Greeting all

Have SLS 94 with a 4.9. When got home, notice engine was running rough. Check all wires etc, all was fine. Change, plugs, wires, distrubutor cap, rotor, pcv,air cleaner. No changes. used a can of fuel injector cleaner, nothing positive, problems remain the same. When I changed spark plug, noticed #5 cylinder's plug being dark ( carbon) not oil. When to the dealer with it, all checked okay. Cleaned throttle body with cleaner, found both tubes full of deposit, cleaned that. No change. Gave it a good ride ( high speed) no change. Check vacum steady in the green. Decided to pull spark plug on cyl #5 same black deposit. While engine was running pulled spark plug wire from the spark plug, noticed no changes in engine, same constant rough running. It reminded me of cars in the 70 when you had a valve burn etc. Well it does the same...About to pull heads off and get them redone. Any advise prior I do this. ( also done, the toilet paper check at the tail pipe, I can hear a missing piston, )
I love this car!! ( sorry it is a 93)

Uh oh....how's the coolant looking?

Before you start pulling heads, check the injectors. On your car you can do a cylinder balance test by shutting down one injector at a time through the onboard diagnostics to see if there is any change in the idle, or use a noid light, or stethescope to check them.

Coolant is fine, discovered a tiny crack on exhaust manifold, will get that weld

onboard diagnostics ...
I need a quick efficient ''how to do this'' Took me a while to reset, oil indexing reset.
Could you do this for me please

To enter the diagnostics, simultaneously press and hold the OFF & WARMER buttons. Once in the diagnostics HIGH FAN = yes and LOW FAN = no. Beyond that it gets a little more involved to go into the peramiter and override modes. A FSM would be worth it's weight in gold.