I have a 1988 ford ranger with a 2.3l 4cyl fi. When it is idling it surges and then goes down until it dies. I searched but found no vacum leaks. I unhooked the map sensor and it will idle fine but will not high idle. I need some help. Is the map sensor the problem or something else and can you test the map sensor.
try cleaning your Inlet air temperature sensor with some throttle body cleaner. also while you are there clean the throttle body and the intake manifold. this condition could be caused by carbon buildup
Key ON engine OFF. use a DVOM with Hz reading. ground black lead. push red lead into center connector. should read 140-150 Hz.
Can also test by bleeding in excess air. MAP would see vacuum drop, and try to richen mixture assuming that the engine was under load. Hz would change.
Failed MAP sensors cause the idle to wander all over the place. Slow. fast, weak, wobbly. All over the place with no rhyme nor reason. ('Cuz the mixture is being diddled with like a drunken sailor....)
I'd check fuel pressure regulator also.
Hope this helps...