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 »

DickieG wrote: As a thought have you conducted a compression test? Worth eliminating as it's very easy/cheap to do and really should be one of the first places to start when carrying out diagnosis on issues such as the one you describe.
No that's a good point, I haven't yet.

Would low compressions cause the oxygen sensor to read a lean mixture under load as I'm seeing though ? Also wouldn't it cause a poor vacuum reading, while my vacuum reading looks good ? (I suppose only one cylinder with low compression might not affect the vacuum too much)

I'm also not sure that compression could account for intermitent symptoms where the car is sluggish one day and zippy the next. (although its only been sluggish lately, its been a few weeks since it was last zippy)

Definitely worth checking though, I better get a compression gauge as well. Although since it means removing the inlet manifold to check the rear bank (not a good job to do outdoors in a car park in the middle of winter) I'd rather follow up the fuel pressure diagnosis first as that's something that's easy and quick to do in poor working conditions...

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

Post by DickieG »

Mandrake wrote:I'd rather follow up the fuel pressure diagnosis first as that's something that's easy and quick to do in poor working conditions...
Quick and easy one to do there Simon, check that the correct pressure regulator is fitted by reading the bar pressure stamped on its housing, I've come across a Xantia which had a 3.0 bar instead of the correct 2.0 bar, not sure what one a V6 should have though.

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

Post by Mandrake »

DickieG wrote:
Mandrake wrote:I'd rather follow up the fuel pressure diagnosis first as that's something that's easy and quick to do in poor working conditions...
Quick and easy one to do there Simon, check that the correct pressure regulator is fitted by reading the bar pressure stamped on its housing, I've come across a Xantia which had a 3.0 bar instead of the correct 2.0 bar, not sure what one a V6 should have though.
Isn't the regulator tucked in underneath the inlet manifold, inaccessible without removing the manifold ? It seems unlikely that the wrong regulator is fitted. Much more likely a blocked filter, faulty fuel pump or ruptured diaphgram in the regulator...

According to the Citroen data the fuel pressure should be 2.5 bars at idle and 3 bars at wide open throttle.

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

Post by DickieG »

Mandrake wrote:It seems unlikely that the wrong regulator is fitted.
That's what I thought until I discovered it :lol:

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

Post by Mandrake »

DickieG wrote:
Mandrake wrote:It seems unlikely that the wrong regulator is fitted.
That's what I thought until I discovered it :lol:
Fair enough. :) If the measurements show a problem with the pressure I'll inspect it to make sure its the right one. (And since I think the manifold has to come off to get at it, I'd check the compressions while I have the inlet manifold off)

Does anyone know what the normal range for compression is for the V6 ?

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Post by addo »

Gauge readings vary enormously. Best approach is to take a middle cylinder warm engine reading off a car that's demonstrably running well and use that as your reference. I suggest any gauge ought to show between 180 and 220 pounds hot, but you need to also understand how the gauge should move as it approaches max pressure.

Fuel pressure is easier to read with the manifold vacuum lead plugged, then there's no idle pressure drop. My observation of FPRs is that the rubber fails progressively; the usual first symptom is hard starting after sitting as the pressure no longer holds up in the fuel rails.

I don't believe you have yet subjected the car to real "load", from what you've described. There's a long way between looking at data with the throttle open and putting 450kg in it (which it will handle easily in good health) and dragging off up a hill. This tests fuelling, cooling, throughflow much more validly.

What I did with one experiment was to weld in a 1/8" NPT (female) steel bung ahead of the converter, on the passenger side. This gave me a pressure takeoff point and it required no removal of any items to establish. My error with the next step was to use soft solder on the copper tube - silver solder or braze would've been smarter! What is screwed into the female port, is a 1/8 NPT to hose barb fitting, with about a foot of copper tube (coiled) mounted on it. Your pressure gauge can then be fed by ordinary "rubber" hose clamped onto the copper end - the copper is long enough to dissipate critical heat levels.

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Re:

Post by Mandrake »

addo wrote:Gauge readings vary enormously. Best approach is to take a middle cylinder warm engine reading off a car that's demonstrably running well and use that as your reference. I suggest any gauge ought to show between 180 and 220 pounds hot, but you need to also understand how the gauge should move as it approaches max pressure.
I may be visiting David this weekend so if he has a compression gauge we may be able to measure one of the front cylinders of his V6, which could give a reference figure....
Fuel pressure is easier to read with the manifold vacuum lead plugged, then there's no idle pressure drop. My observation of FPRs is that the rubber fails progressively; the usual first symptom is hard starting after sitting as the pressure no longer holds up in the fuel rails.
Yep, that was going to be one of my tests, comparing the pressure with the vacuum line to the regulator connected and disconnected.
I don't believe you have yet subjected the car to real "load", from what you've described. There's a long way between looking at data with the throttle open and putting 450kg in it (which it will handle easily in good health) and dragging off up a hill. This tests fuelling, cooling, throughflow much more validly.
Maybe so, but the point of my test was to see whether it was running lean or rich at WOT at low rpm where the car has been struggling for power and pickup for months, and the result is that it is running lean when it should be rich. How lean I don't know since the oxygen sensor is narrowband...it could be very lean indeed. It also has at least 20 degrees of torque reduction retard in the same conditions.

If it's stuck in a lean condition with 50% - 100% throttle at only 1500rpm when it should be rich, there is no way its not going to be lean on wide throttle at higher rpm when even more fuel is required - that's just a bit harder to test safely when driving on the road without a data logger or better realtime display than a Lexia. The way I see it there is a major fuel shortage under load that is present even during low rpm demands, and that needs to be fixed first.

It's a shame I can't graph rpm, throttle opening, oxygen sensor reading and ignition torque retard together on a graph - if I could go for a varied test drive and log all of those to look at afterwards I could easily confirm what I suspect across a wider range of engine load.
What I did with one experiment was to weld in a 1/8" NPT (female) steel bung ahead of the converter, on the passenger side. This gave me a pressure takeoff point and it required no removal of any items to establish. My error with the next step was to use soft solder on the copper tube - silver solder or braze would've been smarter! What is screwed into the female port, is a 1/8 NPT to hose barb fitting, with about a foot of copper tube (coiled) mounted on it. Your pressure gauge can then be fed by ordinary "rubber" hose clamped onto the copper end - the copper is long enough to dissipate critical heat levels.
Yeah, the adaptors I've seen show an adaptor with a flexible metal tube at least a couple of feet long before it goes to the gauge to isolate it from the high temperatures, so I would use something like that.

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Post by addo »

That bit of testing cost me about $50. $9 for the female NPT, $20 for the welding, balance on the pressure gauge and fittings.

You only need to test the reg with vacuum supply blanked off; if the bar are correct then you need go no further. Especially as you apparently are not running rich. Service Citroën should state the rail pressure in Bar for the reg specified.

I can tell you that since the new cat was fitted by Mr Prybar, there has not been a second of lagginess or overheating like before. I have slight oxidisation on the kickdown contacts, but that's known to me and a double stab on the pedal usually clears it. Getting some beautiful understeer drifts happening on warm smooth roads.

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

Post by RichardW »

For a MK2 V6 service.citroen says:

1985 05 01 PETROL PRESSURE REGULATOR
3 BARS

And

Part Number Description Quantity Price excl. VAT Price incl. VAT
0000198505 PETROL PRESSURE REGULATOR 68.44 GBP 82.13 GBP

It shows as being on the RH bottom end of the front rail - and interestingly has a vacuum connection to it.

Fuel pump is in tank, shared with all petrol variants, but costs £188 from Citroen. Filter is in line under the car at the rear. Feed is to rear rail, return from font rail, above the regulator.

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Post by addo »

The vac connection is normal, many novices get caught out by how much it drops the supply pressure at idle when they're trying to "read" the feedline pressure. Checking pressure on the rail side of the reg is a back-up procedure.

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

Post by CitroJim »

If you look at the two fuel rails (one for each bank) you will see that both appear to have a pressure regulator on them. However the one without a vacuum pipe attachment is in fact a surge arrestor and what Citroen describe as a 'Fuel Pulsation Buffer' under part number 1563 G4

I've never known one go or what sort of symptoms it would throw up if it did but it's worth knowing it's there.

I believe it's the one you see on the RH end of the engine as you look at it whereas the regulator proper is on the LH (Cambelt) end of the engine and quite well hidden...

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

Post by Mandrake »

Thanks for the info on the fuel system guys...I've looked up the Citroen data for the injection system so I've got a fairly good idea of whats what now, see attached file below. :)

The price of a new fuel pump is fairly sobering Richard, I'd want to be very certain it was faulty before replacing it. :shock:
CitroJim wrote:If you look at the two fuel rails (one for each bank) you will see that both appear to have a pressure regulator on them. However the one without a vacuum pipe attachment is in fact a surge arrestor and what Citroen describe as a 'Fuel Pulsation Buffer' under part number 1563 G4

I've never known one go or what sort of symptoms it would throw up if it did but it's worth knowing it's there.
I'm guessing that its just a diaphragm with a gas pocket behind it - basically a fuel version of an accumulator sphere to help smooth out the pressure fluctuations that would otherwise occur between individual injector pulses.... That's a good question, what WOULD the symptoms be if it was faulty ? :? I imagine that the pressure would sag during each injection pulse, what that would do for drivability I don't know. It must be there for a reason or it would have been left out to save money...
Attachments
Description Fuel Supply System.zip
(64.54 KiB) Downloaded 53 times

addo
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Post by addo »

My understanding is it works the other way about, damping pressure spikes when a bank of injectors snap shut.

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Re:

Post by Mandrake »

addo wrote:My understanding is it works the other way about, damping pressure spikes when a bank of injectors snap shut.
It would do both. Reduce peak spikes when injectors close and reduce pressure sag when they open. It's just a pressure accumulator after all helping to smooth out pressure variations in response to changes in delivery.

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

Post by DHallworth »

Simon,

Have you actually tried changing the fuel filter?

Our V6 was once running a little sluggish and it turned out that the old fuel filter might of been blocked as the new filter made the car run massively better.

David.