Engine valves can have some very subtle and difficult t o diagnose issues.
Loose valve seats will cause some real headaches.The only method for seeing these issues in real time is via the in cylinder waveform.
The conventional compression test is useless in revealing these issues
There are many non invasive techniques for testing your engines components.
Checking the fuel pumps current pull using an amp clamp and an oscilloscope is a well known one.
Rather than rummage under the car and start taking up seats etc,finding the fuel pump relay and bridging the power pins then clamping the cable is way easier.
What is known commonly as a blown head gasket,can it be diagnosed easily....big hmm Condensation build up from short journeys is a big trap to fal into,make no mistake head gasket failures are expensive,so its a not a diagnosis we just jump at
A great many problems crawl out the typical wiring loom.
Age heat oil fuel etc can and do create some real headaches
Feul straight from the forecourt tanks is pretty lousy ful of sediment,water and many other contaminents.
Without a fuel filter your injectors would rapidly blog and die.
So fuel replacement is vital.Its an item often missed by some dodgy garages.If you are unsure ask the garage.Did you change the fuel filter,and get hem to show you the old one.hmm just look if its new,dont know where it is google it.
this is stil avery powerfull technique for diagnostics.Almost a lost art but still very relevant to todays cars