By fuel pump, I meant the mechanical injection pump that is mounted under front of intake manifold. Pump looks like this
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located under front of intake manifold. Think the average rebuilt is around $630 or so nowdays, new bosch injectors are around $42 each. The mechanical lift pump is around $54, should be mounted on side of block just like a gas engine. The lift pumps can fail too, best tell is your engine oil level getting higher from diaphragm leaking into the pan. You can check that by just opening the drain line on the fuel filter and cranking engine with pump power wire pulled to see how much fuel you get out...by the way, the filter is another thing that you should change regularly.
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I used a set of diesel injection pressure gauges to check out my pump pressure when it was hot, pulled the wire off the pump and cranked to see what I had.
Here is a document about the 6.2l that might be helpful.
http://matveynator.ru/f/9efda83135c6d53e16e47aab9a0122c4.pdf
Another item with the fuels nowdays being ultra low sulfur, you absolutely need to use a good fuel additive because these pumps and injectors were designed for the lubrication of moving parts to be from the sulfur that used to be in there. Without the lubrication, the needles and seats in the injectors and the injection pump moving internals wear much faster. I use Stanadyne fuel additive in my old diesels and my 05 Cummins.
Last thing I like to do on all the older diesels is upgrade the filter with spin on filters for better flow and can get better micron filtration than factory filter, here is an example.
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There is no end of ways to play with your truck, lol.