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Rough Idle Fix - The what and why (archive)
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Posted by Scott Brown on September 14, 2001 at 20:31:31:

As promised:

Fix was installation of a new water pump and thermostat. It was probably just the water pump, but while they were in the neighborhood... Here's the explanation I received: There's a pezio-electric (spelling?) sensor under the hood listening for ingnition knock. One of the cooling lines/pipes runs right by it so it sensed the water pump's death knell as knocking and misadjusted the timing. Sounds good, but why no problem with the A/C off? Water pump is still doing its thing, n'est pas? No good answer here, but my favorite mechanic in Melbourne FL, Owen Tamburr, offers that perhaps the extra load created by the A/C compressor helps establish just the right harmonic for the sensor to react to. Whatever, my enjoyment meter is way to the right again!

FYI - I asked for the parts back and have inspected the water pump. No obvious signs of failure - impeller blades are good, housing sound, etc. But it is extremely hard to turn by hand. Thoughts??

SWB
97 740iL
46,000 miles

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