After doing some more searching on the forum, I found a way to test the lift pump by gauging the flow of fuel from the return line and looking for air in the fuel. I think that the lift pump is okay. One thing to note is that I installed this engine last Summer with Charmalu and this wasn't an issue with the old engine, so I think that the fuel tank strainer is fine. It's a great running engine with lots of power and starts up cold in the morning on the first bump. It idles well when cold, just having issues once it gets hot after running for a while.

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'81 300SD