91 chevy ecm in limp home mode??? cant figure it out. how smart are you?

91 chevy silverado 5.7L runs like crap i have replaced the following parts to no avail. plugs, cap,rotor,wires,igntion module,electronic spark control module, map sensor, ecm, coolant temp sensor. it says my ecm is running in limp home mode. but when i unplug the map sensor it seems to run good but then it gets bad again after a few minutes. any ideas? oh and i changed the pickup coil too.|||have you done a computer relearn or had the ecm flashed with the proper info