5.7 Stumbles when Oil fill cap removed

Buddy of mine has a 98 c1500 like mine. He has a bad miss on cyl 6. He removed his oil fill cap with the engine running for some reason I can't remember and it stumbled like you removed the vac hose going to the brake booster. It recovers and runs like it did with the cap on. When I remove my cap with the engine running, no change. We checked the EGR valve. Any other ideas?????
Tks in advance.

check pcv system, especially the valve.

PCV valve already replaced / swapped with mine. Keep the ideas coming:biggrin:

Do a compression test on this skipping cylinder, You may find a blown head gasket.

Make sure the vent hose isn't plugged.