Oldsmobile/Buick Repair: 1992 Buick Lesabre/stalling, air control valve, vacuum leak

Question
Hi Van, I checked, cleaned & replaced the Idle Air Control Valve, wasn't sure how to test it. Also, I checked the vacuum hose to the master cylinder, it's fine.

However, I removed each spark plug to check their condition and found one plug showing no signs of firing or combustion burn. The plug looks brand new. No smell of gas and no carbon present. The wire checks out fine and spark is good from the coil. I could not tell if the fuel injector for this cylinder is working. I am not sure why the cylinder is not firing, but I believe its the cause of rough idling & stall problem. please help. Thanks.
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Followup To
Question -
Hi Van, I have a '92 Buick Lesabre, 3.8 engine, 168,000 miles. I recently replaced converter on exhaust. Engine idles rough & stalls sometimes while car is in drive waiting for traffic light or to make a turn. Runs fine when gas pedal is pressed while parked or driving. I have replaced spark plugs, wires, coils & fuel filter. This improved slightly. Master brake cyclinder needs to be replaced, per Midas. However, I need to solve idle & stall problem first. Please HELP! Thanks.
Answer -
Hi Maurice,
Disconnect the vacuum hose to the master cylinder, plug it, and see if it runs better.
If you drive it that way, keep in mind that you won't have POWER brakes, and must push very hard to stop.
If it is leaking vacuum, that can make the car idle bad.
Any other vacuum leak will also.
If vacuum is fine, check the idle air control valve, and clean it.
Van

Answer
Hi Maurice,
Sounds like you have done a bunch of testing.
The injector is a good possibility, and they can be checked outside the engine.
They can electrically be checked in the engine, but if the pintle actually isn't opening inside it, or if it is just plugged up, there is no way to tell but pull it out.
Another possibility is the intake valve isn't opening, or the exhaust valve is bent, stuck open, or burned. A compression test could eliminate several of those possibilities.
A "noid" light can tell if power is getting to the injector.
Another possibility is the plug isn't firing...stick a different one in there and try it.
Good luck,
Van