2000 Jeep Cherokee, 4.0.
For 3 years it's never missed a beat. Never failed to start, always ran like a top.
Parked it at the house the other night, next
morning it cranks but won't fire.

I removed the air snorkel and sprayed some starting fluid in the throttle body. It would
then *sort* of fire - weekly. Not like a brief
rev up like you usually get with ether. Still,
there was some sort of ignition - very brief,
and never after key was released from the start
position.

There's a "test" schrader valve on the fuel rail.
I pressed it and a little fuel came out under some pressure, but less than I expected.

Any theories?

Try banging on the fuel tank while cranking. May have a bad fuel pump.

sounds like a bad fuel pump...check to make sure you got good fuel pressure, with a gauge.

If it cranks but won't fire it could also be the cam and/or crank sensor gone goofy. The engine controller needs to "see" where each (cam & crank) are before it will send fuel to the injectors. IIRC the engine will start and run (not optimally) with a missing cam sensor, but needs to "see" the crank for sure to fire.
Hope it helps!

Very common for crank shaft sensor.

That CPS sensor is on the outside back of the Transmission at about the 11 o,clock position where the tranny bolts up to the engine block.

I have to say, this board rocks.

Bolted in a new crank position sensor and
she fired right up. Many thanks.

Glad to hear of your success