This customer has a vehicle with extremely worn crankshaft seals. This caused the engine to not accept the PCV cap. And (as we all well know!) WITHOUT capping the PCV, likelihood of a mileage increase is minimal.
What happens when you cap off the PCV valve on engines with extremely worn seals is they begin to leak oil from the front seal. This is NOT caused by the groove, but normal wear-and-tear on the engine. The vacuum was the only thing keeping the oil in the pan!
(See Service Bulletins for PCV Capping Issues)
Customer refused to replace the seal, and was (understandably) disappointed in his gains as there were none!
