Group:
I checked the archives, but found no answer to any condition such as mine.

My ’98 S90 with 133 K miles idles rough when cold.
By rough, it is not a cylinder missing, but is instead just a vibration.
Like uneven firing position or uneven mixture in each cylinder.
It used to be smoother than a 5 cylinder car, but is actually rougher than a 4-cylinder.
It shows up the worst around 1200 rpm and at 2400 rpm.
In driving the engine still seems rough until it is warm. It is not just the idle condition.
This is after the air pump shuts off and lasts until the water is fully warm.
(As we all know, the temp gauge does not show water temp, just that it is in the normal range.)
This can take 10 miles at 30-40 degrees.
It was rough even in 60-70 degree morning weather.
The hot air intake hose is hooked up.
No codes have ever been set.
Everything is the same with regular gas or premium gas.
The thermostat is good.

Here is what I have checked in the year that it has been a concern:
Vacuum tree and other hoses are good. I sprayed carb cleaner all around it with no effect.
It has the good color intake gasket and carb cleaner sprayed around makes no difference.
The AMM was swapped with another car, with no change.
The FPR has been checked by local indie Volvo expert who found the fuel pressure was exactly right. (I do not know what the pressure should be.) I have the expensive FPR so I hate to experiment.
The vacuum at a cold start idle is 16-17 in, which the gauge says is on the edge of late timing.

Here is an FPR oddity: I disconnected the vacuum hose from the FPR and it made no difference whatsoever in the idle. In fact, last summer I drove nearly a thousand miles with it off and saw no difference in operation or mileage. As I understand it, no hose connected should put the fuel pressure at maximum, and it ought to be rich. Don’t know why it seems to in-effective. Idling and driving with the vacuum hose open and sucking air makes no noticeable difference. It will be a separate question. As a test I sprayed carb cleaner into the hose and the engine died, as expected.

I do not think it is related to the plugs or coil packs. Coils and plugs tend to fail when the engine is under load and warm. They also get worse with time, which does not describe my situation, existing over a year’s time.
I think it may be affecting the fuel mileage, or maybe the car is just thirsty.

The bad component in my case is equally bad when in the open-loop mode as well as the closed-loop mode. It is not unique to either mode. Therefore it is not something that the ECU can adjust or correct for in the closed loop mode when the catalytic converter is up to temperature.

I do not want to go to the dealer and come back with $2000 worth of new parts plus labor.
Anyone have suggestions for replacing the most likely component?
My own #1 guess is the crank position sensor.
My #2 guess is the water temp sensor that reports to the ECU.

What do you guys think?
Thanks for your input.

S’ V’