See my reply above.
The tickover is controlled by the ecu opening & closing the throttle to maintain idle speed.
If you have an air leak in the pipework on the inlet manifold, it can allow extra air in which bypasses the throttle body and can cause idle speed to increase or become unstable.
The "fuel sensors" you refer to are lambda (o2) sensors and will not cause your problem, nor would the valve covers.
Diagnostic machine should point you in the right direction and save you money in the long run, but check for air leaks first.
Let us know how you get on.
206cc 1.6ltr (Wifes toy)
Mercedes ML (workhorse)
Corvette C3 (my toy)
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