Tuning - DIY

Engine must be at normal operating temperature with fan running. 

Before attempting to set the ignition timing or base idle speed, the ECU must be put into 'service adjustment mode'. This is done by making sure the engine is thoroughly warm and then disconnecting the multiplug from the blue coolant temperature sensor while the engine is running. If all is well with the management system the engine RPM should increase slightly when the plug is disconnected.

If the engine stalls at any point while making adjustments to the timing or base idle speed the multiplug MUST be refitted to the blue coolant temperature sensor before attempting to re-start the engine. Failure to do this will cause the ECU to enter 'limp home mode'. Adjustments will then not be possible until you have cleared the fault/s from the ECU memory.

Ignition Timing

The procedure described in the Haynes manual will give a rough timing setting. To set the timing accurately the following procedure must be followed. Digifant has an unusual procedure for setting the timing so please follow the instructions carefully.

With the ECU in service adjustment mode blip the throttle so that the engine RPM exceeds 2500. Do this four times.

Increase the engine speed from idle to between 2000 and 2500 RPM and check the timing. It should be 6º ± 1º. If adjustment is needed, slacken the distributor fixing bolts and turn the distributor until you have the correct timing.

Allow the engine to return to idle speed and reconnect the multiplug to the blue coolant temperature sensor. Blip the throttle three times, making sure the engine speed exceeds 3000 RPM each time. (the blipping is suppose to clear any BTS reading the ECU may have stored)

Check the ignition timing now advances with increased RPM by slowly increasing the engine speed while watching the timing marks. Without an expensive timing light you will not be able to accurately measure the advance, but it should be around 40º at 2800rpm.

Idle Speed

Before attempting to adjust the idle speed make sure the ignition timing is correct. It is also worth taking the ISV (idle stabilization valve) off the engine and thoroughly spraying it with carburetor cleaner. The valve is located beneath the inlet manifold, bolted to the steal water pipe.

The regulated engine speed is controlled by the ECU and is not adjustable. The ECU regulates the idle speed by opening and closing the ISV and also by making continuous small adjustments to the ignition timing. However for this to work properly the base idle speed must be set correctly.

To set the base idle speed, remove the breather hose from the cam cover, and plug the hose. Start the engine and allow it to idle for a couple of minutes, then put the ECU into service adjustment mode.

Blip the throttle three times, ensuring the RPM exceeds 3000 each time. Allow the engine to return to idle and then use the idle adjustment screw on the throttle body to set the idle speed to 920 RPM ± 25 RPM.

Re-connect the coolant temperature sensor multi-plug, then unplug and reconnect the breather hose. The engine speed should now settle down to the correct regulated figure of 900 - 950 RPM.

Speaking from experience - doing this can help determine faults with other sensors or components of the engine management system. Hopefully this will help those of you with poor running GT's get them sorted!

original article here