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[Olympia5776]

Just happened this evening so haven't had a chance to investigate fully.
Lumpy at idle ,not hunting and at circa 750 rpm as normal.
Hasn't stalled and no  lights on. Accelleration is perfect with no missfire anywhere in the rev range. Just changed the cam cover gasket ,fitted new plugs and eliminated a bad missfire on no 1 plug lead by replacing with a good used item. Waiting on new plug leads which should be here tomorrow.
Any ideas ?

[Olympia5776]

Well this has me baffled.
I had a code reader connected and it showed no faults . It was done at a friends garage so I'm happy it was done correctly.
The only thing we found on the reader was a fluctuating oxygen read out from the Lambda sensor.
Unsure if that is a symptom or the fault.
I thought it was down on power very very slightly when I was driving it up to the garage but I'd put that down to the uneven running . When the throttle is slowly opened you can hear the uneven running through the rev range . I'd hesitate to call it a miss because there was a miss on it last week when I found a faulty lead and it's not as severe as that.
I removed both hoses from the cam cover and there is a distinct vacuum on the small one and the larger one also has a vacuum and there is a clicking to be heard down the pipe also ( ICV ) I cleaned these out about 4 thousand miles ago.
I also thought it was heating up quicker than normal so dismantled the front timing case cover and checked the timing and then went the hole hog and removed the timing belt and checked the water pump and pulleys . All were replaced 4 K miles ago and are OK. Belt was refitted and tensioned .
Replaced all four plug leads with a used set which I believe are OK but will know better when a new set arrive tomorrow. Changed all 4 plugs for a good set .
I also examined all the air ducting and found no leaks or damage.
 Problem still there.
It's got me baffled and unsure where to go next. 
Any ideas ?

[Shallow Al]

The only troubles I have had with the idle on mine has been down to the ICV and the breathers. But yours seems to be all up the rev range.

[Olympia5776]

I'm becomming more convinced it's a Lambda sensor failing  .
The fluctuating read out I mentioned is correct on a Titania sensor as per fitted to GM Ecotec .
The rough idle is a symptom and I thought the instant MPG read out was down a little when driving it the other day. No soot and plugs are clean though.....
It would also seem that the EML light is not activated until it fails and if it was " failing " no fault codes would be reported.
Has anyone suffered similar symptoms ?

Anyone out there .........?

[Abiton]

Has the EGR valve ever been looked at?

If it were very sooty and failing to close correctly, that might be giving your symptoms, I think.  Wouldn't throw any codes either.

[Olympia5776]

Has the EGR valve ever been looked at?

If it were very sooty and failing to close correctly, that might be giving your symptoms, I think.  Wouldn't throw any codes either.

That's a strong possibility . I dismantled the timing belt last night to examine the water pump as I felt the temp. rise was faster than normal although it never overheated ,the fans seemed to cut in quicker than normal. There is also a light pinging noise from the engine somewhere ( external ) which would /could suggest a sticking EGR valve ,was most noticable below the car suggesting transmission down the exhaust pipe.
Correct symptoms .
I'm on it ........

[maracus]

I had a similar thing with my cav a while ago, I thought the lambda was to blame,
...until I unplugged the AFM, suddenly, as the ecu was getting no signal as appose to a wrong signal it went into it's pre planned guesstimate mode or whatever it does and idled spot on. Plugged it back it fell on it's arse again!

Annoying when ya can't just remove a carb, clean and adjust! Pesky modern cars...

[Olympia5776]

Has the EGR valve ever been looked at?

If it were very sooty and failing to close correctly, that might be giving your symptoms, I think.  Wouldn't throw any codes either.

You were absolutley 100% correct . 
Stuck partially open due to carbon deposits . Cleaned and cleared but it wouldn't seat properly and was still passing when closed.
It's blanked off now .
Well worth the effort to clean and I'd advise all X20Xev owners to do it . 45 minute job .
Turbine smooth again , Thanks for all the help.