Citroen XM Forum

Yes because im running out of things i have not checked that can cause my mid-range misfire.
ECU is good
Compression is good on all cylinders
Vac is perfect
Injectors are good
all wiring is good
All sensors are good
O2 sensor ok
all ignition components are new and checked
fuel pressure is good
MAP sensor is good

All i have left is to suspect the ignition module and the TPS, even though the later now does what its supposed to do with pretty much the correct range and then i am out of ideas.

D

Are you able to run a Lexia check on fuelling and ignition timing figures while the car is being driven, and is faltering mid-range? Might show up something fluttering.

On knock sensors, what happens if both of these are disconnected/disabled? They are listening for pinking, so if none is detected, ign timing should (I guess) be left unretarded.

Fuel quality?

Sort of Chris yes, my laptop battery lasts about 3 minutes after being unplugged from the mains but that is the next plan of action.
With the Knock sensors unplugged it still misses.
TPS unplugged idle sits at 1400rpm and is much rough and jerky to drive.

D

I can see this leaves you stumped and cause you sleepless nights. I have been there. I have been following this thread with much interest and am amazed at your perseverance. I would have quit long ago and got a Prius - which I actually did, but came back to the XM which is currently giving me a bit of a headache with, yeah, an idling quirk. I hope you'll get to the bottom of this.

Jan-hendrik wrote:

Unfortunately, the fuel pressure is spot on at 42psi and the pressure regulator is working fine taking the pressure up to 52 psi with no vac applied.

Unfortunately

Dean has a problem with hesitation at medium throttle openings. He suspected fuel pressure.

Dean has a problem with hesitation at medium throttle openings. He suspected fuel pressure.

The fuel pressure is not the problem then, so problem not solved. ECU and wiring do not conform and it is back to basics. Easy for me to say.

Hi Dean,
I was sorry to hear about your continuing problems that I am sure will be resolved in the end! Just to confirm, has the running issue always been present since the rebuild and if yes has it become worse over time?
With regards,
Matthew T.

In answer to you question Matthew yes it kind of has always done it, the first couple of weeks it was fine apart from other misshaps and the jerky throttle transitions due to an incorrectly wired and positioned TPS but once all that was sorted out ish it was fine, started off feeling a bit flat around 2k then sometimes it would hesitate and sometimes it would not, over the year the probelm has become worse gradually to the point where it is no longer intermittant, the issue is always present as what i would call a soft missfire f all cylinders at once or more accurately a hesitation of varying severity.

It has now come to the point where the hesitation is appearing below 2k rpm.

Slowly rolling into the throttle produces the worst hesitation (like the ecu with a fly-by-wire throttle is going in and out of limp mode) at varying intervals and periods of time, like someone is pulling the ht lead on and off the coil a couple of times a second i guess. At 5k rpm it clears for the most part and she pulls cleanly at any throttle position.

If you stamp on the throttle and induce a dowshift it pulls fine through the gears even as it changes up.

I have no mechanical fueling issue and as far as i have diagnosed i have no mechanical ignition issue, all plugs look the same and show a perfect mixture and running with just a very pale pink hue on the insulator, probably from the booster in high octane fuel.

To be honest, to me it feels like a TPS problem especially after spending such time looking for problems in every other aspect of the engine, but the live data on the Lexia shows it now does exactly what it should, i have .64v at idle state, switching to MID thottle at .69v and up to a max voltage of 4.3v switching to 'FULL' throttle state at around 4.26v with perfect idle and smooth throttle transitions. The TPS itself gives a perfect .01v incrimental change of voltage through its whole sweep as seen in live data in Lexia and if measuring resistance off the car it gives a smooth reading with no dead spots in the track, all three TPS' have been tried on the car (two for a V6 that came with the engines and one from my 2.0i) which dispite having differing part numbers are identical in function.

Obviously there is a problem and i will find it but at this stage it must be a component i have already checked as i have tested the lot . Tonight i am going to perform a vac test to make sure i have no timing issues, valve problems etc and i have a brand new ignition module on order as i am not sure how to test them for correct function and that is the main switching eliment for the coil, driven by the ECU, it is the only component not proven and tested to be working perfectly so far.

After this, if there is no fault found or improvement i shall turn my attention back to the TPS and would be interested to know which of the three sensors listed are fitted to other cars, i have two types and wonder if my setup requires the third but this is clutching at straws by this stage.

Its good fun though.

D

The Vacuum test looks a bit low and unstable but that could be normal for this engine with an ICV as the idle speed does move about a little, every joint from the throttle plate to the head has been doused in butane and i cannot see anything like a rich signal from the O2 sensor when doing this and no change in how the engine runs so still assume i have no vac leak, would assume a vac leak would need to be huge to cause a mid-range problem on this system anyway.
https://youtu.be/hivtrFx4cG0" onclick="window.open(this.href);return false;

The ignition timing was hovering around 8 degrees but the O2 signal is suggesting its kind of lean bouncing around from 0-.6/.75v but what would now days be called STFT is running at +20% and for a no-load condition at what is in effect a fast idle i find this slightly suspect. These are the same readings i had before fitting new injectors too although now the engine is a lot smoother. Fuel pressure was up at about 53psi during this test as i was using the pressure regulator VAC port for the guage.
Live data was pretty much identical to this
https://youtu.be/BgWgwYkGQ5s" onclick="window.open(this.href);return false;

The good thing is the valves looked good, and the engine looks healthy mechanically according to the VAC test which is nice.

D

Jan-hendrik wrote:I can see this leaves you stumped and cause you sleepless nights. I have been there. I have been following this thread with much interest and am amazed at your perseverance. I would have quit long ago and got a Prius - which I actually did, but came back to the XM which is currently giving me a bit of a headache with, yeah, an idling quirk. I hope you'll get to the bottom of this.

Thats very nice of you to say Jan but what you do not seem to understand is that stupid does not know when it is time to give up

D