Low idle when defrost is on

[mpickett]

My XC90 just did something very weird. On the way to work I turned on the defrost because of fog. no problem. Go to work, parked and took care of a few things inside. About an hour or so later, I had to go to the bank. When I turned the engine on, the defrost was on full blast and I could not figure out how to turn it off immedieatly but the stranger thing is that the engine was pulsing almost, it felt like someone was reving the engine and it was lurching slightly. It sounded as if it were going to die. I finally got the defrost off ( it was on auto though I did not put it on that setting) Once I got the setting back to manual, the engine seemed to go back to normal but the check engine light is on. As a side note, the car has also had a shutter sometimes when I accelerate - not consistent but it is there. Just bought this car used. 100,000 miles on it with no major repairs at this point....

[jimmy57]

The check engine light means there is a code stored and that code will likely tell the tale.
If you dial the knob to defrost and do not manually turn the fan to a lower speed the default for defrost is max fan speed.

I have a scientific wild ass guess but more things need to be verified.
The guess? You have a weak battery and the climate reset your previously dialed down blower speed during cranking and also the Engine Control Module lost its control memory of electric throttle. The fan defaulted to high with the mode control in defrost and the idle speed was the ECM struggling to control engine speed with its control history lost. Did the idle speed settle to normal after you drove it?