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

[lexi]

Maybe the vacuum gauge is only good to find out if any of The Coatbridge Kids have stuck a potato in Simon's exhaust tail pipes .

After all this.........there has been a Maris Piper malfunction

[Mandrake]

It may work with better with a simple non cat exhaust............like old Landy?

What signs do you look for in the vacuum reading when the exhaust is blocked ?

None of the ones you list Simon according to this rather simple guide. http://www.autospeed.com/cms/A_2393/article.html" onclick="window.open(this.href);return false;

This is what your page says for blocked exhaust:

"High reading at first but suddenly returns to zero then builds slowly back to about 16 inches of mercury."

Huh ? Under what conditions ? Idle ?

I'm not seeing any of the symptoms listed on that vacuum page.

[addo]

That would be true for a chronically restricted exhaust (eg, the potato trick, or a crushed pipe). What you're chasing is an intermittent fault - which if it were a restricted exhaust - is typically brought about by pieces moving around and at times converging to appreciably block gas flow.

[Mandrake]

The 3000RPM test reading will need to be driving, with a solid load. Maybe Lexi can loan you a bootful of duchess slates?

Maintaining 3000 RPM under load requires more air, more fuel - it makes more waste gas (by volume) so the effect of restriction is more apparent.

Obviously driving under load will give a higher back pressure reading, however the fact that the engine is not under high load during the test is already factored into the 2 psi figure which has been empirically derived from the testing of many blocked and not blocked exhaust systems across a range of vehicles...

The same engine that generates over 2 psi back pressure under a 3000rpm no load test condition will produce as much as 10 psi or more under load or snapping the throttle.

Think of trying to measure a resistor (our blockage) by applying a current (exhaust flow) and measuring the voltage drop (pressure drop between before the blockage and after the blockage) it doesn't matter exactly what current you test at as long as its consistent and you know what voltage reading you're expecting to see. Testing it again at a higher flow doesn't tell you anything new.

[Mandrake]

That would be true for a chronically restricted exhaust (eg, the potato trick, or a crushed pipe).

Exactly what I said earlier - a vacuum reading might show a chronically blocked exhaust where the car can barely drive, but that's not what I have, its blocked enough to be down on power but not chronically blocked. Only a back pressure test will show the more subtle blockage it might have.

What you're chasing is an intermittent fault - which if it were a restricted exhaust - is typically brought about by pieces moving around and at times converging to appreciably block gas flow.

It was intermittent a few months ago, but its pretty much permanently bad now.

It does vary a small amount in response to driving situations - opening it up on the motorway does seem to "clear it out" a little bit, temporarily, but the change is small. I get a much bigger increase simply disconnecting the MAP sensor, although the car doesn't like to run without it when cold - when its fully warmed up it runs ok without it and has a lot more power, which is why I'm thinking a partial restriction is upsetting manifold vacuum cues to the ECU, such that most of the power loss is actually the ECU changing timing and fueling rather than the effect of the blockage itself.

We'll know for sure in a few days, you might be right that it's the cat that is blocked, my gut feeling (or hope) is that its one of the mufflers...that the mesh that holds the fibreglass wadding in the muffler has rusted away and allowed the wadding to get loose inside where it moves about and partially blocks the exit pipe, but I guess we'll see.

Anyone else want to place their bets on whether there is a blockage and if so exactly where it is ?

[Northern_Mike]

my gut feeling (or hope) is that its one of the mufflers...that the mesh that holds the fibreglass wadding in the muffler has rusted away and allowed the wadding to get loose inside where it moves about

Why haven't you taken it or them off and ran the car without it then? Seriously, it took me about 5 mins (without a ramp, just a spanner, suspension on high, and an axle stand) to remove the split rear box on my Activa, and it ran fine, and quiet enough without it... I can't imagine it would be much longer to remove the centre box too..

[addo]

I've personally encountered on other cars:

Infiniti V6 - blocked precats
C5 I - blocked cat
Xantia CT - blocked cat; loose pieces then blocked mid-unit
406 - blocked cat

A reliable source had the midlength silencer fail inwards on a 306 petrol.

The C5 was sent away; the owner had no money. The Infiniti was gutted, the CT and 406 had high-flow generic replacements fitted; the latter car instantly returned better fuel figures.

With not the centre or rear mufflers, the cars are quite noisy on the road, but with just the rear off it's only a "boy racer" type loudness.

[Mandrake]

my gut feeling (or hope) is that its one of the mufflers...that the mesh that holds the fibreglass wadding in the muffler has rusted away and allowed the wadding to get loose inside where it moves about

Why haven't you taken it or them off and ran the car without it then? Seriously, it took me about 5 mins (without a ramp, just a spanner, suspension on high, and an axle stand) to remove the split rear box on my Activa, and it ran fine, and quiet enough without it... I can't imagine it would be much longer to remove the centre box too..

A few reasons Mike...

My back pressure adapter only arrived yesterday. Disconnecting parts of the exhaust is all very well but if I can't measure the change in back pressure what have I gained ?

The engine doesn't run properly with the exhaust terminating at the cat tail - I know because when the exhaust broke last year (starting this thread) I had to limp home noisily a mile or so with the cat and center silencer disconnected hanging free, and the car had absolutely NO power, I'm not joking when I say it was taking nearly full throttle in 2nd gear to get up a modest hill at 30mph. To use an old phrase, it couldn't pull the skin off a rice pudding.

Anything more than about 30% throttle just produced a lot more noise but no more power. I don't know WHY it has no power when the rest of the exhaust is not connected (doesn't make sense to me) but it doesn't. So disconnecting half the exhaust and going for a test drive would be pointless in judging whether the performance improved. Better to just measure the back pressure before and after swapping over the rear exhaust. (and maybe while its disconnected too, to get a cat only reading)

[RichardW]

Didn't it run like a complete dog after the exhaust broke? Which leads me to think the cat is OK?

Edit - you've posted in the meantime, and yes it did run like a dog!!

BTW - Addo is going to be insufferable if this is what's wrong with it.....

[Mandrake]

BTW - Addo is going to be insufferable if this is what's wrong with it.....

I know, I'm bracing myself already.

To be honest I've got to the point where if it does turn out to be the cat, so be it, I just want the damn thing fixed and working. If I have to find an after market cat I'll cross that bridge when I get to it...

PS I don't quite follow the reasoning where it running like a dog with the exhaust disconnected from the cat means the cat is ok ?

[Mandrake]

The 3000RPM test reading will need to be driving, with a solid load. Maybe Lexi can loan you a bootful of duchess slates?

Maintaining 3000 RPM under load requires more air, more fuel - it makes more waste gas (by volume) so the effect of restriction is more apparent.

Out of interest, here's the info I'm basing today's exhaust test criteria on:



I think its quite helpful to see the dynamic behaviour of the gauge on a known good and known bad car rather than just a threshold figure.

[Mandrake]

I've personally encountered on other cars:

Infiniti V6 - blocked precats
C5 I - blocked cat
Xantia CT - blocked cat; loose pieces then blocked mid-unit
406 - blocked cat

A reliable source had the midlength silencer fail inwards on a 306 petrol.

The C5 was sent away; the owner had no money. The Infiniti was gutted, the CT and 406 had high-flow generic replacements fitted; the latter car instantly returned better fuel figures.

With not the centre or rear mufflers, the cars are quite noisy on the road, but with just the rear off it's only a "boy racer" type loudness.

In your experience of intermittently blocked cats where the blockage is presumably broken away chunks of honeycomb that are moving about, would you say that the broken bits usually end up on the input side of the cat or the output side ?

It seems more likely to me that the breaks would occur on the output side of the cat where it runs the hottest, also that seems a more likely place for a blockage to occur where the exhaust flow would push the broken bits up against the output pipe where the diameter reduces down from the cat diameter to the output pipe diameter... if the broken bits were in front of the main honeycomb I wouldn't have thought they'd cause as much blockage unless they had crumbled down to dust...

The reason I ask is that the cat tail is short and straight so I should be able to look inside the tail pipe with a torch once its disconnected to see if there are any visual signs of chunks of honeycomb loose or whether the honeycomb looks normal, at least the back end of it.

[lexi]

Don't worry Simon, I can open any end of the cat you want with my Stihl cutquick........just for a looky

[addo]

The only one with failure at the rear was a CT that in fairness had probably smacked a few rocks on the farm driveway. Otherwise they've looked pristine on the outlet side, even when chunks were rattling about at the front. The catalyst is in two "packs" and it was the front one failing.

[Mandrake]

Too late at night and too tired to post a full write up of what was found and what was done today, (will do so tomorrow) but here's a small spoiler. No exhaust back pressure at all either before or after replacing the rear exhaust. The exhaust including the cat was definitely NOT blocked at all...not even slightly...