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 »

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I’ve only shown a picture of the dirty side of the plug, as mentioned the opposite side of the insulation is perfectly clean on the two dirty plugs.

So is this amount fouling likely to be causing an intermittent misfire?

What would the cause be? I can think of two possibilities… I did run the car with the oxygen sensor disconnected for nearly a month, this would have led to the engine running a bit rich all the time, maybe enough to foul the plugs?

The other thing I can think of is that with the oxygen sensor connected, if one cylinder misfires the free oxygen makes the oxygen sensor read lean, the engine ECU will then richen the mixture to all cylinders in an attempt to compensate, causing all the cylinders that AREN’T misfiring to run rich and foul the plugs…

Which might finger cylinder one as being the misfiring culprit, as the plug is clean while the other two are dirty? (I haven’t checked the rear spark plugs for obvious reasons) The fact that Cylinder one also has a weak sounding spark is also a bit of a suspicious co-incidence?

Another curious thing I noticed about the insulators on the plugs:
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Again they’re in order 3,2,1 as fitted to the front of the car – the clean plug is on the right, both the dirty plugs have a yellow mark around the base of the top insulator that looks like they have been overheating?

Why would that yellow mark be there? Is the absence of the mark on the right hand cylinder 1 also a sign that that cylinder has not been firing under load and therefore not heating up the plug as much? (Not that I think the other two should be yellow…)

Here is a picture of some of the carbon as I was part way through scraping it off: (This is from plug number 1 which does not have carbon on the insulator)
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Quite a coating for plugs that have been there only since August…

When refitting I swapped plugs 1 and 2 to see if the “weak” sounding spark changed position but it did not…

I went for a quick test drive after refitting the plugs etc and there was a small but noticeable improvement but given how intermittent the problem has been before I don’t think it was statistically valid…

Next stop, fuel rail pressure and vacuum testing…

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

Post by Mandrake »

This was done with a fairly warm engine (not a good idea I know) after a test drive where the performance had been average and was back to being sluggish again by the time I got home.

Fuel pressure when the pump is manually operated without the engine running – 2.6 – 2.7 bars. It should be 3.0 bars.

While idling 2.1 bars – should be 2.5 bars. Snap throttle or disconnecting the vacuum line both gave about 2.6 bars and should be 3.0 bars. With the engine doing 2000rpm with no load the pressure was 2.0 bars.

The pressure does seem a bit on the low side, and I did notice some inconsistency in the readings between when I started the test and finished the test, like it was a bit intermittent.

Removing the fuel pump fuse caused the pressure to drop rapidly; the car would run for about 5 seconds before spluttering to a stop.

My gut feeling is the fuel rail pressure is too low, not low enough to stop it running, but maybe low enough to hurt performance, and it did seem a bit inconsistent. Is there any possibility that the regulator could have a tiny split in the diaphragm, which is causing intermittent variations in fuel pressure?

Pressure regulation is critical because the ECU learns through feedback from the oxygen sensor how much injector opening time (in ms) gives a certain dose of fuel to achieve the correct mixture, but the fuel dose for a given opening time depends also on pressure, so if the pressure is fluctuating when it shouldn’t the ECU is going to be running around in circles trying to adapt all the time…too lean, too rich etc…it will be too stupid to realize that the fuel pressure is fluctuating, as it doesn’t measure it.

All it knows is it fires the injectors for a certain amount of time that should by past experience give the optimal mixture but instead gets too rich or too lean.

On a test drive today I watched the oxygen sensor again and found that it is still reading constant lean (low voltage) with wide-open throttle below about 2500rpm. Above about 3000 rpm it switches to reading rich with wide open throttle and you can feel the performance pick up. With light throttle it alternates lean/rich a few times a second as it should.

I tried pinching the fuel return line for a few seconds and the pump is easily able to deliver about 5 bars, so I think that rules out the fuel filter (new anyway) and the pump.

I tried to do a pressure drop test on the injectors but unfortunately wasn’t able to. Fuel pressure is steady after manually operating the fuel pump then stopping it, however the Lexia function that pulses the injectors to test them unfortunately does not pulse them anywhere near fast or often enough – over a 10 second period it only pulses them 5 times causing a pressure drop of only 1 psi. To measure the pressure drop you really need a total drop of about 10psi. So I could only test them if I had a suitable exciter.

So I’m definitely suspicious of the fuel rail pressure and regulation stability, and whether the injectors are working properly and balanced is still an open question as I wasn’t able to do a drop test.

Next onto the vacuum
Last edited by Mandrake on 17 Feb 2013, 23:59, edited 1 time in total.

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

Post by Mandrake »

I tee-d in a gauge at the feed to the fuel regulator (I didn’t want to mess with the blanking plug at the rear) and got the following readings, all of which are in inches of mercury:

Vacuum at hot idle (640rpm) was a bit less than I expected, at 17” HG, however it varied with precise idle speed so varied between 15 and 18” HG at idle. At 2000 rpm it rises to about 20” HG. At high revs it spikes up to 23” HG when the throttle is suddenly closed.

The snap throttle test was interesting – instantaneous reduction in vacuum to zero vacuum with no lag at all. Everything that I’ve read says that a blocked exhaust will cause the vacuum to drop less and/or more sluggishly during a snap throttle test (even though intuitively I would have thought it was the other way around) so if that’s the case, it passed a snap throttle test.

After doing the fuel pressure and vacuum tests I took it for a second test drive and for some of that drive the performance was markedly better than the previous test after refitting the plugs, especially at higher rpm where it was really flying up hills… but by the time I got home again it was a bit sluggish again. In other words still intermittent even within a single 20 minute drive.

So where do we stand? Intermittent low fuel pressure / fuel delivery?

Fouled plugs causing an intermittent misfire under load?

Despite everything testing ok, a weak spark to cylinder 1 causing a misfire under load ? An as yet undiagnosed faulty injector? You tell me… :lol:

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Post by addo »

What load was the engine under when you were testing the throttle - or was it stationary and idling?

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Re:

Post by Mandrake »

addo wrote:What load was the engine under when you were testing the throttle - or was it stationary and idling?
During which tests ? Fuel pressure ? Vacuum ?

All the tests were done stationary, the only engine load was the snap throttle testing. The only way I could potentially do a constant engine load test would be to run the car in drive against the foot brake and I wasn't keen to do that on my somewhat dodgy gearbox...

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Post by addo »

Behaviour under load can be quite different to zero load situations.

I can say confidently, you can put 700kg in the back with seats folded down - spread the load on boards. 30-35 bags of gravel would see you right (hey, you can return them for a full refund if unopened)...

Or you could go to a dyno tuner and buy a simple flying run.

This weekend just gone, I was hired a generator with blown coil. Symptoms appeared under load (ie, when we needed to use it). At first the hire shop denied there was a problem - they simply listened to it run at nil load. When I made them idle it twenty minutes, then load the bugger up - hey, it struggled and almost died! Just like we found it to do, onsite.

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

Post by CitroJim »

Those plugs aren’t fouled Simon...

Fuel pressure has to be worth a follow-up. First thing to check is the little sense pipe from the inlet manifold to the pressure regulator capsule. If that's leaking then it'll not regulate against changes in engine vacuum...

My XM just feels livelier and more eager...

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

Post by Mandrake »

So you don't think the carbon visible on the insulator is sufficient fouling to affect running ? I did give the electrodes a wire brush to clean them up and it didn't really make any difference to the running.

However playing around with the fuel rail testing its pressure, bleeding it suddenly with the dump valve on the gauge a few times etc did give a temporary boost in performance, for at least 10 minutes of driving it was running far better, and this is also exactly what I did the weekend I drove to David's when the performance was so fantastic. (Although that time it lasted over a week)

Here's a theory - could one or more of the injectors have its input filter mesh blocked with crud ? Could the instantaneous drop of fuel rail pressure when pressing the dump button on the gauge dislodge or move some crud blocking one of the injector input filters ?

The best test to confirm a blocked injector would be to do injector pressure drop tests, but I would need to obtain or make a pulsed exciter to test them as the Lexia won't do it.

Or perhaps there is some crud lodged in the pressure regulator, normally fuel would only flow in one direction through the regulator but dumping the pressure through the schraeder valve would due to its location cause a reverse flow from the regulator, perhaps dislodging something, at least temporarily.

Could I finally be homing in on the problem ? Maybe some petrol injector cleaner in the tank would be worthwhile ? (Anyone have any brand recommendations ?) Although injectors are expensive one small mercy is that a blocked injector misfire is far less likely to damage the cat than a missed spark misfire since the amount of unburnt fuel passing through would be a lot less...

(The vacuum hose for the pressure regulator is not leaking by the way and seems to give the anticipated change in pressure with vacuum)

BTW the tire saga is becoming a bit of a farce, I dropped the car off this morning, then got a phone call at 2:30 to say that they couldn't balance the wheel because their balance machine had broken down! :roll: The receptionist I was talking to then said that they had swapped the front tires over and that that should help a bit in the meantime. WTF ? Is this the kind of drivel that people get fed.... [-X

Furthermore when I picked the car up tonight (after the mechanics had gone home unfortunately) I discovered that they've lost the black plastic "grabber" for the security bolt caps, (that lives in the security bolt box) as well as the caps covering the front security bolts. #-o

Yet one more bizarre thing - they left my car parked in the pickup area with the suspension lowered right down... why ?? I noticed it right away so set it to normal height and waited for it to lift, but less savvy owners would have driven off down the road with the suspension bashing and crashing its way along, likely risking a failed strut top... Clowns... :roll:

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

Post by lexi »

My Land Rover shows that black fluff on one side of a plug at times. Usually when whipped out after idling. That is 50's tech with a Solex carb.

Tyre places are like hospitals..............the less time spent there the better! Those Diddys probably thought they were putting your car in Slam Mode for better handling :shock: I can just see you going in there to find them around your car Simon, and you walk in with a Tannoy Loudhailer " Please step back from the Citroen and keep your hands in the air" :-D

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

Post by Mandrake »

lexi wrote:My Land Rover shows that black fluff on one side of a plug at times. Usually when whipped out after idling. That is 50's tech with a Solex carb.
Interesting. The plugs were taken out after the engine had been idling for a while, and yes one side of the insulator was white and clean with the other side dirty as pictured.

I find it interesting that the number one cylinder plug had no trace of black stuff on the insulator at all, (compared to 2 and 3 that had equal coatings) and also lacked the yellow ring that is presumably from heat...

So if there is a blocked injector it could well be cylinder 1...whether its missing spark or missing fuel it seems that cylinder 1 is not pulling its weight otherwise the plug condition would be the same. I don't know what the rear plugs are like mind you, too difficult to check just on a whim.
Tyre places are like hospitals..............the less time spent there the better! Those Diddys probably thought they were putting your car in Slam Mode for better handling :shock: I can just see you going in there to find them around your car Simon, and you walk in with a Tannoy Loudhailer " Please step back from the Citroën and keep your hands in the air" :-D
:-D

I HATE taking a car of mine into a garage or tire shop, I think I'd rather go to the dentist than leave my car with monkeys... :roll:

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

Post by lexi »

The porcelain section of plug should be kept clean as tracking and resistance builds up there. They are often dirty. Alchohol or similar to clean. We used to use those Colourtune plugs and check for shade while firing. That and Vacuum Gauge was all you needed. Would a faulty TPS sensor or similar not give dramatic problems?

Just to edit that, it buids at top of plug cap and travels the dirt and carbon at top of Ceramic.
Did you fire any injector cleaner in? Not a solution mind you but may give clues.

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

Post by CitroJim »

Mandrake wrote: I HATE taking a car of mine into a garage or tire shop, I think I'd rather go to the dentist than leave my car with monkeys... :roll:
My dentist, definitely! Although I'm quite happy for Costco (of all people) to do my tyres...

Did you check the little sense pipe to the fuel regulator Simon?

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

Post by Mandrake »

Keep up you two, the answers to both your questions are in my second to last post! (21:43) :lol: :lol: :-D

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

Post by CitroJim »

Mandrake wrote:Keep up you two, the answers to both your questions are in my second to last post! (21:43) :lol: :lol: :-D
Sorry :oops:

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

Post by xantia_v6 »

I think that the discolouration on the spark plug electrodes is ash, not carbon, likely to be from combustion of oil fumes, probably ingested via the crancase ventilation system, so no cause for concern.

Have you replaced the fuel pressure regulator yet? Low pressure with the engine stopped can't really be caused by anything other than a faulty regulator.