Communicating with my Car

Discovering how to diagnose automotive ECU faults

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Thursday, 27 April 2017

RENAULT CLIO 2 — Erratic idling and stalling

Renault Clio electrics

Intermittent idling problem

Must be possible to deduce the cause

Having consulted the internet this problem does seem to be relatively common. However, there is, as usual, no consensus of opinion and, as usual, lots of people have spent lots of money and time replacing all manner of things with no clear conclusion or feedback. So my thinking is «it must be possible to isolate the problem by thorough investigation». Especially since speculative parts replacement is so potentially expensive.

The first thing to say is I’ve not fixed the problem yet. Unfortunately, every time I have access to the car it runs faultlessly, except for once. Fortunately, that one time, I gained some information using my OBD2 reader and Android Torque app.

This is what I’ve learnt

OBD data when engine was running badly

  • ECU running in Open Loop even though the car was at normal running temperature.
  • No Short Term Fuel Trim data being reported.
  • O2 sensor above 0.9V
  • Reported throttle position anywhere between 5 — 13%
  • Idling RPM about 1700 rpm but very erratic.
  • Manifold pressure appears to respond to changes in reported throttle position.

OBD data when engine was running normally

  • Short Term Fuel Trim data available and converges in the range -0.6% to 13%
  • O2 sensor voltage falls and oscillates between 0.1V (lean) and 0.8V(rich) approx.
  • Throttle position settles at 7%

What conclusions can be drawn from these observations?

Why Open Loop Mode?

Reasons to run badly — Sensor Data

Reasons to run badly — Fuel and Ignition

Crankshaft Pulse Sensor

MAP Sensor

Throttle Body

Some of the internet posts I’ve read report similar symptoms and suggest the throttle control valve could be faulty and they recommend removal of the intake manifold (not a 2 minute job!) to either clean the valve or replace it (an expensive part to speculate on!).

Through Torque I can observe changes in manifold pressure following ECU requested changes in throttle position. Therefore, if it was the throttle control valve causing the problems, would I observe this? I don’t think so.

Fuel Injection System

I’ve got no way of testing the fuel injection system but Torque does show a reported O2 figure from the exhaust sensor that indicates a very rich mixture when the engine is running badly. From this I infer that there is sufficient fuel and so I’m not inclined to suspect this area as a cause.

An interesting monitoring period was when the engine settled down to normal idling. It came out of Open Loop operation and into Closed Loop. This was evidenced by the reporting of the STFT (Short Term Fuel Trim) data and the O2 figures responding accordingly.

Ignition Pack

Some background on Fuel Trim and O2 control.

In Closed Loop operation the ECU attempts to keep the air:fuel ratio at its optimum of 14.7:1 (In practice, probably a little richer). It adjusts fuel flow by adjusting the time the fuel injector is open and this is called the Short Term Fuel Trim. (It’s called short term because it’s a correction factor to the Long Term Fuel Trim which is adjusted over a much longer time period).

The ECU measures the effect of the fuel trim adjustment by means of the O2 (sometimes called lambda) sensor. A high voltage, say 0.9V means the mixture is rich i.e. too much fuel and so the trim is reduced. A low voltage means too weak i.e. too little fuel and so the trim value is increased. The optimum voltage is around 0.45V for cars such as the Clio ( Note: There are narrow band and wideband sensors. The latter are most likely to be found in more modern high economy cars but the control principal is the same).

Next Steps

It’s difficult to make progress identifying the fault when the car is running OK! One strategy could be to try to create the fault by ‘playing’ with the various sensor components.

(I did also wonder if the O2 sensor was OK since this is a common source of poor engine performance. To test it I followed a procedure of letting excess air into the intake manifold by removing a pipe followed by injecting propane into the air intake (from a plumbing torch). These steps have the effect of making the air:fuel mixture weak and rich respectively. Using Torque the O2 readings can be observed changing accordingly. To all intent and purposes the reading should respond immediately, which they did).

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