure things like air filter, sparkplugs, belt tightness, MAP sensor, throttle body cleanliness, etc are all in good order.
Even brand new S2000s and ones in perfect running order and well maintained, will do what you have seen. We've got a pretty high output, highly tuned engine and although Honda did a pretty good job at making this a pretty decent street car, idling is not always the best thing it knows how to do. Even out of VTEC, this engine has pretty "racey" cams. Such cams makes idling a real chore and the ECU does a fairly good job of it. However, under certain conditions, both governed by the driving conditions at the time and by the input of the driver, the idle will dip. On the most part, the ECU "catches" it and makes it all better. It uses several sensor inputs to sustain idle and at times, the ECU simply has a bit of a brain fart. What makes it worse is the way some drivers get alarmed by it and try to either "help" it by blipping throttle to prevent it from dipping down so low or they do something really goofy, like tighten up the throttle cable so tight that it artificially holds the idle speed higher.
The smart S owner will just leave it well enough alone and give the ECU the chance to learn. Stabbing the throttle and sending new throttle position input signals only serve to confuse it more. Let it stumble. Don't touch the gas peddle when it does this. Once the idle stabilizes, then go about your business. Avoid revving it at the stop light like a crazed motorcyclist. After a few episodes, it should learn and the occurances will be minimized. One more trick ............ As you slow down and are downshifting (you are downshifting, aren't you?), don't be in such a hurry to push the clutch in. Most people will go as low as 2nd gear and not go into 1st till they're stopped. Learn to watch the tach and listen to the engine sound so you stay off the clutch till the revs get close to idle (~1000 rpm) before disengaging the clutch for the stop. You will find that this technique is very much appreciated by the ECU as it gives more time for it to hold idle.
As you've already noted, having the A/C on can aggravate it, especially if the compressor clutch locks up the pulley just at the moment the revs are falling. Again, don't help it. The ECU knows the revs have dropped below acceptable levels and will do its best to catch it. In all but very rare cases, the ECU will catch it. Also, high intake temps will also aggravate this. As we approach the summer heat, this will be more common. Again, give the ECU a chance to learn.
Typically, it's the "newish" S2000 owner that posts about this. Long time owners rarely mention this. They've figured it out.

[Alacrity]

:willynilly: < that emote doesn't have anything to do with my response, i just thought it was funny.

right on repiv- i wish i would have read this when i first got my s2! I have slowly and through a process of elimination figured out all of those tricks by myself, and they help a lot- especially the bit about letting the idle drop before punching the clutch out at stops. the only thing i've missed is my map sensor- perhaps i'll take a look at that :)

[Hoop]

the idle is between 900 and 1000 when stable.

Thanks for the advice.

[casinova]

This pretty much answers my question i was about to post. I grew up on muscle cars that arent as complex and as highly engeneered as the engine in my car. I'm no stranger to a wrench though and this takes all the guess work out of diagnosing the proble. Will check the map sensor and leave it all be if nothing is wrong. Its most likely just my shifting habits (or lack of). Thanks anyone. And although i hate honda i am starting to grow quite fond of Stacy ( 01 yellow S2k). Later.:poke: