You could have good vacuum at idle, but enough blowby at highway speeds to overwhelm the PCV. The breather at the rear of the valve cover should be connected to the air cleaner by a fairly fat hose. Is that the case, or is there just a little air filter there?
One time, as a test, I made a little plastic manifold to allow running two PCV valves in parallel. It didn't make any difference in my case, but you might try it.
One time, as a test, I made a little plastic manifold to allow running two PCV valves in parallel. It didn't make any difference in my case, but you might try it.
