Diagnostics Doctor

Your diagnostics queries sorted.

I could do with some advice on a long-term problem with my 1997 Peugeot 806 Mk1 with the 2.1 turbodiesel XUD 11 engine and Lucas fuel injection system. It has a high mileage of about 240,000 miles.

For the last year or so, the main problem is rough idling. When cold, it starts fine, but as soon as it’s warm, the idle is up and down all the time, making the whole car shake. It starts OK when hot. Under 2000rpm, when pulling lightly in gear, it tends to hunt from time to time, but with your foot down it pulls really well.

I have looked at all sorts of things but can’t improve the situation. Increasing the slow running will not cure the problem as I have tried to hold the accelerator at 900rpm when still, but the unevenness remains. Idle speed is set at about 750rpm. When pulling in gear, there is a whistle from the turbo area, but I have not been able to locate the source. Richard The car is too old for OBD fault code reading, While it does have a code reading facility, it can only be read by expensive Oem-type readers.

Check both the operation, wiring and connectors for the accelerator pedal position sensor (particularly for glitchfree linear gradient output), as well as the coolant temperature sensor for correct output and the injection timing solenoid in the main pump.

The whistle from the turbo could be due to an air leak, so check all hoses and connectors, preferably with a smoke tester. The car has now started to cut out at junctions. There was air getting into the fuel system, which I thought initially could have contributed to the problem, but I solved that and it’s no different. You mention ‘a glitch-free linear gradient output’ – how on earth would I go about checking that? Richard You need to use a multimeter to measure the voltage on the signal wire of the accelerator position sensor while slowly operating the pedal through its entire range with the ignition on, engine off. You are looking for a smooth progression from about 0.45V-4.5V – any deviation from this is a glitch from a faulty sensor. The point of measurement is best done at the management ECU, terminal 10, by back-probing the ECU multi-plug. The wire concerned should have the code of 1392 printed on it, though sometimes this is omitted.

For the CTS, apply the same backprobing procedure at terminal 17 of the ECU, wire code 1336, while the engine is warming up. Expect a voltage of about 3.3V at 20°, falling glitch-free to about 1.0V at 80°.

In general, a digital multimeter is used for measurement work concerning ECUS, but an analogue meter is superior for your particular needs, because it makes glitch-detection easier.