When the temp sensor on the head is no longer functional, the engine is only affected during cold startup. I know because I broke it 2 days ago and the change was very obvious. Once the engine warmed up I didn't notice any difference. I got a used one today and the change back was obvious. Having the 87 model, we get the fragile green 2 prong sensor instead of the plug and it will crack easy if any pressure is applied. Once cracked its toast because the wire that goes through is molded to the plastic with no slack so any cracks and the wire tears.

I would look into the FPR first as MB Doc said.

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2016 Monsoon Gray Audi Allroad - 21k
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2003 Gray Mercedes ML350 - 122k