1998 F150 256 V6 5spd, 4wd, with aluminum plenuum.

I threw the code 1538 (bank 2 IMRC stuck open). So, I take the plenuum off and check the bushing. They seem to be ok from what I can see with a mirror. I put the vacuum guage on the driver's side and the butterfly valves close at 9 in/Hg. I put the vacuum guage on the passenger side (bank 1) and the valves close at 9 in/Hg. I trace back to the vacuum solenoid where the vacuum line originates for the IMRC actuators and 9 in/Hg vacuum closes both sides of butterfly valves. I trace even further back on the vacuum line (which goes into the fender) and no leaks. The vacuum solenoid is normally closed so I could not check to see if it was bleeding off vacuum but I would think if it were the problem, then both IMRC acutuators would not be getting enough vacuum and both sets of valves (banks 1 and 2) would be stuck open. I'm thinking that I'm either getting a bad signal from the acuator to the PCM saying it is open when it is not, or something is wrong with the PCM.

Again: Only one code for the driver's side acuator, vacuum lines are intact and holding a vacuum, both acuators close the butterfly valves when 9 in/Hg vacuum applied separately or together, bushing are good on actuator rods.

Anyone got any other ideas before I put this back together?

Thanks.