5.7 question.
<hr style="color:#000000; background-color:#000000" size="1"> I put this in the General tech section, sorry, mod's please remove. Thank you

This is a new problem to me. I thought I ran up against most issues, apparently not.
Here is the spec's:
1994 C-25 two wheel dr.
TBI
5.7 automatic with A/C
Truck is used at the camp ground i work for. It has sat for over a year because no one wanted to tackle the problem.
Issue:
FYI, firing order 1-8-4-3-6-5-7-2 ( I am sure you all know just a FYI)
Engine is only running on 5 cyc's. # 1,4 & 6 are lazy they do not want to fire. I can/have pulled 1,4 &6 plug wires off the plugs,(at the same time) engine sits there with no change. Pull any other off and it tries to jump out of the truck.
What I have done:
Checked and confirmed the firing order on dist.
Replace cap and rotor
Swapped out a known good plug from a firing cyc to one that is not firing.
No change engine runs the same.
Put bad cyc plug on wire, grounded plug, works fine.
Non firing plug is NOT wet, black but not wet. Firing plugs is a nice tan color.
Removed the cat, thinking maybe a plugged system. Nope not that.
Took off oil filler cap, visual confirmation # 1 exhaust valve is working, cannot see intake valve. (suspect cam is ok) Did not pull v/c off.
Can put finger on all dead cyc's exhaust manifold, cool not hot.
Put oil on intake to check for vacuum leak, nope, oil sat there and did not disappear.
Took EGR valve off and cleaned, making sure the valve was seated, YEP is Ok.
Have considered O2 sensor, but not really sure what the O2 would cause for problems.
This one has me stumped. Any suggestions or help sure would be appreciated.

 

You put a good plug on each of the 3 non-firing wires with the plug out of the engine and they fire or dont?

 

Correct, took the non firing cyc's plugs one at a time, plug wire on them, used jumper wire to ground plug, the all fire fine.
The bad cyc's plugs are black, not wet but black, the good cyc's plugs are a light chocolate brown.
I can actually touch the exhaust manifold of the bad cyc;s, it is not hot.

 

Have you tried a compression check? could be stuck piston rings or even a couple of stuck valves!

 

Compression is 150 lbs on each. I have checked all the obvious things, kinda up against a wall at the moment. Parked it to think this out.

 

i would pull the valve covers off and see if there's something up with the valve train on those cyl's

 

I have thought of that, reluctant to pull them off, this truck has A/C, so the right bank is full of "garbage" that has to be removed just to get at the covers. Thanks for all your inputs, this weird problem.
Does any know what a bad O2 sensor would act like?

 

I tend to think sensors would affect all cylinders together, You said it had been standing some time, anything built a nest in part of the induction?? sound fuel related, is it carb or injection? If injection are all the injectors working, Is there a central injection point? or are there several injection points, I seem to remember that my Astro had multi injectors in a common area & they fed different areas of the induction manifold (I know the Astro is a V6 but the idea is the same!)

 

Thank you for your imput. This unit has TBI, both injectors are/seem to working good. Going to clean all the ground's and check connection.

 

Anyone check what kind of codes are coming up on the ECM/OBDII? Is the check engine light on at all?