Solid lifters are just that, solid hunks of metal that follow the lobes of the cam and transfer the movement to the valves(through the use of pushrods and rocker arms). They have a slightly convex bottom which follows the lobe of the cam.
Hydraulic lifters are basically the same except they use oil to control the expansion of the valve train through the use of a metering valve and oil to control the thermal expansion and contraction in the valve train- with solid lifters you must set the lash(sort of like a spark plug gap for valves) to compensate for the thermal expansion and contraction.
That said , Tonys Explorer really does not use either one. The explorers (4.0 pushrod) use a hybrid of the hydraulic lifter with a roller on the bottom(part that rides on the cam)- a hydraulic roller cam, quite common on engines these days. The roller follows the cam lobe.
How to check what kind of lifters are installed I am not sure. You could pull the valve covers, and intake and pull a lifter out to look at it- should have a tab on one side and a wheel (the roller) on the bottom. If its flat then they used the wrong lifters. I doubt that the cam is the problem, Comp Cams only makes two cams for the 4.0 and both are Hydraulic roller grinds. You could check the part number it should be engraved on the rear of the cam-you have to take it out though.
Looks like the engine builder is looking for an out. I would suggest a second opinion by another shop in the area. Get the shop to get you the invoices for the materials they used, take a look and see what they put in
One question is did they use new lifters? The explorer lifters over 75k miles seem to have probs of their own!
Hope this helps a little- I know this has been a long and frustrating problem.
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Steve VB
91 Navajo
2 1/2" Rancho,
31" Goodyear AT/S
