The cam should be visually inspected, same with rocker arms and valve stems. Cam lobes are usually miked for excessive wear and loss of lift. If cam is worn or rocker arms are damaged, there is usually a loss of performance that may or may not be noticeable depending on the degree of damage. Make sure the dealer is clearly able to detail the exact problem before you sink a ton of cash into the repair. If it's the rocker arms/oilers have them show you the problem. Same with the cam. You describe a smooth running engine, so I'm still leaning towards a hung lifter. At least for your sake and your wallet. Hope it turns out OK.

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE