Xantia V6 broken exhaust (update: and rough low rpm running)

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Well, I decided to take a punt on the £70 coil pack above, so it's now on it's way, expected early next week. :)

It's most likely either the coil pack, rear spark plug leads, or rear spark plugs, with no easy way to tell which except substitution as its not a complete loss of spark, so I've taken the cowards approach and gone for the easy to change coil pack first. :lol:

If I wait I may never find one at that price again anyway - the only others I've seen are £140 and up. If it doesn't fix the problem I'll tackle the spark plugs then...and at least I'll have a new, uncracked coil pack that's unlikely to ever give me trouble in the future...

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

That was quick, it's here already! :)

It came in a box that is covered with citroen / peugeot original parts in various languages, however the brand name is Sagem, same as the one on the car - is that actually the OEM brand after all, or is it just a pattern part in a legitimate looking box ?

It looks in perfect condition, the only real difference from the one on the car is that the potting is semi-translucent instead of fully opaque so you can faintly see some of the insides...I'll try to fit it this weekend (weather looks rubbish) and report back. If its a good-un then there are still 5 left on the same ebay listing if any of you V6 owners are looking to hoard a spare one...

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CitroJim
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

The new Sagem I bought when the coil died on my original V6 was semi-translucent Simon. Must be from the same batch :roll:

Good call that he's got five left. Very, very tempted...

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Post by addo »

It is 100% genuine; Sagem are an OEM supplier. They furnish PSA with many ECUs. The seller is trustworthy.

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Well it wasn't raining, so I decided to nip out and swap it over tonight since its due to rain tomorrow.

At first I thought there was an improvement but after driving for a while I realised there is no real change. So it looks like the coil pack has been ruled out. :(

The new one reads slightly differently on the Lexia - the 3 coils measure more closely matched for dwell time but there is still one coil that is different to the others - this time the coil for plug 3/4, instead of 1/5 so I think the variations that the Lexia is showing between the coils (on the order of 0.1ms for the old coil and 0.05ms for the new one) are just normal tolerances and aren't significant at all, in other words a red herring.

Looks like the old cracked coil pack still has plenty of life left in it, so I'll hang onto it as a spare. (The good news is those £70 coil packs listed on ebay seem to be good, if anyone wants or needs one)

I also re-did the missing lead test for each of the three rear plugs this time with the disconnected output from the coil feeding a substitute spark gap (so as not to upset the firing of the front cylinders) and the result predictably is that there is no significant difference in how rough the engine becomes regardless of which of the three rear leads are disconnected - any one of them leads to the engine rocking on its mounts at 1200rpm, so there doesn't appear to be any significant difference in the firing of any of the rear plugs.

Other things I noticed tonight are that the car seems to run better when cold (it seemed almost normal at first while cold) and deteriorates as it warms up, there seems to be plenty of power at high revs, its only at low revs in the 1000-1600rpm range or so where it struggles.

Also there is still a surging behaviour at moderate revs and light throttle - while doing 60mph on the motorway with a steady throttle the engine is doing around 2300rpm, but I could audibly hear it surging up and down about 200rpm in a regular pattern of about once per second, and see it on the rev counter - exactly the same thing it does stationary with no load with the throttle open to 1500rpm.

This behaviour didn't exist a few months ago and started at the same time that the rough low RPM running started. I can't see how plugs or plug leads could cause this cycling behaviour, so I'm starting to lean back towards dodgy sensor data causing the ECU to vary the timing or fuelling in a way that it shouldn't.

The Lexia clearly showed the timing varying from 30 degrees to 15 and back again every second with the throttle held steady at approx 1500rpm. I can't see any reason why this should happen.

Back to the drawing board I guess... :roll:

One thought I have is what if the lambda sensor is clogged up and reading wrong - by design the ECU uses the lambda sensor to constantly cycle the mixture between slightly lean and slightly rich, at about the same rate that I notice the RPM surging. What if instead of that it is cycling it between too lean and even leaner ? In my MOT the lambda reading was a bit high at 1.02, meaning lean, and that was before the engine was even running badly.

Does anyone know what the fallback response from the ECU is if the lambda sensor is temporarily disconnected ? Will it bring on a warning light, or will the ECU try to conservatively work around the fault ? (At the expense of increased emissions) Is it worth me trying to unplug the sensor temporarily to see if the running improves or worsens ? If so where is the plug ?

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by xantia_v6 »

Be aware that if you have a misfire (on any cylinder), it will pump unburnt fuel and air to the oxygen sensor, the sensor will detect the oxygen and the ECU will command more fuel to correct the apparrently lean mixture.

You need to fix any misfire before hunting for mixture or O2 problems.

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Yes I understand that, but how can I tell apart a misfire on an individual cylinder which would as you say cause the other cylinders to run rich when the ECU is working in closed loop mode, from misfiring in all cylinders due to the ECU fuelling the cylinders too lean ?

The more recent tests I've done seem to suggest that there is no individual cylinder that is running worse than any other, and since this problem appeared suddenly in the space of a week I can't believe that it's gradual wear and tear of spark plugs or plug leads. One week it was running fine, the next week it was doing what it is now.

One reason I'm thinking about the Lambda sensor is that when the throttle is open wide at higher revs the ECU switches into open loop mode - and as far as I can tell the engine runs smoothly and performance is fine in this state. It's when you're cruising with a low steady throttle that the problem occurs, both the staggering, and the oscillation in engine revs with a steady throttle, and that's when it would be in closed loop mode.

It really does feel to me like its just running very lean with low throttle openings causing intermittent misfiring of all cylinders.

I guess I should still do the rear plugs and measure the plug leads to see they're ok, (new ones are £70) but I'm really not looking forward to that job, and my gut feeling is it will not fix the problem.

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CitroJim
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

Plug leads are not resistive Simon so they should read very low...

I'd go for a new temperature sensor if having the manifold off and replacing the rear bank plugs does not crack it.

Then, and only then worry about the lambda sensor. It not too bad a job but don't do it with the cat hot. I still bear the scars. Cats get very hot and retain heat for longer than you'd think!

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by RichardW »

Had another thought earlier on - much has been said about the ignition side, but what about the fuelling - what's the fuel rail pressure saying / doing? Odd that such a clear fault hasn't put the EML on - at least that would let you know where to look!

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

Good idea Richard, but I don't believe the Lexia reports the fuel rail pressure on a petrol engine ? I went back and checked the Lexia screen shots I posted earlier in the thread and none of them show fuel rail pressure. About all it shows regarding fuelling is the injector pulse time in ms.

Are you suggesting it could be something like a blocked fuel filter ? I would have thought that would affect wide throttle high rpm performance more though, this seems to only affect low rpm.

As for no engine management fault light - I can leave a plug lead disconnected causing severe misfiring and not only does the EML light not come on, no faults are logged and there is no change in the dwell readings! Therefore I'm not sure that fault codes are all they're cracked up to be when it comes to diagnosing a subtle fault that doesn't actually stop the engine running entirely... :roll:

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by xantia_v6 »

Mandrake wrote:Yes I understand that, but how can I tell apart a misfire on an individual cylinder which would as you say cause the other cylinders to run rich when the ECU is working in closed loop mode, from misfiring in all cylinders due to the ECU fuelling the cylinders too lean ?
disconnect the oxygen sensor. It will default to running a bit rich, but not enough to cause a misfire.
I would still recommend checking the rear plugs for oil-immersion.

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

xantia_v6 wrote:
Mandrake wrote:Yes I understand that, but how can I tell apart a misfire on an individual cylinder which would as you say cause the other cylinders to run rich when the ECU is working in closed loop mode, from misfiring in all cylinders due to the ECU fuelling the cylinders too lean ?
disconnect the oxygen sensor. It will default to running a bit rich, but not enough to cause a misfire.
I would still recommend checking the rear plugs for oil-immersion.
Unplugging the oxygen sensor (is that the lambda sensor on the pipe between the exhaust manifold and the cat ?) as a quick test is what I suggested earlier - if its leading the ECU to run the engine too lean and disconnecting it improves the running due to richening the mixture a bit I could have a contaminated lambda sensor ?

(See: http://www.lambdapower.co.uk/Diagnosis/ ... _index.asp" onclick="window.open(this.href);return false; )

If it was a rear spark problem unplugging the lambda sensor wouldn't help, it would still misfire I presume despite the richer mixture ?

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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by RichardW »

Oxygen sensor = lambda sensor.

I'd have thought it if was a spark problem then unplugging it shouldn't affect the problem. I was thinking about perhaps a problem with the fuel pressure regulator causing low rail pressure, and hence low injection, bu then if you are OK at wide throttle openings perhaps it isn't that....!

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Mandrake
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by Mandrake »

RichardW wrote:Oxygen sensor = lambda sensor.

I'd have thought it if was a spark problem then unplugging it shouldn't affect the problem.
My thoughts too, if it was ignition problems disconnecting the lambda sensor won't improve it, but if is running too lean due to incorrect sensor data it may.

I've been reading the tech notes about how a lambda sensor works on that site I linked to, this page in particular is interesting:

http://www.lambdapower.co.uk/TechNotes/Tech-5.asp" onclick="window.open(this.href);return false;

Basically it's saying that the most common failure mode of old or contaminated lambda sensors is not complete failure, but that it either switches from high to low voltage at the wrong air/fuel mixture, (instead of 14.7/1) or that its response time becomes sluggish. (Up to 1.5 seconds compared to 0.2)

So the fact that the O2 sensor is cycling voltage up and down on the Lexia only proves that the closed loop mode is working, not that it's regulating to the correct air/fuel mixture, nor that the sensor is responding fast enough for the regulation to work properly without excessively overshooting the correct mixture on each cycle.
I was thinking about perhaps a problem with the fuel pressure regulator causing low rail pressure, and hence low injection, bu then if you are OK at wide throttle openings perhaps it isn't that....!
It does seem ok at higher revs but bear in mind I haven't driven another V6 before so I don't have a frame of reference to judge the performance... :)

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CitroJim
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Re: Xantia V6 broken exhaust (update: and rough low rpm runn

Post by CitroJim »

Unplugging the oxygen sensor should, if the ECU behaves like a TCT one, cause the ECU to disregard it and determine mixture purely on the other parameters it can read. Worth doing as a test for sure and you may find it does not bring the EML on.

My XM is currently running without an oxygen sensor and has for some while. It's runs fine and has passed MOTs on the cat test with it inoperative.

I'm not saying the V6 MP7 will be as happy as the TCT's MP3.2 but it's got to be worth a shot, just to see.

You'll need to clear a stored fault with the Lexia if you do.