I recently did similar work on my 95 E300D, and ended-up with similar symptoms. In my case it turned out to be trapped air in the system. Took close to 300 Miles, over an 6 day period to get rids of it. Coincidentally, my 1 year old Rad cap gave-up at the same time, would not keep pressure.

Try the following: 1) squeeze the top of the radiator hose when its hot. Should be pressurized. If not than its the cap. 2) Squeeze it when its stone cold. Is it soft and spongy, do you feel air bubbles?

So if your problem is trapped air, be patient, it will escape on its own.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]