Too fast idle

Well I'm convinced the issue I s the pcv tube. Do you remember that tube I replaced three months ago? It looks like the hose I got from autozone. So I've ordered the hose from a gm parts store. Will let you know how it turns out.

It's not holding a seal where it goes into the valve cover

Can't be clamped. I think the aftermarket hoses do not have the right dimensions for the hole the tube "t" goes in. The oem plug part is rubber and has a better fit. The aftermarket hose has a plastic "finned" plug that does not offer a tight enough fit for the hole, so there must be a grommet needed if you use the aftermarket hose. The problem with trying this, is you would have to take the throttle body off, and remove a clamp to do all this. Then I'd need a gasket and other things. I've ordered another hose, but this one is supposed to be oem. I will see when it gets here on Monday.

Well replaced the line and as always the initial start sounds good, then the random misfiring starts up. It seems that anytime something is replaced, it changes but not really fixed. It seem that when I replace a part, the engine goes thru a high rev that seem to cycle for about four accelerations. I attribute this to the engine computer relearning for idle mixtures. Of course when the engine goes though the rough idle cycles with the misfires, the intake air sounds like its being blocked or rerouted. I'm wondering if my mass airflow sensor is bad. Cause when I unplug it and let the engine run, it seems to settle to normal. I did get to replace the coolant temp sensor but that has not changed anything. I'm almost running out of options.

I unplugged and plugged the sensor, ran engine no change. Cleaned the MAF sensor last month, got no appreciable difference in performance. I'm not ruling out some sort of either gasket leak or possibly maybe even really bad fuel injector clogging. However I have read it will cost me quite a bit for gasket work, and I'm not gonna spend this much money on it. Ive only known about this misfire condition until I fixed the high idle issue. I'm at a loss now. The only part I have not changed is the ICM and Ive read that this component is either good or bad.

Still have the random misfire.I don't have a smoke generator, I don't smoke a cigar or cigarette ( the cheap smoke generator so I read :) ) I have sprayed the top of every seam and gasket of the car with a mild soapy solution to chase a leak, and have not found one. I have read that 99 Buick centuries are famous for clogging fuel injectors, I've run two cans of seafoam in my tank and there has been a slight increase in performance. The ransom misfire and idle stumbling does not happen when the engine is first started. It runs fine for about 20 sec. Then as the engine runs, I guess the electrical signals from whatever goes through the engine computer. The engine then starts the cycle of high revs with the first one higher revving then tapers about every 10 seconds for roughly 5 cycles. Then the engine settles with the misfire. When the engine throttle is moved instead of a smooth acceleration, it stumbles and at a certain point will quickly accelerate. So I think one of two things is going on. Either I have a vac leak still, or I have a horribly clogged fuel injector. The misfire does seem to go away, but not entirely.

Will try again but got no rise in rpms

Replaced the idle air control valve

Merry Christmas.
I'm really at a loss. It seems like what ever I think maybe the issue is not. I am thinking now it is some sort of seal leak. When the engine is cold initially it runs nicely for about a minute, then the random misfires occur, and stay until the engine reaches optimum temperature. Once the engine is allowed to cool, then the misfires come back, then goes away when the engine has reached the optimum temp. When the misfires occur, the engine is very sluggish to the point of stalling. Then once warm, the acceleration is normal, and so is the idle. I'm thinking intake manifold from the throttle body? My intake manifold is NOT plastic, but looks like cast alluminum. Is this gasket normally an issue or is it somewhere else?

Can a bad O2 sensor cause this problem or is it truly a leak?

You are gonna love this.
Had run out of options and tired of buying parts so. I looked at routing of spark wires and all of them were good plug to right coil. However. I did find the #3 plug wire NOT clipped I the top of the plug. So I checked them all and started the car and I had a single hiccup and surge and immediately the idle went away. So for anyone who reads this thread to the end random misfire check all cables for connection. Random multiple misfires DO typically point to something in the ignition/timing discrepancy. It took three months to get to this point and I believe that if I would have changed the ICM I may still have not fixed this. Truly the craziest and most challenging of problems always have the simplest corrective action. I did have some minor initial drive rumbles with acceleration but the ECM quickly returned the engine to normal operating parameters. Thanks for your assistance and suggestions for fixing this idle/rpm misfire dilemma.
Dennis