I have a problem with my engine surging or perhaps I should say loping whenever I am in park or neutral.The car does not seem to have "the pep" it formerely had.I have driven it very seldom in past 6 or 7 months.I added diesel additive to increase the cetane number to no avail.I would appreciate any advice for this problem.Sincerely,Daniel

When is the last time you had a good valve adjustment?

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1982 300SD Light Blue
2002 Honda Accord SE
1974 Toro Wheel Horse Tractor
2000 Toyota Tundra Pickup

Sounds like a candidate for a few diesel tune up tips.
1) Check the fuel rack adjusting bolt. It is mounted on the rear of the IP, in a very narrow space between the oil filter cannister and the ID. Has a locking nut and bolt. Loosen the locking nut and turn the bolt in some with the engine running. It may steady the idle lope. If it turns all the way in with no change it may be worn, or in some cases it was defective. The old silver fuel rack adjusters were replaced with gold tone ones years ago. Still it is possible you had one that was never done.
If you turn it in too far, the engine will die if you rev it an let the pedal drop sharply. Just back it out a little.
2) Get some diesel purge and clean you fuel injectors. There are several sites that show how. Very effective medicine for the diesel engine.
3) check the pressure line coming from the rear of the intake, through a banjo bolt and up through the switchover valve. Several threads here that explain that one also.
4) valve adjustment every 30k miles is advisable