Your lack of heat is 95% caused by the monovalve diaphragm being broken, you buy it as a kit and it's very easy to replace but messy. PN for my 420sel 88 is 000 835 06 44. Symptoms on my car were the same as yours.
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Mihai
1988 420sel
1973 porsche 914 2.0 (Deceased)
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