That's an IAC problem IMO,- low idle, loopy and low upon cold start. Low idle when warm with O2's assisting. So if you replaced the IAC, you still have a bad one or a short in the harness/bad connector.

Disconnecting the battery, then cleaning the MAF is a good idea, but that's more less normal maintenance. MAF and resetting. Giving you the benefit of doubt, -that you have done this already.

 

Ya I here ya.....I've had a couple of MAF's act goofy under 30° on some stangs.....they were replaced .......but that may not be the op's issue.....I'm leaning towards the IAC and or the O2 s.......

 

Right, well, from what I recall, for 180 seconds from cold start the system is Open Loop - fuelling is arranged as a function of temperature, (ECT or CHT if aplicable, IAT) and air input (MAF).

Outside air temp is easily adjusted via program, but it's the IAC that needs to react correctly by command. When they get sticky, this sort of thing or idle problem occur.

The O2's are smoothing once active and as much as possible. When warm idle is below yield (700 rpms) that's another hint it's an IAC problem. It cannot find 700-750 for reasons usually pointing to the plunger that has developed a carbon ring around preventing both cold and warm idle from adjusting correctly.

The only other possibility is that the emissions harness has a blockage (not a leak). That can happen when the 1/8" line is forced to far into the 90 degree elbows, to the point of blockage. But,this is only common after the vehicle owner has screwed around with the emissions harness lol.

 

Maf i can see. How can thermostat do this?

 

Like everyone else, I have been having the same issue. Changed mass airflow sensor, IAC, EGR valve, EGR Pressure sensor, PCV valve, and air temp sensor with no change in condition. After all this, I was expecting a vacuum leak. I took it in today and they hooked it up to the vacuum smoke machine to find any leaks in the system, not a single leak. Found out the the throttle body had a lot of carbon deposits that was blocking the airflow when the engine was at idle. They cleaned the throttle body and performed a professional fuel system cleaning. Running like new again!

Hope this helps!

 

I have the exact same problem as dburgjohn. It first showed up last winter. I have a 2000 f-150 with 5.4l. Had our first -15 Celcius temp this weekend and the engine will die at a stop if the gas pedal is not pressed. One interesting item, in January it was -40 and I was attending college for auto mechanics. Told my engine instructor my problem so he told me to bring my truck into the shop to check it out. The instant my truck was in the college shop it started to idle fine. Ran full diagnostics on the engine and everything passed. Only code was 0300 which is generic misfire. The instant it was outside in the cold again, it would not idle. dburgjohn, have you found a solution yet? I cleaned my maf and replaced my PCV. It is the instant reaction to cold air that has everyone stumped.

 

Here is a case for the use of a good scanner to look at the live data and see what is out of limits or shifted to far.
Then go after the reasons why the data is off center of limits.
Unstable idle is due to the PCM not able to adjust the IAC to a 'shifting' condition and settle down to an idle within +/- 50 rpm of spec
A rubber fitting or hose that moves with varying idle vacuum and lets air in will cause the idle hunt.
As temperature changes occurr the rubber may change pliability near the fault.
As well, the OX sensors detect the shifting air/fuel ratio and feedback there own signals trying to keep the motor from stalling.
Even the fuel regulator being subject to intake vacuum can change the fuel pressure to some extent and add to the whole end result.
Your just missing the fault some place.
Good luck.

 

Thanks bluegrass. I cannot believe none of the mechanics I know, not even my instructor, mentioned this. We have a very good Snap-On scanner at work so I will use it and see what I get for info.

 

During the conditions the fault occurs, unplug the IAC and note the result with the engine running.
If no change, the IAC is not controlling the air bypass function.
If there is a reaction, the IAC is at least working to some extent.
With cold temps. things shrink and get smaller such that a the IAC may not function as well especially if gummed up.
.
Cold start sequence goes like this;
At key on before cranking, the PCM looks at the value of the head temp sensor to tell it the start will be under cold conditions.
This advances the igntiton, opens the IAC fully, richens up the fuel injection according to tables and begins a timer function.
As the OX sensors begin to heat up from a combination of their heaters and the exhaust gas, they begin to output their signals at about 600 degrees and take over automatic fuel control. Normally this takes about 1 minute +/- depending on how cold the outside temp is according to the head and intake air sensor (IAT).
The timer function steps the idle down a little at a time.
Working on this basis, you can look at all the areas that affect the cold start idle and what is different when the motor fully heats up.
The differences are the head temperature sensor resistance, the intake air temp as the engine bay warms the air and ignition timing retards from it's advance from the cold start.
Other items are faults with rubber hardware, gaskets that seal as parts expand with heat etc.
Good luck.

 

As with dburgjohn, my sysmptoms don't change with engine temp. Only changes with air temp. Engine light came on again so a scanner check showed the 300 code again. It has warmed up here so I will have to wait for colder temps again before I can put the scanner on while the engine is running rough. I did notice on the last rough idle that it does idle smooth on high idle. Only when the idle drops to normal does it start missing.

 

The 300 code implicates the Camshaft Position Sensor and/or the EGR valve sticking off it's seat for some reason.
You could clean the EVR solenoid and clean the filter under the round cap.
It may be allowing some vacuum to get to the EGR and lifting it off it's seat. This will upset idle at any time.

 

Well I am in year 6 of the rough idle in cold weather. Any new ideas?

 

Exactly what have you done so far to try to fix the issue since the posts made by Bluegrass? This person knows what he is talking about and when he makes detailed posts, read and reread them until you know exactly what he is talking about.

 

It is possible the intake air temp sensor is out of limits.
The way to tell this is with a Scanner to look at the live data reading for being out of limits.
This often won't set a code.
The code 300 is a random misfire detection.
Could be the crank sensor or it's plugup, the EGR is being held open a bit or even low fuel pressure.
You have to get into true diagnostics to find the root cause over just shot gunning the problem in hopes of getting it.
Good luck..

 

Replaced the following: MAF, IAC (twice), IAT, both upstream O2, DPFE sensor. all soft rubber hoses

Tested: vac test hard hose incl reserve tank, used entire can propane for vac leak test

Dealer: cleaned throttle body