Petrol engines Diagnosing faults

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General advice

General engine diagnostics using a vacuum gauge connect gauge on 2.1 to the distributor retard hose that goes to the plenum chamber, (to identify this side has vacuum at idle) [1]

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.

General approach

A systematic and analytical aapproach from beginning to end usually pays dividends
HM: 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.

New owner/new vehicle?

If the vehicle is new to you, or you haven't owned a T25 before...
HM: Don't assume anything about anything... be a pessimist, until you know better. Go back to basics and work forward


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.

HT leads, rotor arm, coil, ECU,

Carburettor causes

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

Fuel starvation (fuel filter, tank feed blockage, pump failing)

Grossly over fuelling (snatching, bucking, kangarooing) - temp sender/blocked air filter, stuck choke etc

Injection/ECU/AFM/ISU causes

Vanagon syndrome
Lambda sensor (2.1 MV)
Temp sensor
Idle Stabilisation Valve/ IS control unit
Fuel pressure/fuel press. reg manifold balance feed etc Over-run throttle stop adjustment (Hesitation)

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


Fuel starvation (electric pump/ strainer/filter/fuel gauge problem)
ECU malfunction (heat sensitive)
Vanagon syndrome

Bad starting

Starter earthing, +ve feed, solenoid feed /solenoid (stuck/failed) /ignition switch/wiring /starter pinion support bush /bad starter (field/armature etc)
Flooding (carb, stuck needle valve, punctured float)
Flooding (injection, T2 sender, ISV)
No sparks Ign. timing many degrees out (just rebuilt /dizzie removed/replaced), 90 degrees out

Petrol.. non-start, old fuel, small amount of fuel, no fuel, rotten fuel. Drain and re-fill. NB. Even LPGs need 'some' petrol to start ??

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Kangarooing 2.1 DJ