fuel caps and evap

Todays engine  have fuel tanks that vent  back into the engine itself rather than to atmosphere.
In earlier day the vent was simply hung out of the fuel tank  to equalise pressure.
vehicles today  have a closed system that  partly  absorbs metro; vapour and balances atmospheric pressure  through a charcoal canister.
The amount of gases  or vapour drawn through the  canister is dependant on  many things obviously balancing atmospheric  pressure with out  overichening fuel mixture.
Blocked charcoal canisters and leaky pipes or non operative  evap valves are the common failures.
Any issue with the evap can cause issue with engine power and need looking at.
If when you release your fuel cap there is a n intake of air  you definately have issues even without a code or light appearing

Lunar autos methods

Here at the workshop  a car arrives,usually with issues that other garages have long given up on.Not to say they cant fix it but its simply not worth their while  pushing any further work.
In some cases a recalcitrant issue does  end up in the scrap yard before its time.
All  engine running issues,take time to pin down,its rarely  the same day  find it and fix it that isnt the real world.

misfires

sometimes hard to detect but never ignore them big killer of engine power

No codes

No codes!
Actually no codes is the usual case  of many or in many driveability issues.
Live date is mostly the first port of call then  other non intrusive methods.in particular relative compression testing.
Its a simple but powerful test of an engines compression needing no dismantling of anything save  stopping the engine starting

An engines changes in power output

Misfires:are a great killer of power.
These can be mechanical,fuel related,ignition related what ever their cause they need to be detected and rectified.
Using a maths channel is a superb way to do just this

jumping

never presume the same symptoms in the same car are down to the  same issue

exhaust pulse test

A quick powerfull method of checking engine integrity and running state

Cost £45

Rusty fuel tanks

A nice jag that has been stood for a long time was suffering from lack of fuel and avery noisy pump cause rust clogged fuel system

connecting an oscilloscope

Remember an oscilloscope is measuring voltage making the possibility of shorts a ll to  possible.
So common sense prevails

Oil change intervals

Certain makes of cars demand fully synthetic