I'm thinking a lifter not pumping up would be less likely to result in low compression. I believe he stated he has leak at the intake valve located by a leak test. Just like a vacuum gauge a leak test is very helpful. My 03 3.9 threw a P0303 after changing the MAP. The check engine went back out and I cleared the codes and they they have not come back yet, SteveYou may have a collapsed lifter that won't pump up until oil pressure increases at higher RPMs. That could also cause a low compression reading on that cylinder.