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I just bought a 2004 Civic with 118k miles on it. It is the LX edition.

Once the car starts the idle sits about 1.5k rpm. Once warmed up, it drops to about 600-700rpm.

As the rpm drop, the vibration of the car becomes worse and worse. @ 1.5k rpm the vibrating is hardly noticeable, but as it drops to 700rpm it gets significantly worse.

I have replaced the motor mounts, and the exhaust (The original owner had a truck exhaust on it... don't ask me why...) It has a AMF (I think is the mfg.) cold air intake, that comes of the top down to the bottom of the car.

A person who said he worked for a VW dealership noticed that the vibrations continued as the car's rpm increased. I do not know what this is indication of.

Any ideas what else it could be?

 
Rep Power: 235

First, when you replaced the exhaust did you use the rubber hangers? If you have no rubber hangers then the car will vibrate a lot thru the whole rpm range (especially at idle).

If that isn't it, then it sounds like it has a miss. Which would set a code. Scan for codes, Autozone will do it for free. Then post it here.

 
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I had a small shop do the exhaust. It is a small, near factory exhaust. Don't want to give you the impression it is some loud after-market exhaust.

They did a real good job on it, and all the mounts have rubber, no metal contacts. I am not sure of what the mounts are called but they are the square ones, with the rubber in the middle and the hanger goes though that.

I also just replaced the spark plugs. The person I talked too suggested I pull off one spark plug wire at a time and see if that made a difference. I did this, and all four of them made the vibration dramatically worse when I pulled of the wire. Making me think that the spark plugs are not missing.

I am also very confused by this.

EDIT: Could it be the timing belt? I was reading in the manual that the ECU controlled the timing as long as the timing belt was correct. Could someone explain this a little more? And if this is the case, how would I fix it if the ECU was out of whack? Should I pick one up from the junkyard?

 
Rep Power: 285

ECUs ging bad is extremely rare, you could re-set your idle by pulling the ecu fuse while the car is turned off for at least 10 seconds. I suspect your motor mounts are bad.

 
Rep Power: 235

To the OP: Scan for codes.

To everyone else: Who wants to bet me it's the knock sensor?

 
Rep Power: 285

I don't want to take that bet, because thats a definite possibility!

 
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Went to Autozone to check for codes. Nothing came back. There are no lights on. What is the "knock" sensor and where is it located?

I replaced the motor mounts two weeks ago, so it isn't that.

 
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Wouldn't this be the case when I pull off the batt. cables? Or is there a back up battery to the ECU that the fuse would disrupt?

 
Rep Power: 314

Knock sensor is located just underneath the intake manifold between cylinder #2 and #3, in the middle of the engine. You have to get at it from underneath and it helps a lot to remove the oil filter. That being said, I don't think it's the knock sensor, but I'm definitely not taking any bets Oh yea, you can't see the knock sensor when underneath. You have to remove it by feel lol.

Did you use OEM mounts?

 
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I bought the ones from Auto Zone. Not sure of the brand to be honest. But the original motor mounts were demolished from a stupid kid that owned it before. When I replaced the mounts, it did help the vibration, probably a 50% difference or so.

Can using after market mounts really make that much of a difference? If that were the case, as the factory mounts wore out I would think there would be a significant difference in vibration.

 
Rep Power: 314

It's probably still the mounts. If you felt that much of an improvement they are probably stiffer than OEM and thats why you feel the vibration.

 
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Here is a video I made to show the vibration as the RPM's change using a cup of water to show the vibration

http://www.youtube.com/watch?v=3TTQ4fo51nA

Hopes this sheds some insight

 
Rep Power: 235

I noticed mine doing that when I took off the resonator and put on a straight pipe and aftermarket muffler. In short- exhaust vibration. Are all the exhaust hangers good?

 
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Mine civic (India) is also having same vibration problem... Last week Honda replaced the throttle body assembly unit under warranty...but of no use as the problem is same as earlier. Now i am taking legal action on honda. I may need all your guys support. Plz help by giving as much feed back as you can, i am going to put all these feed back as proof in consumer court forum. My email ID is manishparajiya@yahoo.co.in

 
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Guys, these are civics, not lexus. They do develop some vibration with age. To the OP, your vibration doesn't seem bad at all, especially on the hood.

 
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My wife's 2001 Civic has 96, 000 on it and it also vibrates some at idle. Note that these are 4 cylinder engines that only fire twice per crankshaft revolution as opposed to 3 times on a V-6 or 4 times on a V-8. They also do not have a balancing shaft like many engines used in higher priced vehicles do. I just did a full tine up and a timing belt on it and it still idles the same as before. But it runs fine otherwise. My advice is to just keep driving it otherwise get a new car.