W126 warm-up phase
 

Throttle response is great when the engine is completely cold, or fully warmed up. But theres a brief period of several minutes in between when there is a noticable hesitation off idle.

I'm guessing this is the warmup regulator going lean too quickly. System and control pressures are good, though I had to knock the pin down a little to get the cold control pressure where its supposed to be. Where else should I be looking, or should I just leave well enough alone?

looks like you answered yer own question. most likelly the
warm up regulator. shouldn't need to "adjust" it. this is a
sign that its bad. just make sure that yer fuel pressure and
flow is good.

george

Would increasing the system pressure change anything, as long as its still within specs?

no, don't think so. its hard to diagnose over the net
but what messes you up is the control pressure.
also check that yer o2 is working properly.

george

O2 is fine (new). The problem is during the period before closed loop. What other inputs control the warmup regulator, or is it completely stand-alone? I'd rather chase down a bad sensor than replace the WU regulator, which I understand is expensive.

Sure is good to find george on here. can always count on good advice from him. pauljc

if yer fuel pressures are fine then i think yer stuck with the
warm up regulator. you do have other options like eng vac leaks
that need to be found with a smoke machine.
best,

george

What will happen if I uninstalled the O2 completely completly on a 560SEL 1989 car?