Petrol engines Diagnosing faults

=Replacing parts

Is usually part of eliminative diagnostics - but can catch you out.
Aidan: Just because parts are new don't assume they are good (lets face it nowadays what, if anything, is made better than it used to be). If anything is changed, especially if it's not because of a fault, keep the old part to be able to revert if necessary. This applies particularly to engine ignition components e.g. HT leads, rotor arm, etc.

A systematic and analytical aapproach from beginning to end usually pays dividends
HarryMann: Try to work logically, and keep notes. Don't make assumptions at an early stage... do the groundwork in systematically eliminating suspected parts, systems and reasons for the problem.
Always "assumed" is the general good state of the engine internals... sound internally, with +ve evidence (e.g. compression test) must be determined before proceeding to any detailed diagnosis of ancillaries that follows.

Usually, start with fuel and sparks. Fuel usually before sparks, if nothing previously known.

If the vehicle is new to you...
HM: Don't assume anything about anything... be a pessimist, until you know better. Go back to basics and work forward

Misfiring/hesitation

Ignition causes

Aidan: Some may remember Project Caravelle had a problem once upon a time following fitting of new parts during service that he couldn't resolve and ended up throwing in the towel and selling the van at a big loss, then new owner replaced leads with a good old set and it was sorted for a tenner, just because new don't assume they are good, (lets face it nowadays what if anything is made better than it used to be) if anything is changed, especially if it's not because of a fault, keep the old to be able to revert if necessary.

Carburretion causes

Weak misfire/hesitation OR grossly overfuelling Air leaks anywhere between carb and cylinder (flanges/manifolds, vacuum pipes etc)

Injection/ECU/AFM/ISU causes

Vanagon syndrome
Lambda sensor (2.1 MV)
Temp sensor
Idle Stabilisation Valve/ IS control unit

Exhaust causes

Leaks and holes in the standard exhaust system can result in popping back when on the overrun, and slightly weak fuel/air mixtures that may cause hesitation on light throttles. Seriouly open exhausts can cause valve burning or overheating in the longer term, and can require mixture richening/re-jetting up, to get performamnce back and prevent damage. Of the three fuelling types, the injected MVs runnning closed loop (lambda connected) should stay in tune better.

Wear/Damage/Age causes

Burnt exhaust valves