Previously we went through the basic operation of the V6 engine management system, and explained how the ECU (electronic control unit, or computer) reads the signals from the various sensors (INPUTS), and uses these inputs to decide how and when to operate the fuel injectors and ignition coil (OUTPUTS) to enable the engine to run.

But what if it doesn't run properly, or worse still, it doesn't run at all? Most people wouldn't know where to start looking if their V6 stopped, so perhaps we should look at what these engines NEED before they'll run, and how to diagnose problems when they don't.

To begin with, if a V6 has a fuel pump that produces enough pressure, an ECU that works, and the INPUT from one sensor only, that being the distributor, it should run. I repeat, the distributor is really the only INPUT that it needs to make it go.

In other words, if it won't start, forget about the air-flow sensor, throttle position sensor, coolant temperature sensor, etc. - they won't stop it from going.

Why? Because the ECU is smart - whenever it realises that an input is not working (either you unplugged it or it simply pooped itself), the ECU substitutes a value that it believes is correct, taking into account the inputs from the other operational sensors. In other words, if you unplug the airflow sensor for example, the ECU then looks continuously at the throttle position and the engine speed to guess what the airflow sensor signal should be, and then substitutes the missing input with its own estimate. Sure, it's thirsty, and it's got more flat spots than an old Valiant, but it still goes!

Okay, now let's look at a typical no-start scenario. If it won't fire up, the first thing to check is the sparks. Disconnect a spark plug lead, jam a spark plug up it and crank it over.

If it visibly sparks, you obviously have a fuel problem - no fuel pressure (should be around 38 psi), or no injector operation. Try taking off the big air intake hose and manually spray some fuel into the throttle body, crank it over and see if it momentarily fires up. If it does, you're on the right track - no fuel supply. Crack open a fuel line fitting (preferably under the car at the fuel filter), crank it again and see if fuel squirts out. It doesn't? You have a dead fuel pump in the fuel tank! Power-up the pump manually direct from the battery (thus by-passing the control relay and the ECU) - still no pressure? Sorry, the pump's dead. Amen.

Okay, this is only one example, but see how easy it is to work through a problem once you know HOW it works? So many people would throw their arms in the air and say "it's the bloody computer!"

Which, by the way, brings us to the most common causes of no-starts. From my experience, they are, in order,

(a) the crook burglar alarm that you had fitted, which disabled the ignition when it shouldn't have,

(b) fuel pump failure,

(c) ECU or computer failure, usually caused by you (you drowned it).

I referred this question to one of Victoria's busiest automotive computer repairers, and they suggested that the following incidents would account for most cases of ECU damage.

(a) Alternator/regulator failure resulting in uncontrolled alternator output (18 volts plus).

(b) Damage from water entry.

(c) Incorrect use of jumper leads, either incorrect polarity or immediate disconnection of leads after engine start-up, resulting in large and sudden power surge from alternator (relatively rare).

So in a 4WD, it's obvious that the big risk is water damage - this is the recommended method of salvage.

If you drown your vehicle and the ECU gets submerged, don't try to start the engine. Remove the ECU (it's either under the dash above the passenger's feet or behind the plastic panel to their left). Take the top cover off, and if it's been wet inside, wash the ECU out in clean water, then dry it thoroughly - use compressed air, heat from another car's heater, warmth from the campfire, whatever. It must be dry before refitting. This will usually be enough to save it. Get it checked out by an automotive electronics repairer when you get home.

So let's recap what we have discussed so far.

In Part 1, we saw how the engine management system is really a simple system of fuel injection control and ignition timing control, mastered by a computer or ECU, which reacts according to information received from a handful of inputs or sensors, the main ones being the distributor and the airflow sensor.

This article (Part 2) looked further into component failures and how to get around them.

Now I could go on forever about diagnosis of various failures, but as stated earlier, this was meant to be a basic discussion. May I suggest that those who have an appetite for more knowledge in this area should refer to the Mitsubishi manuals in our club's library.

The next article will assume that your EFI Pajero actually goes, but not quite as well as it should - we will look at diagnosis and rectification of tuning problems.