My 98 Camry (105K mileage) has a bumbling problem when A/C is on and transmission in gear while stop in traffic. the engine rpm will up to 750 and drop to 500 and repeat itself. Bum will stop either transmission change to N or P or turn off the A/C switch. It runs fine in highway when RPM is more than 1000. I clean the IAC, idle Air Control valve, which no helps. change spark plugs, air filter and gas treatment but problem still. The air hiss sound from IAC is louder when A/C switch and tranny is in "D" and "R". I think something is confusing the ECU so it send signal to IAC unit for more bypass air which drop the rpm to 500 and it stop the A/C unit, then RPM increase to 700-800, so A/C is ON again which caused the engine bumling. RPM drop .... repeat until traffic light change to green and add more gas and run.. is the IAC is out of range and need replace? the cold start is fine and smooblingth without any problem.

It is bad for A/C since it is on off too frequently when stop in the traffic and feel the car will die or surge ( unsafe feeling)

 

sometimes a sticking or defective EGR valve will do this also. if stuck open then at idle (like at a red light) the engine will stumble. i would try to locate the EGR valve and maybe clean it of the carbon build up (which it will have, being in the exhaust stream) or i'd just replace it. first though, what you can do is, once you locate it, gently tap it with a hammer to un-stick it.........then remove the vacuum line that goes to it, or if it is an electronic EGR valve, remove the wire. then go for a ride and check to see if this cured the problem. if so, then clean or replace, but i'd replace it myself.

 

Thanks. I will try tap the EGR to see the result and remove EGR to clean later.

 

just be sure to disconnect the EGR valve after because if you don't, it will stick open again. i highly suspect its carboned up........

 

Are you mean remove the tube to EGR and plug it while driving?