By Charles Figgins, Technical trainer from Blue Print

The car in question is a 350Z which is a high performance 3.5 V6 Nissan - the owner kept experiencing the MIL (Malfunction Indicator Lamp) coming on, followed by the car going into 'limp home mode', which resulted in it having all the driveability of an old 1.0 Micra - it wasn't much fun to drive!

The owner said these symptoms had been going on for some time, but were intermittent. Diagnostic problems that only show themselves every now and then can be very frustrating from a workshop point of view, but the job needed to be started somewhere.

How Electronic Throttle Control Systems Work:

Gone are the days when there was a simple cable running from the accelerator pedal to the throttle butterfly - vehicles  have all gone fly by wire in one way or another. Now you have an ECU that's responsible for controlling the throttle valve with a motor to control the throttle valve angle; feedback is then given to the ECU by a throttle position sensor - these components together make up the Electronic Throttle Control.

To trigger this device there is an accelerator pedal position sensor, so when the driver puts their foot down the input is registered by two potentiometers. The engine ECU takes this into account, along with engine speed and temperature, plus other information from the ABS ECU. This includes feedback such as wheel spin, which causes the traction control to back the throttle off, even if the accelerator pedal is flat to the floor.

Time to start testing:

first gear

On operating the accelerator pedal several times, I checked for any voltage drop outs or differences in demand between the accelerator and feedback throttle position sensors - I was looking for the sensors to be operating within a range of between 0.36 volts and 4.75volts. If we had an incorrect voltage reading this could be the possible result of an open or short circuit, but at this stage everything looked fine.

The next thing to check was the operation of the throttle valve and again, this seemed to be OK - there was no sticking or jerky operation. Following on from this, I looked to see if there could be a wiring or connector fault; or was the ECU not giving me enough information? Therefore the next step was to check the voltages from the sensor to the ECU with everything still connected.

Note:

Time for a road test:

As everything was checking out under workshop conditions it was time for a road test, so I plugged the G-Scan back in and set it to 'flight record mode' to monitor the accelerator pedal and the throttle position sensors for a good five miles. Again the results showed nothing - this was getting very frustrating! A few miles later, as I was accelerating out of a junction and changing up from first to second, the MIL lamp came on and we went into limp home mode, just as the owner had described.

The G-Scan had recorded an open circuit on TPS Sensor 1, so we had found the reason for the MIL lamp coming on; now all we had to do was find the cause. On inspecting the connectors at the ECU and at the sensor, I found the sealing grommet missing at the sensor end. After finding a suitable replacement, checking all connections and giving them a quick clean with contact cleaner, there was a better connection. But had this fixed the problem?

Accelerator Pedal Released Position Learning:

It must be performed each time the harness connector of the accelerator pedal position sensor or ECU has been disconnected:

Make sure the pedal is fully released.

Make sure the pedal is fully released.

Idle Learning:

Note:

I road tested the vehicle once more, and it was back to its high performance best; it's amazing how such a small problem can cause so much poor drivability!

Although this looked like it was going to be a time consuming job, with a well thought out diagnostic process and the right information, we could logically carry out a test routine to come to a conclusion - with a bit of help from G-Scan!