Hi all, great site, 1st post, head-scratching problem.
I have a 97 E-250 with 4.2 getting P1537 (IMRC stuck open) and P0174 (whatever).
The sweet thing with the E-250 is removing the cowl or cover in the cab and all IMRC components are right there in front of me. All vacuum lines are good, the plastic actuator bushings are good, both actuators operate and the solenoid checks out.

Here is what happens:
Engine off and imrcs are open.
Start engine, imrcs close.
25 secs go by and imrcs open.
imrcs remain open regardless of rpm.
Turn engine off, imrcs remain open.

Everything functions normally during the first 25 secs after starting the engine (imrcs go open above 2500rpm and go closed below 2500rpm) but as soon as 25 secs go by, both actuators revert to the open position and remain there at any rpm. I've scratched my head watching this process over and over enough to know that it is pretty much exactly 25 secs after the engine is started.
I figure these valves being open is causing my lean/rich condition or whatever is causing the P0174.
I'm guessing it could still be an actuator or 2 causing the problem even though they appear to operate normally, but at $150 each it's not the kind of trial and error that I prefer.
Any thoughts or ideas greatly appreciated,
Terry