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I had some nasty rough idle recently and I traced it to the following issue - here is what to verify:

Remove the thin rubber upper PCV valve tube from the valve.
Get a vacuum tester, pull a 20Hg Vac. I guess you can suck on it like a straw, that is pretty ******* though...
If it don't hold for a few minutes(a very slow leakdown is acceptable) - you have two possible issues

1) You Purge solenoid is bad (unlikely)
2) The metal tube or some rubber hoses are deteriorated and letting in air.

Fix:
Get some vac lines or fuel hose (I bought the former, wanted to get it done) that is a bit smaller than 1/4" ID and bypass the metal tube. There will be a plastic tube somewhere under the airbox after the metal tube. You may have to remove the air filter cover and the two sensor connectors to get to it.

Retest. Still failing?

Remove the passenger wheel shroud (just a few screws and clips). Replace ALL the rubber connectors (just short lengths of hose) to the tube, charcoal canister and purge valve. After the purge valve you could still have leaks but they will never show up at idle.

While you are in there you can remove the air silencer elbow for a bit more power!

Of course, you should also trace the large PCV hose to the tee on the manifold, but that is supposedly common knowledge.
 

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1994 Thunderbird LX 4.6 : 99,223 miles
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Adding to this ancient thread in the event someone else is tracing a rough idle or stall on cold days or startup.

did you clean your MAF and/or throttle body recently? Or change your air filter. If so, you may have inadvertently unplugged a temperature sensor that runs into your air filter/cold air intake. Turns out I unplugged it and never plugged it back in. And it may or may not have been throwing a code.

It resembled a bad/failing idle air control: your car can still drive with some coaxing but it really wants to stall on startup and switching from park/reverse/then-drive. So make sure it’s plugged in. And if you left it unplugged for you don’t know how long. Clean it with some electrical contact cleaner and then give it some diaelectirc grease.
 
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