Why does my 2.9 engine start with a sparkplug removed and will not start when put back in

SOURCE: ignition wiring

1991-1994 2.9L and 3.0L Engines Firing Order 1-4-2-5-3-6 1995 And Later 3.0L V-6 With DIS Ignition System Firing Order 1-4-2-5-3-6
4.0L Engines Firing Order: 1-4-2-5-3-6 DIS Ignition System

SOURCE: 2001 Ford Windstar runs rough.

remove the coil pack and look at bottom of it if it shows any cracks coil is bad very common on these vans if you have a noid light check for injector pulse on #4 if you have pulse and good blue spark most likely the injector is bad you have to remove upper plenum to get injectors out let me know how you make out

SOURCE: Ford 150 runs rough when cold

Hi! Either the elbow at back of intake manifold is leaking or your intake gaskets are leaking. A very common occurance on both items and either one will set a lean code and illuminate the check engine light. Hope this info helps!

SOURCE: 91 f150 4.9L no visible timing marks, hard start runs rough.

I have wondered why this happens many times in the past. You would think the marks should line up using #1 cylinder but they don't.
It has been a while but I think if you hook up to #5 ignition wire the marks will come up for you.You will need to undo the "spout" connector before setting it to 10 degrees before top dead center. The spout is a two wire jumper clip that should break out of the harness at the distributor.
Warm the engine up for a while so it doesn't stall as easy while you set the timing.

Testimonial: " Right on, timing it on #5 cylinder worked. I thought I probably had a bad damper but it appears o.k. "

SOURCE: engine is running rough

did you change fuel filter? also your fuel pump is starting to fail.how about air filter.