2002 Optima vibration/ slight knock while idling

[skpklc]

Last few months I notice a slight thud sound, not really something that will shake u up but very small knock kind sound something like engine vibration, when the car is idling at long traffic lights etc. The sound is specially pronounced when AC is on. It bothers me as a driver but most passengers don’t feel it, I guess it not very bad yet.

There has been a lot of stuff happening with my car, recently had a code for input-output speed sensor and got those changed, timing belt was changed; all this makes me wonder if one of these could be the problem. Another thing that bothers me is that once I saw the code for Engine knock sensor (6 months back) but it was gone later when I took it to the mechanic. Could it be the knock sensor acting?

Anyone else experienced similar problem when idling? I appreciate any help on locating the potential cause.

[skpklc]

i tried shifting to neutral when idling and the problem goes away i can see the engine rpm going up too. whats going on ? any ideas...

[joebunda]

Knocking can be caused by a variety of issues, but the most common is your gas and or injectors. Try running a can of "dry gas" or injector cleaner next time you fill up and see if that improves the situation (a cheap first step).

Other things to consider is the quality of your spark. Did you change the plugs when recommended? Check your plug wires for wear/cracks.

The fact that it only happens at low idle indicates it is not a serious issue, but if it gets worse, you need to address it as a bad knock can damage your engine.