Smoking WVO

Help! My 1985 MB 300D which has a coolant heated wvo system is smoking. Mostly on acceleration. The rear tank, which is where I mistakenly put the thermostat, reads 120F so I'm pretty sure I'm getting up to 150-160F up front. I've been checking the engine oil and it doesn't seem to be burning oil. Does anyone have any ideas why it might be smoking?

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