Just wanted to add my solution to this thread on what ended my rough idle...
Same car as Nicks1 here - 1998 E300 TD. Approx 139k on the odo. Symptom: rough idle in park or neutral only after car warms up (hitting roughly 80 on the temp guage) While car was in drive, no problem on idle. And when car was cold (below 80 on the gauge), no problem) - (although it was a bit rough on deceleration around 10 mph also). Anyhow, here's what my journey was to the solution. Rough idle was not fixed until the final repair below..... read on.....
1. Motor mounts were new - just replaced per receipts before I bought the car.
2. I added several doses of diesel kleen to the tank and indulged myself with several italian tune ups.
3. Fuel lines checked for air bubbles several times - nothing
4. Fuel filters, air filter replaced
5. EGR valve checked, not failing
6. Fuel shutoff valve replaced (was leaking)
7. Coolant temp sensor replaced
8. Heat exchanger replaced
9. Probably something else here I'm forgetting. (cocktail impact here)
10. Injector pump - PROBLEM SOLVED

After the new pump was installed, it's smooth on deceleration and idle in neutral or any gear idle is silky smooth also. Just finished a 300 mile road trip and may have exceeded the speed limit by a few mph. You know, 58 or 59.

Hope this is info is helpful. And yes, the cost of the above repairs more or less went from cheapest to most pricey. But I love my E300.

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Barker