Citroen XM Forum

Have you tried running the engine with the vacuum line from the fuel pressure regulator to manifold disconnected and plugged at the manifold end?

My apologies for the belated response to the various further kind suggestions made, while I have made a little more progress. Firstly for Mike I have indeed tried removing and plugging the pressure regulator vacuum line, but there was no improvement.

The good news is that with the aid of my Citroen electrical manual I have check the entire engine loom with critical connections to sensors, injectors etc. and there do not appear to be any faults, the bad news is that the car is still barely driveable!

It was particularly interesting to read the various service sheets kindly attached by John and I actually experienced the issue with the potentiometer poor contacts earlier today, including repeated fault codes, although I do not think this is my only problem.

Fortunately I now have Dave visiting on Monday evening with a Lexia so we will see what that reveals and the comments made by Steve will certainly be kept in mind.

With regards,

Matthew T.

I took the car out earlier today and it is actually sort of driveable, although generally will not rev much above 2,000rpm, which is a bit tricky for an auto! As I have failed to find any faulty components or wiring, I am really now largely dependant on searching live data. I feel ever more convinced that is must be a partially broken or shorting wire, which is not showing up on checks, but hopefully may appear while the engine is running.

It also occurs to me to ask that if the issue is under-fuelling due to a faulty sensor or associated wiring, which sensors would so severely restrict the supply? As the engine struggles when more air is introduced, surely it is only the throttle sensor that controls the flow? If the air and water temperature sensors, oxygen sensor and knock sensors or wiring were absent or faulty the running would be less smooth, but would still rev, while the engine does not run at all without the flywheel sensor and air pressure sensor.

In conclusion I think the issue has to be related to the throttle sensor, while as the sensor and ecu have been changed with no effect, therefore presumably the wiring must be to blame? Something else I could try is installing new wires direct between the throttle sensor and three ecu connections, assuming there is a fault somewhere in the loom.

Regards,

Matthew T.

If you have removed the loom from the engine there is a real possibility there is a short somewhere within the loom wrap itself, the insulation used is very poor and turns to powder after years of heat, the latr looms are improved in this respect.

when running in closed loop (idle, light throttle, cruising) the ECU will probably just use throttle position and or MAP and the O2 sensor to regulate mixture.

In open loop (cold start (until O2 sensor is heated), high engine loads, full throttle) the engine will probably be using the MAP, throttle position, coolant temp and air temp sensor to control mixture and ignor the O2 sensor.

Hopefullythe live data will show something that dosnt look right.

D

The throttle sensor (speaking generically of MAP systems) does not do much in steady state, as the engine fuelling can be calculated accurately from the manifold pressure and the temperature of the the combustible air in the cylinder (a combination of the water and air temperatures).

The fuelling calculation relies on the injector flow rate being known (hence rail pressure is important), and the volumetric efficiency of the engine should be correct to specification, so manifold restrictiveness, valve timing and exhaust back-pressure etc can upset it.

On the balance of the symptoms that you have, I think that the most likely scenario is that you have low fuel pressure due to a restriction somewhere, and I would be checking the rail pressure, and flow rate. Lexia won't help you much there, except that the lambda will be showing a lean mixture.

V6 Mechanics Training Sheets. John

I know with the XU10 engine unplugging the air inlet temp sensor wont do anything, the lexia shows the ECU asumes an air temp of 0 degrees and it just carries on, there is no EML brought on even though the sensor is open circuit and the car will start, run and drive perfectly normally without it.

Timing is one thing that has been disturbed and 1 tooth out on a cam may cause this issue but i wouldnt think adding fuel manually would of made a difference and i would expect to see a lot of spitting back, valve contact and not limiting the engine speed with no load, the video of the tach while reving the engine shows it almost like hitting a rev limiter.

D

Thanks Chaps and just to confirm that the engine wiring loom was not removed on the recent occasion and only disconnected and the insulation tape has been completely replaced. The valve timing has also not been disturbed and I ruled out fuel pressure earlier Mike when I installed a pump above the engine. As the problem occurs under acceleration surely it is only the air flow that really changes, which is effectively measured by the throttle sensor? I am also grateful for the service sheets John that I shall be interested to study. Regards, Matthew T.

it almost like hitting a rev limiter

I understand that rev limiters work by cutting off fuel that is certainly what is happening in this case Dean, as pouring in the petrol dramatically confirmed! Regards, Matthew T.

Airflow is measured by the manifold pressure sensor, the throttle position sensor provides the equivalent of an accelrator pump signal to stop the engine stumbling when the throttle is opened quickly, and it triggers the idling algorithm when the throttle is closed.

Does the engine (on one of your other cars) run reasonably with the throttle potentiometer disconnected?