We have an engine that starts okay, idles smoothly, and goes reasonably well when you plant the thong, but it's not like it used to be. So you decide to get a tune-up.

But now that you all have a good understanding of engine management systems, you're thinking to yourself, "what the hell's in a tune-up these days?" There are no points to set, no distributor advance to fiddle with, no mixture to adjust, no float levels to play with, no choke to disconnect..... Remember those pleasant Sunday afternoons in your youth, fiddling with carburettors that never seemed to work as well when you finished, and there were always those bits left over?

Those days are gone, mate! That trusty ECU in the left footwell has taken over all of the above. So these days, other than spark plug, air and fuel filter replacements, you don't really "tune" anything - you just make sure it all works.

The Self-Diagnosis System

So let's say you have replaced those obvious bits, and it still doesn't run properly, now what? Well you take the easy way out, and ask the vehicle's computer to tell you what's wrong. The ECU has the ability to recognise faults, which it then stores as a fault code in its memory, just waiting for someone to extract it. If it's an NH or later model, it may, with some fault codes, flash the "check engine" light at you to inform you that there is a fault present.

Fault codes can be read with a digital multimeter or LED test lamp from the diagnosis socket under the dash - they are a series of on-off voltage pulses, a little like Morse Code, which you can compare with the fault code chart in the workshop manual. If you have a problem, this is a pretty good place to start - a fault code can lead you directly to a suspect component which you can then individually test. It can save a lot of diagnostic time.

Before you start playing with any of this stuff, get the correct workshop manual from the club's library, and use a quality multimeter - you can spend a lot of money really fast by poking a cheap test-light at the wrong wire!

Common Engine Performance Problems

By far the most common defect is blocked fuel injectors. This usually doesn't occur until 80,000km or above, and the symptoms are a rough idle, along with a hesitation on take-off, especially when cold. Add-in-the-tank fuel injector cleaners are okay for preventative maintenance, but once injectors are blocked, the only cure is removal and ultrasonic cleaning. The ECU will not record any fault code with this defect.

Power loss on hard acceleration is usually caused by a blocked fuel filter or faulty fuel pump. Diagnosis is simple using a fuel pressure test gauge.

Oxygen sensor failure crops up occasionally - this is the sensor mounted in the exhaust pipe which signals oxygen content of exhaust gas to the ECU. Failure results in the ECU substituting its own value, which results in a richer fuel mixture than normal. This is the obvious one to look for if your fuel economy has dropped off and the exhaust gas stinks of sulphur. The ECU will register a fault code, but the "check engine" light will not come on.

Erratic idle speeds are usually caused by a faulty idle speed control motor (stepper motor). A suspect motor can be tested by unplugging it and simply testing continuity of the 4 motor windings with a multimeter.

I would love a dollar for every time I've seen the airflow sensor plug disconnected. This is usually because somebody has cleaned the air filter, refitted the air filter lid then forgot to reconnect the wiring to the sensor which is mounted on or in the air filter body. This will result in several fault codes, as well as flat spots on take-off and poor fuel economy.

And while we are looking at the air filter, make sure there are no air leaks anywhere in the intake system, such as loose hose clamps or vacuum hoses fallen off. This allows incoming air to by-pass the airflow sensor, which give a lean air/fuel mixture, causing rough idle and flat spots.

Remember that there are no fuel mixture adjustments - this is controlled entirely by the ECU.

If you wish to "clear the slate" of any fault codes in the system (you may have generated a few yourself by fiddling around), simply disconnect the battery for a few seconds.