Somewhat Rough Idle

[Blue)(Fusion]

For the last 6 months or so my 2000 GM has been acting a little funny when it comes to idling. It always used to be very smooth and quiet. But it's been getting a little rough while idle. At other times it runs and sounds great.

Recently it got a little bit louder and rougher when idling in gear. It seems to have gotten worse a few hundred miles after my last oil change using Mobil 1 5W-30 full synthetic plus 4oz. of Auto-RX.

Today I decided since I have all day free I'd take a look into it. I took out a spark plug to see if there was oil on it and check the condition (they were replace about 5,000 miles ago). There's no oil/carbon on them and they look to be in new condition. I then decided to hook up the OBD-II scan tool to log some data. From a cold start, the RPMs were logged around 850 RPMs while idling in park. After waiting a few minutes for oil to get flowing and warm, I got in and shifted it through the gears. The RPMs were recorded at about 550 RPMs. shifting into Neutral or Park and the RPMs jump to about 920 RPMs and settle back down to 850 RPMs, so in my limited experience it seems that the idle air control valve is working properly. I also checked the tailpipe for soot and it was squeaky clean, so I don't think there's a problem with burning oil.

Does anybody have any suggestions as to what to look for to see what's causing this?

I replaced the PCV valve when I changed my oil about 300 miles ago. I cleaned the MAF sensor using the cleaner specifically for it 5,000 miles ago and again today since I was under the hood. The transmission was flushed 3,000 miles ago at the dealership. Previous to this oil change I was performing the Auto-RX rinse phase after a clean phase following the instructiuons from the Auto-RX website. The fuel filter and air filter were both changed within the last 1,000 miles. The car now has 79,500 miles on it.

Any help appreciated!

[rhandwor]

I had to clean the intake on my 1997 F-150 5.4L I used Winns Intake cleaner in a can with a hose with a valve on it. I let the vacuum pull it in through a small hose on the air nozzle. You need the valve so you don't stall the engine.This really did a good job. I also cleaned the injectors but the intake cleaner helped the most. I didn't have any codes but did previously change the dpf sensor.

[Blue)(Fusion]

About 2 months ago I did do two SeaFoam treatments of the intake and cylinders as well as in the gas (I use Auto-RX in the oil, so I did not use that treatment). I introduced the SeaFoam via the PCV valve. Would this have done the same thing or does this not clean the intakesystem as well?

[rhandwor]

I have never used sea foam but other people do but you should try to put in in the air going into the engine. The Pcv usually gets gumed by oil so I don't think you cleaned the intake.

[Blue)(Fusion]

From what I understand, and I may be completely wrong as I am still in the novice stages of learning about this stuff, but the PCV valve's vaccuum is from the intake. The PCV had plenty of suction to draw the fluid from a cup so there must not have been any oil blockage. But I'm not sure if the point in the intake it is introduces is poor placement for the cleaning or if the SeaFoam even does that job. I think it's mostly to clean the carbon from the cylinders.

[rhandwor]

The vacuum is from the intake I thought poured it from the other end.

[Blue)(Fusion]

Well I checked it out again today after fixing a bug with the MAF in the ScanXL program. I was mostly listening to the engine this time, though. The best way I can describe it is it sounds like one cylinder isn't firing with as much "oomph" as the others so the RPMs kind of drop off when that cylinder is fired. It used to be a very smooth idle. It may be the compression of a cylinder. I want to get a compression test on it.