Citroen XM Forum

I am sorry Will, but how would you do the metering? With regards, Matthew T.

I know I can't contribute much to this, but bells rang when you reported ECU earthing being made at the inlet manifold.
One of our family had a 1989 BX GTi 8v, which always started ok, but could just die - anywhere - or suffer lack of power.
I could never work out why the numerous ground connections at the engine ECU (under the driver's seat) seemed to run everywhere except to a sound chassis/battery -ve point. Several went . . . to the inlet manifold! Porque?

We remade them all to chassis, and problem went away - except for the appalling circular multiway connector atop the engine, which needed a regular diet of contact cleaner. 1989 for you!

White Exec wrote: 1989 for you!

An excellent year sir

I wonder if its something to do with even resistance or ground continuity between sensors, i notice all the manifold earths are of equal length and guage wire. could be pure coincidence of course and im clutching at straws.

I still think the next step is live data, just need all the forum brain power to decode the information into a fault........

Who wants to bet its something simple in the end though like something unpluged or disconnected or a leaky doodah rather than complex electrical issues like ecu failure or intermittant sensors etc

D

Matthew ...two ideas on the Lexia..
1. Perhaps the Lexia provides an interactive test mode, which could help? So for example, a Ford version from a few back had at least two modes. Key on, engine off (KOEO) , and key on, engine running. SO the test technician would set up the system in one of these modes then run thru a pre-defined test sequence. So the Key on, engine off test might require you to key on, press pedal to full, then release. The tester would expect to see 0 to 100% range on the pedal or throttle sensor or see the throttle 'wide open throttle' switch change state - as appropriate. My point being perhaps the lexia can provide some better diagostics.
2. Another perhaps.....=> at some point in time, these tests units were able to provide real time data.... so if that's possible on your setup, perhaps you could display some key variables like pedal, AFR, RPM, fuel pulse width command... and see what happens to them when the engine start mis-behaving.
3. Then Clutching at straws#1t: does the engine behave any differently if you very slowly open the throttle rather than quickly? (so you'll get to the high engine rpm without the throttle being wide open )
4. and ... at straws #2: If you disconnect the throttle position sensor, and repeat the test, does it make any difference (just to take out of the picture the throttle sensor )

Some background info: The two dominant systems from this era were called alpha-n and speed density. The first uses throttle position (alpha) and RPM to estimate cylinder air chage and hence required fuelling. The latter uses the MAP sensor and RPM to do the same. The systems also need to know when the driver is asking for full throttle so as to richen up the mixture for power! So on an alpha-n system, the throttle sensor is already there to provide that information. On the speed density, I think the "full throttle" information will be from a switch (cheaper), but could also be from a throttle sensor - in which case the ecu can use the throttle data to supplement the MAP data. So from your posting of Wed Feb 15, 2017 7:02 pm , and reference to MAP, it suggest the system is speed-density - so maybe there is something not right aboutthe MAP signal/wiring - not enough to set the EML on but enough to screw up fuelling. ==> really need live lexia data. Good luck.
late add: The HEGO data provides only a trim term to the fuelling control - except at full throttle when it's ignored.

The EMU lamp has three modes of display not two.
Mode 1 - No fault, the lamp goes out after two seconds.
Mode 2 - Serious fault exists, lamp remains lit.
Mode 3 - Minor or intermittent fault, lamp remains lit for 5 seconds then goes out.

The workshop manual also mentions that if you set the RH display to instantaneous fuel consumption then at speeds above 12.5mph you will see how much fuel the ECU thinks it is injecting (based on 12,680 pulses per litre of fuel injected).

Somewhere I think I saw a technical flash about swapped knock injectors or the wiring of them playing havoc with V6 engine running.

John

John, did you mean knock sensors or fuel injectors?

On the older looms the two knock sensor plugs are both black, knock sensor plugs are green and blue, on later engines the wiring harness is also colour coded to make plugging the wrong sensor into the wrong plug impossible but did not tab the plugs to make it truely impossible.

If plugged in back to front the engine ecu would be locked into a death spiral, it would see knock on one bank then retard the ignition on the oposite bank until it reaches the end of its timing adjustment after not seeing the desired result and should bring the EML on, trouble is i cant see any way the engine can adjust ignition timing on one bank as it uses a distributer (please school me if wrong) but i do think the injection loom looks like injection is on a seperate loom for each bank. Trouble is so far as im aware you cannot solve premature ignition by altering fueling can you? and the injection isnt sequential either so....................

Need to read up on this more but i think its worth checking they are the right way round anyway.

Ive not forgotten Steve, just struggling to get to the car atm sir.

D

Info Rapid V6 notes.
John

Dean wrote:
Who wants to bet its something simple in the end though like something unpluged or disconnected or a leaky doodah rather than complex electrical issues like ecu failure or intermittant sensors etc

D

I t will definitely be the Dooda, no doubt about it.

The engine ran fine before Matthew touched it. It will be something done in the process of dealing with the mechanical fault. Just retrace your steps carefully...or sell the bloody thing to me for scrap price.

MTXM wrote:I am sorry Will, but how would you do the metering? With regards, Matthew T.

If you have a decent wiring diagram use a digital meter to check all the feeds and returns to the ECU from each engine control component. The ECU is a pretty simple device and resopnse to inputs. We know the injector trigger and earth returns re good because it runs, so there isn't a component failure in the ecu.

Don't forget the Lambda sensor also controlls mixtrure strength, but I wouldn't expect it to have this much effect.