Does your car have monovalve. If so check that. Sounds like it needs rebuilt. Also does your engine reach 80C after normal warm up? if not it could be the stuck open thermostat( Which I have now in mine)

Read about water recirculating pump posted by me in past do a search under MVK. Lots of info there.

MVK

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One penny saved is three penny earned.

1985 300D Turbo 179,000miles