I don't have a ton of experience in troubleshooting although I have been plagued with problems since we bought our '88 190E. That sounds familiar to me. I was having that problem after I took off the air filter and broke one of the plastic vaccuum things. I would say that since the idle is extremely dependent on the vaccuum system, I would double check all the vaccuum connections under the air filter. Next to the valve cover. Good luck, hope this helps.
1988 190E 2.3 (178,000)
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