IDLE imperfections, or what my miata thinks about Winter

[MANAEUNOS]

Hello gentleman
I am writing from Boston, MA
It's been getting colder every day here
And my Miata clearly dislikes that.

I drive a 96 Montego Blue roadster which never gave me problem, until the temperature outside started to drop under 35 degrees F.

So when I start up the car, it stays at 2k rpms. However even when it warms up (according to the temp gauge) it still stays at around 2k


Then, when I touch the brake pedal rpm needle jumps up and down from 2k to 1500 and back to 2k then back to 1500 until I let the brake pedal go

Once I let the pedal go it just stays at 2k. (only when temperature is below 35).
This keeps happening even 20 minutes after startup

Now to Hypothesis:
I have a bigger Mishimoto radiator installed
Can it be that Miata simply doesn't get warm enough, because massive aftermarket radiator gets a lot of cold air in the cold weather or???

I eliminated Idling control valve, since on the warmer days I have no idle problems.

Will a piece of cardboard covering miata's mouth fix this?

[The Driver]

I don't know what is wrong with your car, but cold weather is not the problem. I started my '96 Montego this monrning @ 31 degrees and it ran like champ, all the way to the office.

[atank]

Could be a vacuum leak, can you post a picture of the engine bay........?

[Agent☣Orange]

I'm thinking a vacuum leak would cause rough idling as opposed to a smooth lower idle.

I don't think it has anything to do with a big radiator. Since it's temperature related, it must be a sensor but IMO the car is working fine given the conditions. It drops back down to 800-1K rpm when fully warmed, yes?

[kung fu jesus]

Idle control valves are finicky animals. They work until they don't. I would just recheck all your vacuum and electrical connections first, see if anything split or cracked. I highly doubt it has anything to do with the radiator, unless you bumped off one of the connections installing it.

[MANAEUNOS]

It was installed on the car,
Will post a pic of the engine bay shortly.
It idles normally after a second or third start of the day. But then idles great steadily at 950rpm
Will start checking the sensors and then get back
It seems like it take is half day to warm up.
Here's a typical Heat cycle of the motor
Drive it for 40 minutes in the morning.
Means:
Warm up for 3 minutes until temp gauge third of the way up
Start Driving it rpms at 2k still not all the way up
After 5-7 minutes of city driving, symptoms described above occur
20-30 minutes driving like that. No change. Turned it off and back on no change.
Gauge appears to be almost at it's usual mark (a bit below)
Park it for Hour and a half
Then start it up, steady

[kung fu jesus]

Are you getting any CELs? A p0505 perhaps?

[MANAEUNOS]

Nope, no CEL. Neither on dash nor on OBD scanner
Its about 45 degrees here right now, it warmed up for 10 min and was running normal. o_O