5.0L low/no vacuum idle / full throttle
11-24-2015, 02:35 PM
#15
I solved this problem and thought it might be helpful to someone in the future.
Truck has been on the road now about 6 weeks and runs like a watch!
A local racing engine builder suggested "valve tension" (he's only seen one other time on a Chevy motor).
This occurs when the head is decked and (due to the shorter head height) without adjustment at rocker or pushrod prematurely opens the valves by putting tension on the valve train. I think the fact the engine passed the Compression Check and Leakdown test is indicative of this problem (i.e. leakdown test occurs at TDC where the valves are closed, and compression tester holds max pressure (doesn't indicate premature valve opening).
I found some Rocker shims at a performance mustang website (.020 and .040). With the .040 shims installed the rocker was visible loose when torqued. The .020 shims got the truck on the road and driveable, but I though the mileage was very poor. I manufactured some .005 shims and tried a total of .030 the rockers were loose, so I backed it down to .025. All the rockers torqued without any "excessive" wiggle, and the pushrods were tight (no vertical movement while they still rotated).
So, reproved the lesson - junkyard parts, you never know what you'll get.
Truck is 13K from 300K miles - guess it's time to start working on those quarters...
Truck has been on the road now about 6 weeks and runs like a watch!
A local racing engine builder suggested "valve tension" (he's only seen one other time on a Chevy motor).
This occurs when the head is decked and (due to the shorter head height) without adjustment at rocker or pushrod prematurely opens the valves by putting tension on the valve train. I think the fact the engine passed the Compression Check and Leakdown test is indicative of this problem (i.e. leakdown test occurs at TDC where the valves are closed, and compression tester holds max pressure (doesn't indicate premature valve opening).
I found some Rocker shims at a performance mustang website (.020 and .040). With the .040 shims installed the rocker was visible loose when torqued. The .020 shims got the truck on the road and driveable, but I though the mileage was very poor. I manufactured some .005 shims and tried a total of .030 the rockers were loose, so I backed it down to .025. All the rockers torqued without any "excessive" wiggle, and the pushrods were tight (no vertical movement while they still rotated).
So, reproved the lesson - junkyard parts, you never know what you'll get.
Truck is 13K from 300K miles - guess it's time to start working on those quarters...
Last edited by ytudrepus; 11-24-2015 at 02:39 PM.
