1999 S70 rpm's pulsing

[wolf]

something really wierd has been happening lately. Everytime I am driving where my rpms are around or below 1000 (going fairly slow or stopped) my rpms pulse to about 1,500 rpms, its like i was pushing a little bit on the gas every second. I have no idea whats going on. I can come to a complete stop, then let go of my breaks, and instead of slowly gaining a mile or two ann hour, it kind of jolts every second as my rpms go up, like there was a ghost pressing my gas peddle. I can also see it in my headlights, they kind of dim every time the rpms go up a little/. If anyone knows anything i could do, i would grealy appreciate it.

[JBRollo2000]

You did not say if the A/C was on or off. Sometimes the A/C cycling on/off can cause RPM problem. In that case check freon (too much or too little), aux fan operation, and high/low pressure sensors.

Assuming A/C is off, you might check out the Throttle Position Sensor. These things usually go bad at the lower RPM positions. There is a voltage specification at idle for most of these. I have never checked out one for a Volvo but generally you measure the voltage with a digital voltmeter at idle. I have used two needles pushed into the wires but I do not reccomend that. Alternatively, you can disconnect the connector and measure resistance. You should have a smooth increase upward when you move the throttle.

On many GM cars, the idle reading voltage should be about 0.5 volts ASSUMING no one has turned up the idle. Don't know the Volvo specs, sorry.
(Turning up the idle stop on a computer controlled engine drives the computer completely nuts and will cause RPM hunting)

Jim Rollins
Cairo, GA