Idling not regular

[PG]

My 1.0 L Corsa does not idle very smoothly. Whilst stationary the idle speed tends to dip briefly every 3 or 4 seconds before picking up again.

I replaced the spark plugs as they were due for a change but this made no difference.

Also, it tends to struggle above 60 mph & I hear rattling from the engine when pushed to 70 mph.

Could this be a problem with ignition timing or is this normal for a 1.0 L car.

 

[Quick Fit]

If you have fuel injection it could be the Throttle Attenuator module.
Quick Fit

 

[jonnyfishing]

I assume that your car is 1997 or later, 3 cylinder DOHC, engine code X10XE ?
The ignition timing,idle speed,or fuel mixture is not adjustable,it is all controlled by the E.C.U.via information received from the management sensors.

It could be a number of things,so I think you should start at the beginning.
1) check compressions
2)check for split or broken rubber pipes,vacuum lines etc.
3)check/change the fuel filter

If no faults found, have it put on a fault code reader( cheaper in the long run).There are many expensive sensors that could cause these symptoms,a readout of the E.C.U. memory will point you in the right direction.It can be very expensive to try and guess the cause and go by trial and error.
Good luck PG