For cars that are older, there's a possibility it is cause by poor idling adjustment. Older car have carburettors, that would allow the adjustment of idling speed ( And the CO and CO2 mixtures).
For most modern cars, most are EFI( Electronic Fuel Injection ) that are control by ECU. Perhaps, in order to eliminate this problem, probably should have the mechanics to check if there could be any fault(s) to be found in the various sensors, such as; Throttle Position Sensor (Electronic control system is hunting (up and down) for the correct air/fuel mixture, and the engine would not respond to throttle input but to input by electronic sensors) and O2 sensor( ECU is constantly altering the injector pulse width, measured in the exhaust by an O2 sensor, in its search for the perfect air/fuel ratio).
Above may not be the complete solution to solve the above problem, hope it help in its little way.
Another idea is that the engine is missing (not firing or not firing correctly) on one or more cylinders while at idle; whereas an increase in engine RPM makes the missing unnoticeable...or to disappear (in the case of a mixture being compressed enough to ignite or ignite more efficiently before having time to decompress via leakage).
Possible reasons for the above might be:
1) Dirty/fouled spark plug(s) that need cleaning/replacing. (Broken and/or worn valve stem seals can cause the respective spark plugs to foul).
2) Burnt valve(s)...(from age, too high of a combustion temperature, inaccurate timing, or using unleaded in an older auto that requires leaded gasoline)
3) Blown head gasket. (Coolant entering cylinder(s) can cause non-combustion at low RPM, and not at higher RPM or non-combustion is not noticeable at higher RPM) (And/or compression loss via blown gasket passage causing non-combustion).
4) Cracked cylinder head. (Missing caused by same process as in #3 above except through passage of the head crack).
5) Cracked piston. (More unlikely).
Hopefully this has helped in giving some more insight. Vullin
Check your motor mounts. Have a friend eye the motor when parked. Apply the brake, put into drive gear, hold the brake and lightly press the gas. Try the same in reverse and watch for excessive twist in the motor.
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