A Mystery that even the Car Talk Guys might be Unable to Solve

Ok. So...
A long time ago, in a city far far away, overnight the air became frigid-the years first frost glazed cars's windows. The following
morning, trouble began. When the car was in park, the idle would be somewhat rough. When it was in drive, the idle would be more rough. The roughness would smooth when the car accelerated, but the car would not accelerate without hesitation. I did a couple of things, and the problem went away; however, it's unclear if anything I did had any effect. I changed a spark wire, and dropped in the tank some dry gas.
But the problem came back. I don't remember the relative temperatures as keenly; I do remember it was cold, and I do remember that the problem again precipitously went away after I did something. I took off the air filter housing, checked my air filter, and replaced its housing; the car had been running for a while.
The problem came back. I don't remember the temperatures, but I do remember it was cold. Without doing anything, the problem went away by
the following morning. Then it came back. I changed all the spark wires, and replaced the filter-which I think I may have been improperly
replaced the aforementioned time I checked it. I think it had been replaced cocked, not forming a seal.
The problem went away, again precipitously, but only after the engine did more driving.
In Buffalo two nights ago, the temperature, after having been unseasonably warm, dropped dramatically, and it snowed. Last night, the temperature still being (more usually) cold, the problem came back.
And again, it came back precipitously: After the car had been sitting for perhaps two hours following a short drive, soon after I began driving it back home, the car suddenly began vibrating at low speeds, vibrating while stopped at lights, and from a stop the car began accelerating hesitantly.
I had someone check the sensors once. They said that they were fine-that they had plugged in a diagnostic tool and read the sensor outputs, and that they were correct. But this same person was retarded, so he could have been wrong. (When I drove my car away after
he had done his work, I noticed the problem was worse, and then noticed
he forget to reconnect two of my spark wires.)
There is no check engine light: there never has been. When the problem is active, there is pungent, more voluminous exhaust.
My car is a 1995 Chevrolet Lumina Sedan, 3.1 V-6.
Thanks alot, Sean.
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snipped-for-privacy@gmail.com wrote:

Did the person who used the diagnostic tool attempt to pull codes?
Does the check-engine light come on when you turn the key to 'on' without starting the engine?
Is the exhaust pipe black on the inside?
Has the fuel pressure been checked?
Could the fuel press. regulator be dumping gas into the vacuum hose that goes to it (this car may not have that type of regulator).
I think its possible to have a fault when running open-loop (before the engine warms up) that won't show up on the trouble codes if it clears before the engine goes closed-loop, but I'm not sure.
Could be a lot of things...
Dave
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My guess is that your cold-start injector got lazy over the summer (common in Dallas). Buy some Techron fuel injector cleaner in the black bottle and follow the instructions to the letter. Should be O.K. after a treatment or two.

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Ed Fisher wrote:

Hmmm ... my old Saab with it's mechanial injection system had a cold start injector, but I would not think that a "newer vehicle" with presumably EFI would have a cold start injector (it should instaed just boost the flow a bit when cold) ???
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bowgus wrote:

My only data points are my '92 ranger and my wife's '99 minivan. No cold-start injector on either.
Dave
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snipped-for-privacy@gmail.com Wrote:

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If I had to guess, I'd say spark plugs or Intake Air Temp Sensor. I don't think there is a cold start injector, I believe the PCM uses the IAT sensor to enrichen the fuel mixture when cold. It might not hurt to run another bottle of heet in the fuel tank. BTW, what temp is the car running when it starts acting up?
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I would check cold fuel pressure and warm fuel pressure to see what you come up with. If the IAC motor has any moisture that freezes it could also cause this problem temporarily.
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snipped-for-privacy@gmail.com wrote:

Its the choke pull-off (default Car Talk answer ;-)).

Look for an air leak which is temperature sensitive. Maybe a small crack in the intake manifold which closes up when the engine warms up.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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