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This trend is starting to beg the question why, and who makes a good lifter.
I believe I know the primary reason, two valves per lifter. But as a secondary cause, I'm really starting to think about the low oil pressure, fully hot at idle situation. The ambulances and certain service trucks would be prone to the worst oil volume issues.
I think you have a point, but throw in the mix that the 7.3 used the same lifter and I only have seen one damaged lifter on one and that roller was still in one piece. That one looked like the cam was harmed before the lifter. But that is a 2 valve per not a 4. I also have a pile of d Max's in the fleet with roller cams and 4 valves per, and no lifter issues yet. I wish the older e series had an hour meter on them to compare to. I see the 6.6 with 100k on em and over 12k hours on them. One of my 10 E350's is in now with 150k and 18k hours on it waiting for an oil cooler. I leaning to the oil cooler as a primary problem, especially since Ford added the P252F to the programming.
With the 6.0 and 6.4 is where it gets to open 2 valves per lifter. The 7.3 makes the point.
I don't see it as the oil cooler, although I don't despute poor cooling of the oil could be a factor. My concern is how low the oil pressure goes from reports of those that installed oil pressure gauges. Unfortunately, I didn't so I don't have a history to reveal. From others, cold start goes to 75 psi, hot can be around 10 psi.
My failed lifter is I believe internal to the hydraulic valving from day 1, not a roller bearing issue. But idle rpm doesn't change the compression loading on a lifters roller, but it sure seems to alter the pressurization which could alter the oil spray patters within the block.
Another thought is intake or exhaust lifter failure. Most of them that I have seen is intake roller failure, with the poof poof in the air filter like the 6.4. But, the 06 I just finished was an exhaust roller. Came in for a hard start. When engine was cranking, it would slow on one cyl. Acted like too much compression on one. On the oil press thought, I have only put a gauge on one. That one had the oil press lamp on at hot idle, found the new to us engine was a train wreck.
