I had a similar issue once, the culprit turned out to be a couple of cracked air hoses. When the car got hot, the cracks opened up letting air in and causing a low and rough idle.

Easy test is to spray (sparingly) carb cleaner around suspected areas. Watch for idle surges. On that motor, pay attention to the cold-start injector hose, (under intake manifold), idle air hose, and all the rubber parts that you can't see such as the boot on the bottom of the air meter. Also, the vacuum hose to the valve cover can crack and leak on the underside and look fine from above.

Also wouldn't hurt to check the idle air valve, if it's unable to change as the car warms up, that could give a similar symptom.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition