Ok my brother came home today with his engine running rough. I been out of the industry for many years now. I hadn't had much time to work on it and i'm disabled. So I will be more less a guide.
He said it ran fine until he went to leave work and the engine light was blinking and it ran rough he claimed knocking.
I pulled the codes, had the normal O2 codes we had since my dad purchased it. But we did have a new code P0300 misfire. I believe all the codes were 300, 420 440, 135, 137.
I seen it was misfiring externally heavily on the #4 and #5 cylinders. Under the hood I found the PVC line at the top of the intake removed. It's times for wires and plugs as well.
I reset the codes and got the 300 back. The O2 codes will come back and I can't rule them to be part of the problem as they been their for years.
Emissions code may be fixed, havn't drove vehicle yet.
Tomorrow I plan on putting a compression tester on, bore scope in their, and fuel pressure test and probably a coolant pressure test (no evidence of a coolen't leak yet). We did notice alot of smoke, but have to wait till day to see what color it is.
I did notice that it smooth's out when I bring the idle up and misfires drop off to zero at high enough RPM. But 4 and 5 were the only ones misfiring and only at lower RPMs.
Anyways, anyone see this on these engines? if their a common problem to be looking at? How is 4 and 5 tied together? It's possible 5 always misfired and now 4 is too (5 is the hard plug to get to, not sure if anyone's done it) I know years ago we couldn't get to it which should have been right through the fender well. For some reason we didn't have the clearance on this vehicle.
Well, ill find out more in the mourning, but maybe someone already knows of a common problem here?

 

I am curious on what your fuel pressure and leak down tests show before I go on any farther.

Key on engine off for the fuel pressure and leak down test, we need to know the initial pressure and what it is after 5-10 min.

 

AC Delco 41-993 plugs, .060" gap.
AC Delco cap & rotor, silicone dielectric on each terminal, inside and out, of the cap.
A good quality set of plug wires.


#3 plug is the rough one to get to, (driver side, center plug). Disconnecting the steering shaft from the gearbox makes all the difference in the world. You can go straight in through the wheel well, don't even need to remove the tire.


Anyway, replace the plugs, cap, rotor, and wires, and report back.

 

Definatly would change the cap, rotor, and plugs first