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Hans and All:
Appreciating the info.
Car has 133 k miles.
To address the causes you list:
Temp sensor - Gauge works as it should so not that
Cheap gas - same on any fuel, have tried multiple tanks of both, so not that.
Knock sensors - maybe and it will be put on the B list
IAC - idle speed never changes because it is regulated properly.
Intake hose - Have seen that on other Volvos, will check carefully.
MAF - swapped with another like car, no change to either car operation.
Vacuum leaks - checked several times, would not be so uniformly temp sensitive
EGR - Not sure S90 has one but air pump works fine
Air filter - clean, and would not be temp sensitive.
Fuel filter - many thousand miles pulling a trailer runs OK. Only 70K on current filter.
My suspicion of poor fuel mileage is based on other owner reports, and may be within specifications of the car. This inquiry is only about the rough idle when cold.
S' V'
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