Shuddering at idle

[Flutie7NFL]

My 2001 Xterra has 58,000 miles on it. With the trans in drive and foot on the brake, the idle seems too low to me. It reads no more than 700 and there is a constant, minor shuddering. If I push the gas pedal a hair to up the revs the vibration is gone. Coupled with this is most times after about 10 to 15 seconds at a traffic signal the rpms drop suddenly to about 550 and the shuddering is very evident. Then it might jump back up a bit to almost 700 again. Sometimes drops again. Two questions: I have been told to not mess with the idle screws as the computer handles any idle considerations. True? And, it has been suggested that a sensor is going bad causing this idle fluctuation. The service engine light is NOT on. The drivetrain warranty is up soon so I am considering having the dealer investigate even if it costs me some $. The engine runs perfectly otherwise. Has anyone else noticed similar behaviors with their X? Thanks!

[Speednis]

Problems such as these are often hard to diagnose. Was there a particular point at which this started happening? Was any work performed prior to the symptoms? Did the engine bay get cleaned, or the battery changed? Take advantage of your warranty while you still have it. You don't want to wait until it runs out and then decide to fix it.

Does the symptom occur if the engine is in park, or just when in drive? Does the AC being on make a difference? If its only when in drive, its likely a transmission issue. If it happens when you switch on the A/C in park as well, it may be the ISC. If it happens regardless, keep reading.

From the experience I have with cars, I would tell you the same thing, don't mess around with the idle set screw. They usually don't adjust themselves. I haven't experienced this problem in my 2000 Xterra, but it could be a number of things. I would initially examine all your plugs and wires. First, if all the wires are in snug, pull out each plug and see if any of them look fouled. At that mileage, they shouldn't look too bad. If everything appeared in order there, I would begin suspecting a bad sensor. I would begin with the water temp sensor (which is on the water outlet housing - good luck) since it is the cheapest. There is a smaller sensor next to it that is actually a switch that is used by the ECU to shut off the AC compressor if the engine gets too hot. If your AC is working fine, don't mess with it. Anyways, there is a way to test the water temp sensor if you pull it out with a volt meter, but I don't know the exact procedure off the top of my head. If you really need the procedure, PM me. Or you could just replace it since its only like a $30 part (estimating here).

If that sensor checked out OK, I would check the MAF sensor. It usually measures things like air temperature, barrometric pressure, and how much air is going into the engine. Its the expensive one inside the air intake hose. Its probably not your problem, since it would usually throw a CEL if there was a problem with it (YMMV).

Also, make sure your oil level is good and you are getting good oil pressure.

[OffroadX]

Idle is supposed to be 750 RPM

[Flutie7NFL]

Thanks for the responses. There has been no work performed or unusual event that seems to have brought this on.This was something I noticed once in while since I first bought the X last March. I was wondering if I was just being too finicky, but I have to say it more pronounced than it used to be. I asked a couple of strangers if they noticed their X's have the RPM's drop like this and they both said, "Yeah, once in awhile." That is why I posted here to see if others noticed this. My intent is to go to the dealer and see what they think. Interestingly enough, last summer I had occasion to drive a 2004 Altima with very low mileage and it had the 2.5 four cylinder. That car exhibited the same sort of "resonance" and vibration that is more associated with 4 cyl. cars. I would not expect this with the 3.3 in the X, and certainly not the r.p.m. drop. If I switch the A/C off, the revs will sometimes drop even lower. I am uncertain of when I can get an appointment and get there, but I will report back the dealer's findings when that happens. Hopefully it will be of help to others out there.

[Flutie7NFL]

I finally took a vacation day and set up an appointment at the dealer for them to diagnose the condition described. I was fortunate to end up with a mechanic that enjoys a great reputation around the dealership. Anyway, I was hoping that it would be something with a warrantee issue. It was not. They had warned me it would be $92 just for the diagnostic if a warrantee issue was not found. They said that after checking everything it was decided that I need a "air/fuel induction service" to remove carbon buildup from the fuel we use. Since I was in for $92 already I had them do it and the walk out the door bill was about $230. The bottom line is that the idle is smooth now with no shudder or rpm drop so evidently they got it right. I am thinking that it might make sense to use a fuel additive/cleaner with some regularity after this. I wanted to provide the update in the hope that it might help others out there.