When I am slowing down from 40mph to 10mph the rpm will become 400rpm to 500rpm, I could hear the engine almost stalling. Then when the vehicle comes to idle the rpm will become 900rpm. During winter time, sometimes the car will stall when I am slowing down at 10mph, but it starts so easy after the stall.

Is it normal to have 900rpm on idle? Is it normal for the rpm to go down as far as 400 to 500rpm when slowing down at 5 to 10mph after driving 40mph?

What is wrong with the car? Is it vacuum leak? Or, transmission torque converter or the whole transmission problem? Is it EGR, EGMor PCV valve problem? Or the whole engine?

By the way, my spark plugs,fuel filter, air filterare new and have oil change every 7500miles with synthetic oil, and I'm using premium or super gasoline.

 

Could be a vacuum leak, sometimes vacuum leaks can give erratic idle as well. On a S70 unless it is turbo, they dont use the EGR system. I know PCV problems, leaks, blockage, can cause stalling. But it sounds like your problem is more likely in the Idle air control valve, or TPS sensor. IAC malfunction would definatley stall vehicle under certain conditions, and a bad TPS sensor can pretty much memich a bad IAC, but will usually trigger a CEL condition,(at least on GM). IAC will also help compensate for engine loads when in idle, or drive, such as A/C compressor, A/T, Engine coolant temp., engine RPM's, etc.

With that being said, (a short novel), I would pull IAC and clean, check for valve stickiness, or anything else that may catch your eye. Also please specify, year model engine mileage etc.

 

A good throttle cleaning usually fixes that.

 

my 98 volvo s70 has an erratic idle, I have cleaned the Throttle body, mass airflow sensor and idle control valve, also checked the plugs seemed ok. The car has power and runs OK