Reduced power after warmup

Immediately after a cold start my 1990 XJ I6 feels like has more power and better throttle response than it does when it's warm.

It's not a huge difference. But it's noticeable off the line. Those first few stop signs leaving my house in the morning; feels like it really twists right up and flies off the line when I mat it out of those intersections.

Then when it's hot the throttle feels a bit sluggish; like you floor it and it takes it a second or two to build power. It always bothers me at this one intersection where I have to cross a steep hill; people always come flying down it and I want to get through that intersection ASAP and the seeming hesitation there drives me nuts.

Could this just be the difference between cold and hot air in the intake?

Or a result of colder more viscous tranny fluid? I have the AW4.

Maybe the problem starts when it goes into closed-loop mode? O2 sensor?

Any known issues with sluggish response when warm?

Maybe I'm just crazy?

It sounds like it's running good at all RPMs.

O2 sensor reading at idle looks good on my analog voltmeter; that means it's OK and the ECU is in closed loop right?

You need a scanner to know when it has entered closed loop

My guess its running rich at cold start

If it was in open loop wouldn't the O2 sensor reading be constant?

I am pretty sure that O2 readiness isn't the only input (I might be wrong and I don't have the book for Renix)

I'll have to test this tomorrow morning at cold start.

Do you have the FSM? There is a discussion in the fuel management chapter I think