I have reviewed your fault codes in the Chilton's Caravan/Voyager manual, part #8155, page 4-17, for 1987-1990 MFI engines. Here is what the manual states:
13 - MAP Pneumatic Signal/MAP Pneumatic Change - No variation in MAP sensor signal is detected./No difference is recognized between the engine MAP reading and the stored barometric pressure reading.
This fault indicates that there is a problem with the barometric air pressure reading read by the MAP. You may either have a problem with the MAP itself or the pressure measuring device the MAP reads.
54 - Sync Pick-Up Signal - No high data rate signal detected during engine rotation.
I'm not sure what this is, but you may have another faulty sensor in the engine.
55- Completion of fault code display on the Check Enginer Lamp.
I hope this information helps you. You definitely have a problem with the MAP and some other sensor in your engine. Good luck with your trouble-shooting and let us know what you find.