Missing badly after checking fuel pressure.

2004 Buick Rainier with the 4.2 has 170K on it. Bought it with 140K and it's been pretty much trouble free. New spark plugs a year ago. I started getting the P0171 and P0172 several months ago. They would only show up while driving up and down a steep canyon every once and a while. Now they are coming on a few times a week along with the P0106 during normal driving. It has been running smoothly but seems less powerful. (It's the wifes car, so I don't drive it too often) After pulling the FP relay to de-pressurize the line, I connected a fuel pressure gauge. FP was at about 50-55 psi, and didn't drop after sitting 20 minutes. Taped the gauge to the drivers window and went for a drive and it stayed steady at 55 psi under full throttle. I pulled the relay again to remove line pressure and disconnected the gauge. When I restarted the car it was missing badly at idle. Took it for a drive and it ran like a slug...still missing badly and gave a P0171 code. The FP filter looked old and rusty so I changed it. With the new fuel filter, fuel pressure is now at 60lbs, but it is still missing badly. Runs like it's only on 5 cylinders, and is giving a P0300. When I disconnect the vacuum line from the FP regulator with the engine running, the FP does not change, and there seems to be little or no vacuum coming from where the vacuum line connects to the intake when I put my finger on it. I'm wondering if my FP regulator is bad, or if and injector got plugged during the process. ???

What year?

With the vac line connected, at idle you get a min fuel pressure........ when you disconnect it, fuel pressure should increase 5-10 psi......

Vac in line at idle should be around 18", with a gage.......

You have a misfire, so you need to find out which cylinder is misfiring......you have coil on plug setup.......one at a time, disconnect the harness to each coil.....there should be a noticeable drop in rpm........if there isn't, that is the cylinder with a problem.......

You put in new plugs....were they OEM? If yes, once you found the cylinder, that is misfiring, swap a coil with another cylinder.....if the misfire moves to the swapped cylinder, you need a coil.....if the misfire stays at the same cylinder, then try swapping a plug...if the misfire still stays at the same cylinder, you need to check compression.....you also need to check for a pulse signal from the PCM.....if all this checks out, you have a possible injector problem....

What year?

With the vac line connected, at idle you get a min fuel pressure........ when you disconnect it, fuel pressure should increase 5-10 psi......

Vac in line at idle should be around 18", with a gage.......

You have a misfire, so you need to find out which cylinder is misfiring......you have coil on plug setup.......one at a time, disconnect the harness to each coil.....there should be a noticeable drop in rpm........if there isn't, that is the cylinder with a problem.......

You put in new plugs....were they OEM? If yes, once you found the cylinder, that is misfiring, swap a coil with another cylinder.....if the misfire moves to the swapped cylinder, you need a coil.....if the misfire stays at the same cylinder, then try swapping a plug...if the misfire still stays at the same cylinder, you need to check compression.....you also need to check for a pulse signal from the PCM.....if all this checks out, you have a possible injector problem....

Thanks for the reply....
2004 is the year. Fuel pressure did not change when removing the vacuum line from the FP regulator. I don't have a vacuum tester. New plugs were OEM. The CEL is P0300 now since checking the FP and replacing the filter. I disconnected the front 4 coils, one at a time and it didn't change the rough running. The problem seems more fuel related, and I can smell gas, but not see a leak. The missing problem only started after checking the FP. Unplugging the FP relay to remove line pressure caused the engine to cough and sputter before stopping. Could that have done something??

Problem Solved. Turns out that during the process of checking the fuel pressure, and replacing a dirty fuel filter (was very difficult to blow thru) due to recurring P0171, P0172 & P0106 CEL's.....two of the original ignition coils went out, giving a P0300. I had a friend with a scan tool diagnose it and replaced the two bad coils related to the missing cylinders on his scan tool.