Xsara running lean

LeFrog · Post by **LeFrog** » 29 Aug 2005, 04:09

My Xsara drives extremely well. However, when idling, it's too lean, making the engine sound rather erratic. I've been advised that the SL96 ECU I have does not allow you to program the mixture, so I'm stuck with it.
Any tips or tricks for making it run richer, particularly when idle?

jeremy · Post by **jeremy** » 29 Aug 2005, 15:12

Is it running at the correct temperature? - eg thermostat stuck open?
Are there any seals round the injectors - and if so have they failed?
Are any breather pipes connecting to the inlet manifold in good condition and FIRMLY attached - they may have perished and let air in.
Jeremy

LeFrog · Post by **LeFrog** » 29 Aug 2005, 15:22

Thanks for the reply. All these things have been checked by several mechanics.

Post by **RichardW** » 29 Aug 2005, 16:21

You can't make it run 'richer' since it is designed to operate at stoichiometric ratio - 14.7:1. This is controlled by the Ecu with input from the Lambda sensor (which actually isn't a lambda sensor at all, but an oxygen sensor). It actually can't control at the correct ratio, so what happens is that the mixture varies up and down - the sensor sees it's too weak and tells the ECU to richen it up, then it sees it's too rich and tells the ECU to weaken it, then is sees it's too weak.... ad infinitum. Now, if the sensor is not reading correctly it could give an offset in mixture making it too lean (or too weak) - this would show up in an exhaust gas test. However, it is also possible for the sensor to go 'slow' whereby it is slow to respond to the changes in the oxygen content of the exhaust gas - in this case the idle speed tends to hunt up and down as the mixture changes. Can't remember what the frequency of the changes is supposed to be, but it's quite fast. You really need to get an oscilliscope on the sensor to see what the output is doing.
There are of course other causes of weak mixture, like failed coolant temp sensor or gummed up or broken idle control valve to name but 2.
What sort of eractic idle are you getting, and what does the gas test say?

LeFrog · Post by **LeFrog** » 30 Aug 2005, 02:11

The oxygen sensor has been replaced and it made no difference.
The Idle control device is spotless, as it the whole throttle body. Spark plugs have been replaced - no change.
The thermostat has been replaced. The Coolant Temp Sensor seems to be doing its job in that the output voltage changes with temp and doesn't jump about.
I'm totally out of ideas. My mechanics have said, "that's just the way it is." Citroen would never sell them if they were like that out of the showroom.