First question is why? Unless you have a broken chain (caused by other failures such as a seized cam) then they should be good for the life of the engine meaning 300,000 miles and even more. To just change them as a regular maintenance concern is not necessary. This should help if you do need to replace them. http://www.f150online.com/forums/art...r-engines.html
Your chain tensioner will/should take up any normal slack and it would be extremely rare for the chain to be stretched even close to affecting timing. I've seen these chains with well over 300,000 miles still be in factory spec. I'm with ONELOWF what codes or other indications do you have of a timing issue?
my vacuum pressure is at 15psi and there's no other indication of a leak anywhere else. It got no pickup acceleration and feel like a few cylinder not working,
also given the age and mileage of the truck...
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