I heartily endorse the recommendations for catching up on all the typical "tune-up" jobs (valves, fuel and air filters, etc). Rough idle can have numerous causes, so start simple/cheap and work your way up. Checking the timing chain, checking the glow plugs, checking the fuel injectors; all these could be the cause or several together are contributing to the rough idle.

Valve adjustment and compression: You cannot get meaningful compression numbers until AFTER the valves have been properly adjusted, because a tight valve or two will CAUSE low compression, and the normal life of a valve is to GET TIGHT over time.

When you have the valve cover off for the valve adjustment, you can also quickly perform a rough check of the timing chain stretch by rotating the crankshaft until the camshaft mark lines up with the marks on the camshaft bearing towers. The camshaft mark is on the shoulder just behind the camshaft gear. Read the chain stretch on the crankshaft balancer timing marks. Search on this forum for how-to's / tutorials / guides, the job has been covered many times.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC