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I have a 99 Cadillac STS 4.9L VIN 9 that came in with a P0300. The only cylinder showing a misfire count is Cyl#3. I've scoped the ignition pattern looked ok on #3 & #8 its companion cyl. I've checked compression on all cylinders in spec ranges from 165-170 and on a running compression test they are all 85. I've swapped injectors, plugs, coil, wires & module. Ive smoke tested for a vacumm leak none found. Vacuum is 17 and steady.

I've driven the car and the miss is only at idle; off idles, its gone. Ive checked backpressure on the exhuast system and its 0. EGR valve is operating OK and not sticking.

Had one recently that acted the same way. I had used ig coil so I stuck it on for testing purpose and the miss was gone. Evidently something about the coil. It tested ok but misfired. That was same cylinders. Good luck.

i swapped coils

Have also seen vacuum leaks on them intake manifolds that will be difficult to track. On the one I saw some time back the vacuum leak was on the inside and was extremely hard to find. I'd think that vacuum should be more than 17". Perhaps there is a valve or piston ring issue, but on second thought a compression check should reveal that. What is fuel pressure?

a worn cam or other valve related isses will give you all the same readings and iaaues that you're experiencing.

If this car is not fixed yet I would do a cylinder leakage test to determine if you have a valve sealing problem. A leakage test will show that but a compression test will not if it is slight enough. How severe is the miss? I had a missfire on a Chrysler a while back that set a code but could not be felt by the driver. I ran Seafoam through it and killed the engine and let it soak. I then ran it hard. It cured the problem and the light never came back on. Hope this helps.

The Cadillac From 1999 Should Be A 4.6L. My Only Advice Is To Checkl For A Vacuum Leak Near The Power Steering Pump. The Backfire Flap Is Bad About Having A Vacum Leak That Only Shows Up As A Misfire At Times.