Xantia V6 running rich.

[Allmostfrench]

Thanks SO much everyone for your input!
It turns out that it WAS the Throttle Position Sensor, she's running as smooth as silk now.
I'm thinking that I might change the Lambda Sensor while I'm at it, since she has covered 200 000km now and I don't know when it was last changed,

Does anyone know at what revs or throttle opening it changes from closed loop to open loop?
Cheers, Chris.

[Mandrake]

Thanks SO much everyone for your input!
It turns out that it WAS the Throttle Position Sensor, she's running as smooth as silk now.

Glad you got it sorted - I replaced the TPS on my first Xantia V6, I'm starting to wonder if I need to replace it on my current one now as it does stumble quite a bit sometimes but not at other times. (Hard to tell as it hardly gets driven so when I do drive it it has always been sitting for a long time)

I'm thinking that I might change the Lambda Sensor while I'm at it, since she has covered 200 000km now and I don't know when it was last changed,

Changing the Lambda sensor can be a bit of a fiddly job, I wouldn't change it unless you have some evidence that it's faulty or you strongly suspect it has been "poisoned" by running rich all the time. I changed the one on my first V6 when I was chasing running problems but in hindsight it wasn't faulty.

You can test it either with a Lexia or an oscilloscope - or even a fast responding volt meter for that matter.

With the car warmed up and idling you should see the voltage alternate between <0.2v and >0.8v about once every two seconds. If you hold the engine at 2000 rpm it should alternate between high and low at a faster rate - about once per second.

If it does, it's fine. You can connect an oscilloscope or volt meter by "backprobing" the connector which is on the rear corner of the engine near the LHM tank. I used to have a backprobing cable I made for myself for such jobs - essentially it was two sewing pins soldered to a shielded audio cable with some heat shrink over the soldered joints. You then just carefully push the pins down into the plug along side the two wires between the wire and the rubber potting and they make contact inside the rubberised part of the plug where the wires connect to the pins. Doing it this way leaves no holes in the wire to start corrosion, and you also get a mechanically secure connection during the test.

Another way to check the Lamba sensor is working is to get the Lambda reading checked with a gas analyser - eg an MOT emissions test. If the Lamba sensor is not working there is no way it will pass! The car will run rich if the sensor is not working.

Does anyone know at what revs or throttle opening it changes from closed loop to open loop?
Cheers, Chris.

Revs don't really factor into this - it can run both open and closed loop mode at any rpm. It's more about load.

In short, you'll see open loop mode in these situations -

1) Full throttle or near full throttle
2) Momentarily every time you increase the throttle opening quickly - for about a second then it will go back to closed loop mode
3) During about the first two minutes of running on a cold engine - it takes this amount of time for the lambda sensor to heat up and start working
4) If the Lambda sensor is faulty.

If you watch the voltage under these conditions you'll easily see whether the ECU is in open or closed loop mode.

Unfortunately this ECU will not set a fault code if the Lambda sensor is faulty and/or the engine is running rich, the only clue in the Lexia that the Lambda sensor is faulty would be to watch for the cyclical high/low voltage switching reported for the sensor and if it's totally absent (stuck high or low) even when warmed up and idling then the sensor is not working.

[Allmostfrench]

Wow!
That's a lot of useful information.
Hopefully, it might help others, too.
Thanks again.