My recently purchased 90 D-150 5.2L has a miss to it but doesn't run terribly. I decided to do a compression test on it hoping that there were no problems in the bottom end. Here are the results:
Cyl 1 - 135psi
Cyl 2 - 140psi
Cyl 3 - 135 psi
Cyl 4 - 125psi
Cyl 5 - 140psi
Cyl 6 - 125psi
Cyl 7 - 140psi
Cyl 8 - 140psi
According to my manual, as long as they are over 100 psi and within +-25psi of each other, they are fine. Can someone with more experience chime in and confirm my thoughts?
Also, did a cap/rotor and plug change today. It still has a shake/miss to it. The plug wires looked fine so I did not change. Any other ideas of where to look to figure out my miss/shake?

As far as you went so far I would change the wires as well.
When running,pull one wire at a time and see if it changes,if no change check that cylinder(plug,wire,cap,etc.).

I would change the wires or if you have more time than budget take an ohm meter and see if your wire have much resistance, if they even need to be replaced.

Might be the ECM.

Cam lobe going flat? Egr sticking causing rough idle?

You da Man!!!

Exact problem with my '90 when it was a year or so old. Exhaust pipe went sooty black too.

OP: Pull the hose of the EGR thing on the valvecover and plug the hose with a golf tee or screw. You'll be able to tell right away.

I just left mine plugged.

Well, tried the EGR but not much change.I guess the plug wires need changing. They all seemed to have spark but maybe I'm missing something. Just about every other ignition part has been changed. Wires are next!

Tee a gauge into the fuel line on the back of the throttle body and check the fuel pressure. I forget the exact specs but it should be about 15-17psi when running and hold good pressure for a couple of minutes after being shut off. My 90 was like a new truck after I changed the fuel pump. It would run ok, but not great and be hard starting sometimes. Another common culprit on these trucks is the pickup sensor in the distributor.

Just a note: If the EGR valve is stuck open, disconnecting the vacuum hose and plugging it won't make any difference.

You would need to remove the EGR from the manifold, and put a plate over the two holes on the intake to close them off.

Replace the plug wires, you have no idea of their history. They can develop breaks anyplace along their length and short out the spark to a different wire or to a ground source. I've even seen them short out inside the plug boots!

Just for the sake of weird, I found an old big block that had a inconsistent miss, the plugs, car rotor and wires had been changed several times chasing the miss. What I discovered was there was too much antiseeze on the plug threads for them to be a good ground for the spark. I cleaned the threads in the heads and the threads on the plugs with carb cleaner, dried everything off, added just a touch of antiseeze and reinstalled the plugs and the miss was gone.

If your 318 has evidence of oil surrounding around one or more plugs, it might be worth cleaning and drying the plugs and the heads around the plugs, add just a touch of antiseeze, reinstall the plugs and see if the miss goes away (after you replace the plug wires). Gene

Well, I haven't done too much on the truck this week. I need to order some new spark plug wires and hopefully that will cure my engine miss. I did get to fix the tailgate latch. One of the plastic clips that hold the linkage rods had dry rotted. Also, my ram hood ornament was a little loose and got that corrected today. I will update once I put on the new spark pkug wires.

Make certain that they are installed in the correct pattern! All of us at one time or another have installed them absolutely correctly, just to learn that we messed up anyway! LOL!

Well, I have changed the spark plug wire but no change. The lopey miss seems to get a little worse as the engine gets warmer also. I have not pulled off the EGR yet but I guess that is my next task to see if it is stuck. Any other ideas are welcome!!