Quick diagnosis please, sudden engine roughness

1989 Cherokee 4.0, 200K miles

Just started running rough one day, like running on 5 cylinders.

I pulled off the fuel injector plugs one at a time, and all resulted in worse running except cylinder #6, which gave no change in performance. I used a stethoscope at each fuel injector to find clicking on all cylinders, so the solenoids must be operating. And it does have spark at cylinder #6.

Does this mean it must be a plugged-up fuel injector on #6?

Not necessarily, did you pull the plug wire off #6? Did you pull all the plugs and do a comparison? You might also have a vacuum leak. Have done a compression check or a leak down test. You might also have a problem in the valvalvetrain.

I pulled the plug wire off of #6 to insert a spare plug and make sure I had spark.

I pulled out spark plug #6 and #3 to compare. #3 was black, not very tan like it should be, but #6 was quite a bit worse, although not horrible, I still think it would fire. The Jeep sits a lot and does not get driven much anymore. But I cleaned both plugs and switched them, but no change in running condition.

I don't see how a vacuum leak would affect only one cylinder.

I have not done a compression or leak down test yet because I'm being lazy there is so much crap in the way it is hard to even pull spark plugs. The Jeep had been running OK, I just feel a valvetrain problem is unlikely, and a plugged injector is more likely, but I'm here to see what you people say.

A vucumn leak at the manifold is not uncommon at all! You have lots of things to check. I would only buy injectors as a set so their not a cheap "just throw parts at it" cure.

A plugged injector will sound muffled through the stethoscope, while an operational one will have a metallic click. I would also suggest a compression test, just to rule out engine mechanical issues.

Interesting. As I recall, some of the injectors had a metallic click, some of them were muffled to a degree, and #6 was somewhere in between. Could be more than one injector is clogged to an extent.

I'll probably have to pull the fuel rail and injectors out and see how well they squirt, as well as doing a compression test.

Also switch #6 injector with another one and see if the problem follows. Could also be burnt valve, carbon build up, collapsed lifter. Compression test could help.

UPDATE: Engine fixed
I checked compression, all cylinders close to 150 psi.
I ran the engine straight from a can of carburetor cleaner connected to the fuel rail with a homemade adapter as per YouTube videos. Did not help.
I pulled all the injectors and noticed #6 had tan deposits around the tip almost like calcium deposits. I shot carburetor cleaner down each injector (as per YouTube) which seemed to improve the spray pattern on four of the original injectors (including #6), but two of the remaining injectors that had been replaced many years ago actually got worse and essentially stopped flowing altogether. I can't explain that. But all injectors seemed to keep clicking when connected to a battery however.
I wasn't going to reinstall bad and mismatched injectors, and decided to buy a set of upgraded and matched four hole Bosch injectors.
After installation, I immediately had a huge fuel leak from the 34 year old fuel pressure regulator O-ring and then noticed it's diaphragm was also shot. So I had to wait again for parts.
Installed the new regulator, replaced a couple of cracked and leaking vacuum lines I noticed along the way, put everything back together, and now she runs.
The End