Suddenly began running rough on drive home from work. Idles poorly and loud, but does not stall or backfire. Accelerates so poorly it can’t be driven safely.

I popped the hood and noticed the fuel vapor return tube that should connect to EVAP canister was completely off due to rubber boot dry-rotting. I have no idea how long it may have been disconnected. It’s a pre-formed black plastic tube, with custom bends, that connects evap canister to somewhere under throttle body, on firewall side. I can’t see the throttle end but it “seems” to be secured on the throttle side, when I give it a light tug.

More details:
- A mechanical knocking or vibrating sound seems to come from underside of engine—or at least that’s where I hear it.
- Liquid drips from exhaust pipe –seems to be water
- After it idles for a few minutes (or less), when I turn it off, I can hear “fast dripping” sound at topside of engine area for the next 15-30 seconds—like liquid is dripping somewhere. But nothing is dripping on the ground—so that is internal to something. (or maybe that sound has always been there and I never noticed)
- I have never changed vacuum hoses on this car. So it’s possible the rubber hoses are due to break down soon—although most look pretty good.
- The starter died on me about 3 week earlier. New starter went in with no problems at all and truck ran fine for a couple of weeks until this. Don’t think I could have done anything that contributed, but am paranoid about it.

What I have done:
- No check engine light or codes come up
- Reinstalled air hose back in place on the evap canister side. Note I did not do a thorough check on this tube at other end, because I can’t see back below the throttle body where connects. I probably need to give this more attention but have been distracted due to other things below.

- Put a timing light on each plug wire, right off the coils. Did not check actual timing, just watched light pulses to check for anything unusual. The coilpack near front of car sends signals on a very regular basis to all 4 wires. The coilpack behind it (nearest firewall) seems to pulse much quicker and slightly erratic. Along with that it seems the timing light pulses more dimly from all 4 of those wires (passenger side plugs of engine block), and I can see the light ‘missing’ sometimes. I tried to catch this on video but digital video loses some of this activity. Should the timing light look same for all plug wires, and on both coil packs?
- Removed all plug wires from both coils and checked resistance on coils. Looks good on both pairs, from both coil packs.
- I assumed that timing signals should be similar on both coil packs and since coil packs seemed ok, I replaced Ignition Module without doing a full diagnosis—hoping for an easy fix. Did not fix.
- Installed vacuum gauge and at idle, needle bounces VERY quickly between 14-17 inches. Read online this could be caused by stuck valve. But since this engine fires plugs on exhaust stroke also, I am guessing it could be any normal plug or plug wire problem on any 1 cylinder. Is that correct?

I hope this is not too much info to share. Between the vacuum reading and the funky timing light differences between the 2 coil packs, I don’t know if I should be chasing a possible vacuum leak or an electrical issue. I am guessing an electrical issue is causing the vacuum issue but really don't know. Could it be the other way around? I would appreciate any suggestions.