I have one Cylinder that has 50 psi lower compression then the rest and it nails slightly when hot and more so when cold.

When I used to rebuild Fuel Injection Pumps in a Shop Delivery Valve Spring breakage was rare but I have seen it a few times.

What is going to be fun in this case is that if the Delivery Valve Seals are replaced people have said all of them nails for a few 100 miles.

Some People have claim issues with the Delivery Valves themselves that were solved by replacing them but I don't remember the specifics of the symptoms they were having.

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84 300D, 82 Volvo 244Gl Diesel