I have found a 98 5.4 3.73 with tow package, Gibson exhaust, aftermarket crazy stero, and the old 2H/AW/4H/4L system with only 29,000 miles on it. It is in great condition.

My big question is I think I can hear a slight knock under WOT between 3-4K. Could this be piston slap, or another serious problem?

 

may not be a bad idea to get it checked out before you buy

 

I bought my '98 two years ago with 27,000 miles on it. Now I've got 57,000, and mine's done the same thing the whole time. It's not consistent, but every now and then while WFO it'll do it. Also, it's not very loud, just barely noticable.

 

Could this be "piston slap?" And yes, I'm getting it checked out, but mechs can only see so much-and seem never able to to hear anything to diagnose it correctly.

Does piston slap occur only during cold startups? Would 89 octane help solve the problem?

 

I have a '98 with the 5.4 and piston slap. Octane won't help. It sounds horrible in the winter, but not all that noticeable during the summer. The engine is very strong, even with 129,000 miles on it. Personally, I wouldn't worry about it. I may have missed something,( and someone please inform me if I have), but I have yet to see where piston slap has actually damaged anyone's engine. From what I can tell, it's just embarassing as hell to drive a $35k rig that emits sounds like that from under the hood. Good luck with the purchase. Sounds like a nice ride.

 

Sounds like you've got a great deal on your hands... My 98 has a bit of this and that too, but with 92K on her... She just gets up and goes... I love my 98 Expy... Buy it, Buy it!