Vacuum drop or misfire on one cylinder??

2001 Jeep Cherokee Limited, 4.0, auto, NP231, with 81k miles
How do I tell if I have a bad coil pack (coil on cylinder set up) or a intake vac leak? Would an intake vacuum leak cause only one cylinder to "hiccup"?

Slightly rough idle when in gear and stopped, it doesn't want to die like when an Idle Air Control (IAC) is bad. It idles at around 900 RPM but dips to about 800 RPM for a half second then back up to 900 RPM for about 1-2 seconds or so, then repeats. I checked for noticeable vacuum leaks but found none. I pulled all vac hoses off and tested each vac line with a vacuum tool but I didn't find any noticeable bleed down. I plugged all vac lines into the intake manifold and put vac gauge on intake manifold, I see a drop in vac for a half second that matches my idle drop. Compression test was solid on all cylinders. I also know that I have low vacuum at stoplights because my vent wants to blow out the defrost vents until I accelerate from stop.

Any help would be great! Thanks!!!

Oh yeah, no engine codes are being thrown currently or over last months but I did get a "1495 - Leak detection pump solenoid" one morning but it went away the next day and never came on for last 4 days.

There is an ancient tool called a vacuum gauge that will tell you tons about your motor if you know how to read it.

old_man said:

There is an ancient tool called a vacuum gauge that will tell you tons about your motor if you know how to read it.

Click to expand...

And how to read it: http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm