I have a 90 model G20 van with the 5.7 litre.The van has a new engine and ran great for a few months.It started to idle rough.We did a complete tune up and i had a mechanic look at it.When the map sensor is plugged into the clip it runs rough but when you unplug the map sensor it runs great.So we bought a new map sensor and installed it thinking this would correct the problem but it still exists.We also tried a new computer with no luck.Any help would be greatly appreciated.Also has a new fuel pump.

Sounds like the engine is running lean, possible one injector is dead, low fuel pressure or a vacuum leak somewhere. Disconnecting the map forces it to richen up.