Surges On Idle

I am having a problem with my 1987 3.0 Aerostar. When starting the rpm will not stay constant. It will surge then stumble and die. I have cleaned the Idle air valve, changed the throttle position sensor, changed the O2 sensor, placed the cap and rotor (maintenance), and searched for any vacuum leaks. I am running out of ideas. There is a constant voltage to the idle air valve. Could this be a bad sensor? Tach maybe?

I ran codes and got:

Code 18 Ignition diagnostic monitor IDM circuit failure, loss of rpm signal or spout circuit grounded.

Can anyone tell me where the problem is?

I replaced the MAP sensor and that seems to have solved the problem.