2000 Volvo S70 GLT SE 187k. Started getting check engine fault on very cold days (below 15°). Vehicle had original MAF sensor so I suspect it was at least 15 years old. Symptoms were erratic idle while waiting stopped or hunting tachometer while making turns under 15mph. After tiring of spraying MAF cleaner on unit I bought a new MAF sensor; installed, but still getting tach hunting (between 400-700rpm) but not consistently. Sometimes (60%) It idles just fine (although after braking it will dip to 600 and come back to 850-900. Any ideas what to look at next and thanks in advance.

The P0101 didn't come back?

Check for a small vacuum leak, any unmetered air getting into the intake manifold is bad news. Don't forget the lines to/from the turbo.

Check the check valve at the brake booster, new for 2000 it tends to fall apart.
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Keeping it running is better than buying new

Thanks for responses. This is all very new to me; anybody got diagrams or photo help to indicate where I can find these things? Again, TIA!

"...after braking it will dip to 600 and come back to 850-900...."

What about the possibility of a vacuum leak somewhere in the brake booster system? It would cost nothing to try capping the hose as a test.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Not sure about your model but the 93-98 cars, the temperature sensor is very important to the idle. The 960 when cold would raise and drop the RPMs like crazy: It was hunting for the proper idle.