1988 Jeep Cherokee Idles ok, but misfires totally if gas is

1988 JEEP CHEROKEE
250,000 MILES • 6 CYL • 2WD • AUTOMATIC
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PHYSMECH
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4.0L engine, have checked air, gas, and spark and all look good. Engine will idle ok, but seems to misfire completely once throttle is applied, with complete absence of power.

Plugs and distributor cap & rotor are fine and spark checks out ok on all 6 wires. Fuel pressure at fuel rail is right on spec with fuel pressure meter. New fuel filter also. Exhaust has been tried with muffler/catalytic removed. Catalytic was clear. Airbox is clear and air filter is new. No vacuum leaks that I can see.

Tried to check trottle position sensor, but was unable to get a reading. Can this sensor or some other sensor be causing this?

This problem started being intermittent at first, and would usually start once the car first got warmed up, then would settle down after misfiring for a minute or two. Now it messes up all the time.

Another question - how much play in the rotor (side to side) is ok? There is about 5-10 degrees of play when I checked the cap/rotor.

Other things I've thought about are fuel injection system or worn out timing chain?...
Jun 6, 2008 at 3:26 PM
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RASMATAZ
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Test the throttle position and manifold absolute pressure sensors/ignition coil primary and secondary resistances.
Jun 6, 2008 at 3:46 PM
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PHYSMECH
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The rotor is secure and tight on the distributor shaft, but you can move the distributor (rotor) about 3/8 to 1/2 an inch back and forth. Cap is tight and contacts are not worn or scored much. Asked someone today about this, and they said the distributor is probably worn out. 250,000 miles on this thing, could they be correct?
Jun 6, 2008 at 10:18 PM
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RASMATAZ
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Could be the distributor mechanical/centrifugal advance spring weights are sprung
Jun 7, 2008 at 2:28 AM
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