Since no one else has answered (I'm no mechanic so hopefully a real one will) the most common thing is a sticky hydraulic valve lifter ( due to oil deposits, low oil level or most likely a combo of both)(needs oil based engine cleaners though if the lifter's really stuck the nothings circulating through the lifter so they don't work well, I've had to just replace them, manuals sometimes specify that after certain a milage) or to wide a gap setting on non-hydraulic setups, there are a quite a few different types but whatever you have that is usually wear and tear and is a normal advanced maintenance-tuneup issue. I'm sure there are other possibilities but they're beyond my expertise. If you want better answer might help to include more relevant info: Make model year engine type mileage, etc etc FYI engines are often "gummed up" because people mix oil brands. Modern multi-viscosity oils are complex packages that react chemically in unpredictable was with other brands. Corporate fleet managers sometimes stick with straight 30 wieght to avoid this and get hundreds of thousands of miles out of their vehicles. If you've mixed brands, flush or have flushed your engine thoroughly and then pick a brand and use nothing else. If you have to use other wise in emergency use straight wieght no more than necessary