Usually my 05 buick lesabre custom 3800 series 2 with 145k starts and runs fine, but after warming up and stopping it has a crazy rough idle at start bouncing between 500 and 1200rpm and sounding terrible, and occasionally at red lights, stopping, etc. Pressing the gas pedal does not seem to have any effect when this happens, if it does it is delayed a few seconds. I usually shut it down and restart it several times until i can get it rolling.

My mechanic said it throws no codes and likely had bad fuel. So, I ran some fuel system cleaner through it and it has had no change.

I figured I would start with Idle Air Control Valve or Fuel Pressure Regulator, since it runs hours perfect on the highway I figured idle air control first. Then move on to connectors/wires/sensors and get a new air filter, but since it runs 100% fine when not idling i figure most of that unlikely.

I've read a lot of posts/problems about similar but not identical problems. I thought it strange that I havent seen any problems idling when it starts cold, but definitely has problems consistently when warmed up. It has not stalled yet, but being unresponsive at an intersection is not something I'm willing to risk. Any other suggestions?

Replaced the idle air control valve - no change

Replaced the fuel pressure regulator - no change

I was thinking fuel/air because it only has problems during idle, but maybe i should move to the spark/coils next? When it starts the rough idle it seems like its misfiring but it never stalls and with it not responding to the pedal when the throttle opens could be a sign it isn't the fuel/air?

And it is super clean. No signs of corrosion on any wires or connectors.

I'm wondering if its heat related, it might be a heat induced ignition control module. Usually when they go bad, the heat takes them offline and just shuts the engine off. But, never heard of them causing driveability issues.

BTW, there is no such thing as a mechanic in a bottle. If you have bad gas, the only remedy is to replace ALL of the gas.

The service engine light finally came on, i got p0300 multiple cylinder misfire, autozone suggest checking sparkplugs and wires

Well I haven't done any more work, but more research.

Last year the ICM board, coil packs, spark plugs & wires, crank sensor and catalytic converter were replaced with new parts.

Here is a short video of the idle and response:

In the video the idle is only slightly rough, but when the throttle is opened it has a really rough response until the end when it self corrects.

Usually when this happens at a stop sign it doesn't idle slightly rough, it goes straight to dying and catching without touching throttle. Sometimes after 5-10 seconds of throttle it goes back to normal, other times i restart it several times until i get a start where it is normal. It only happens after it has been warmed up, first start and first 30 minutes of driving everyday are 100% fine. Also, it still never stalls, and runs 100% fine on highway for hours. It is acting up less intermittently and more consistently now.

Because a lot of parts were replaced already and there are no codes I'm unsure where to move to next, drain all the fuel and clean the lines and injectors? Why doesn't it ever stall?

I keep thinking someone has had to have seen something similar to this before.

Have your mechanic check for vacuum leaks with a smoke test.

I would like to know the actual fuel pressure.....

Would like to see O2 data during this.....

How does it run with MAF disconnected?

Would also check exhaust back pressure.......

Thanks a lot for the suggestions. I managed to find a decent sized crack on the underside of the vacuum harness coming from the fuel pressure regulator, so i ordered a new one as well as my own scanner tool. Hopefully this is my problem. If not, I'll get the smoke test done and come back with more data.

Ok, didn't get the smoke test yet, but I replaced the cracked hose and read some data that I have a limited understanding of:

When it is acting up at idle:

Ignition timing advance bounces when RPMs drop, usually 0.5-1° at a time and stays between normal 21 and up to 25, though it dropped as low as 18

Intake manifold absolute pressure bounces between 26kPa and the normal 34kPa, when throttle is pressed it climbs to 70-90 until it quits acting up

Short term fuel trim goes to -5 to -10% and then jumps to +15% when throttle is touched
At normal idle it stays between -3 and 3%

Long term fuel trim drops to -25%, then goes to normal 10% when throttle is touched. When it first starts acting up it is bouncing between -20 and +20 but it settles at -25

Mass air flow rate goes from a steady 4g/* to bouncing between 3-15g/* and climbs when throttle is pressed proportional to how much it is opened

Oxygen sensor 1 voltage stays bouncing between 0.02 and 0.9v like normal idle, goes to 0 when throttle is touched

Oxygen sensor 2 drops to 0.1v occasionally but stays around .7-.9v where it stays around 0.7v at normal idle

Calculated load goes from a steady 23% to bouncing between 10 and 70%

It will continue to idle rough indefinitely until i press the throttle and it gets really rough until it usually self corrects after 5-10 seconds or so.

With the MAF unplugged it does not act up.