My rpm's drop then jump while im at a stop light does code PO440 have something to do with this?

Question

I have a 1999 pontiac bonneville with the 3.8, she starts up and drives perfect but when I come to a stop light or stop in general the rpm's will drop to 500 then jump to 1500. My check engine light is on and code PO440 comes up is this the evaporation system doing this or is there another problem?

Answer 1

P0440 evaporative system

This code and the pontiac are common, and it usually turns out to be the fuel tank pressure sensor, if this was related it would have to be the purge line disconnected or plugged, if disconnected you would have a high idle at all times, if this is a supercharge engine it would be a MAP sensor problem, what make this case different is the idle goes low, a catalytic that plugged partially can cause this,fuel pressure or volume would set a lean code, because this condition is not all the time or constant you would not set a code, From experience this was encountered a customer car had a same situation the only code was a P0440, and nothing else, during the test drive there was nothing unusual it was not able to be duplicated and the data stream read nothing out of the ordinary, the vehicle was let go, 2 days later the vehicle came to the shop on tow truck, the catalytic was found completely plugged,