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

[Mandrake]

Is that a 2 litre 16 valve with an AL4 ?

What sort of loss of power characteristics were you noticing due to the exhaust ?

[addo]

Yes, an AL4. Firstly, I had a sensation of "sogginess" that came and went for several years - laggy response when the foot was pressed down, more especially with a load in or behind the car.

Then, from late 2011, a growing tendancy to overheat and struggle a bit when loaded up and pulling hard. Also a slight rattle of the exhaust innards near the motor on cold starts. I checked and rechecked both fans on several occasions to dispel ideas the fans were helping me overheat.

Finally, six weeks ago a significant loss of oomph - it still came and went - that reduced my towing speed uphill to about 40km/h with a trailer, and gauge temps trended upwards of 105°C (indicated). A new exhaust joint hardware kit (£8 pattern part) and a visit to Mr Prybar's Muffler Services saw the restriction sorted. The front half of the cat had formed chunks and powder that restricted gas flow through the second "block" of catalytic material.

[Mandrake]

Does sound very familiar I have to say... Everything except for overheating and rattle of the exhaust. I don't do any towing mind you, and the V6 probably has enough extra oomph in reserve that a power loss is not as catastrophic as a 2 litre.

I presume "Mr Prybar's muffler services" replaced the cat with a straight through pipe ? I'm not sure the MOT man would appreciate that over here, I would have to replace the cat with a "kitten", since a genuine cat would be 3 times the value of the car...

[addo]

There is a rumour that some cunning devils install a 2" 304 stainless straight pipe through the middle of the cat, and weld the tail back on.

[CitroJim]

Simon, sorry to hear the new MAP sensor was a DOA

Hope you can get it replaced with no problem...

[Mandrake]

Simon, sorry to hear the new MAP sensor was a DOA

Hope you can get it replaced with no problem...

Do you see any problem with the hole size and the exposed chip inside though ? If not I'll get the seller to replace it rather than refund.

[CitroJim]

It being so wide open like that is not how I've seen them in the past Simon, all have had quite small holes...
I'm wondering if your duff one was badly made and is leaking and therefore unable to measure accurately...

[Mandrake]

I thought the wide open hole was odd too, hence my concern. From what I understand the method of operation is the small chip that is visible is a solid state strain gauge - the top side is exposed to the manifold pressure, the bottom side has a small sealed cavity of air as a reference, thus changes in pressure on the top side apply strain which is measured.

If the sealed air cavity had a leak it wouldn't work, although its just as likely to be an electrical fault. There is no difference in the mb reading on the Lexia with the engine off or cranking...

The original has a pinhole that is maybe 0.5mm at most, presumably to protect the circuitry inside, on the other hand oil contamination could easily block that small pinhole hole causing sluggishness in response since the oil droplet would have to move with pressure change...

[addo]

Mine - a Bosch, and the original sensor - has no variance in the bore; it is a straight hole.

[Mandrake]

Mine - a Bosch, and the original sensor - has no variance in the bore; it is a straight hole.

As big as the one shown in the picture with visible insides, or about 0.5mm ?

[addo]

I've attached a snapshot of the sensor after hosing out with brake cleaner.

[xantia_v6]

My understanding of MAP sensors is that they have a pinhole orifice and cavity before the sensor chip to perform an acoustic low-pass-filter, to prevent wear and tear of the chip following the fluctuations of pressure due to each inlet valve opening and closing (and resulting aliassing effects due to the finite software sampling rate). So I think Simon that your replacement is a dog.

[Mandrake]

[Mandrake]

On further reflection although I agree the sensor does look kind of cheap, I'm not sure that I buy the acoustic low pass filtering explanation. A small neck with a cavity behind it would form a helmholtz resonator which is a bandpass function not a low pass, and this resonance is likely to have undesirable interactions with the pulsing from the valve openings at certain rpm.

While I also agree that anti-aliasing from the likely low sample rate used could be a problem, I'd be highly surprised if there isn't an analog low pass anti-aliasing filter between the strain gauge and the A/D converter that is chosen to be slow enough to smooth individual valve pulses but still fast enough to get a good throttle response. Failure to include such an anti-aliasing filter would be gross negligence in the design of the sensor as its standard practice for any A/D converter. (Mind you we are talking about a £20 pattern part where every shortcut possible is probably taken)

So while I don't like the fact that we can visibly see the chip and the cavity around it, I don't think the large hole is a problem as such, besides how do we explain Addos factory fitted OEM sensor that also has a large hole. I'm curious what the white substance seen within is though, perhaps a porous filter paper like membrane to keep grit out of the sensor and away from the contacts of the chip...

The problem I have with the pinhole design of my original is that a single drop of oil or muck would clog the hole, so of the three I think I like the look of Addos the best...

[DickieG]

Loads of theory and huffing/puffing Simon but personally I'd simply buy the correct/genuine part and be done with then you know where you are and aren't introducing secondary faults.