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

[Mandrake]

Yes very interesting... a couple of the references I was getting information from:

http://www.aa1car.com/library/map_sensors.htm" onclick="window.open(this.href);return false;

A low intake vacuum reading or excessive backpressure in the exhaust system can trick the MAP sensor into indicating there is a load on the engine. This may result in a rich fuel condition.

http://www.tirereview.com/Article/98309 ... nosis.aspx" onclick="window.open(this.href);return false;

Speed Density System Diagnostics
Speed-density systems found in older vehicles are designed to provide a calculated, rather than direct-reading, value for engine airflow. Speed density calculations generally include data from the throttle position, manifold absolute pressure (MAP) and engine speed sensors.

Keep in mind that the MAP value will begin to equal the atmospheric pressure value as the throttle opens and load increases on the engine. The PCM responds to the increased MAP value by richening the air/fuel ratio. If the exhaust is restricted, the MAP value will equal atmospheric value as soon as the throttle is opened and the engine fails to accelerate. The engine generally tending to run rich and lose power are symptoms of exhaust restriction in a speed-density system.

There were others that contributed too but I cant seem to find them in my bookmarks, including a couple that referred to an excessive calculated engine load value (derived mostly from the MAP reading) causing the ECU to retard the timing. (as well as richening the mixture)

I suspect the retarded timing is the largest contributor to loss of power that I've been seeing, and I have commented before on the apparent tendency for the ECU to apply a 20 degree "torque reduction" timing retard even during normal driving conditions, (when it shouldn't) so it fits in with that observation. Now I think I know WHY the timing was being retarded.

[Mandrake]

A few minutes after getting home today there was a knock at the door, look what I have now:



Not much in the way of packing, the flanges and mounting brackets were wrapped up but apart from that it was just shoved in a cardboard box, so there are a few dents here and there, nothing of significance though. It could also do with a few of the sharp edges rounding off including on the tail pipes, but nothing that a few minutes with a file won't fix.

Some more pictures:





One thing that had me a bit puzzled at first is it looks like the two tail pipes are askew and not facing in the same direction, particularly from this angle:



I seriously thought one of them has been bent (or misaligned when welded) but looking at it from this angle I realised that depending on the rotation of the backbox when mounted the tips arrive parallel at the ends:



I'll have to go out and have a look at the orientation of the one on the car. As mentioned earlier in the thread (sorry I can't remember who) the tailpipes are rather long compared to the originals! They could possibly do with shortening a bit.

Other than that it looks pretty sturdy, I was shocked at how big and heavy the back box is - it doesn't seem that big on the car but I guess it must be... the real test will be whether things fit together well and line up with the route and mounting points on the car correctly, also whether the silencer pipe is a good fit on the cat tail, since we'll be needing to use a slotted clamp arrangement rather than the normal cat joint...

[CitroJim]

Snap! I have one identical to that in storage in the workshop loft for when it's needed..

Yes, the tailpipes are not the neatest and the welds are a bit scruffy in places but for £77 all up it's quite tolerable...

A bit of work will see those tailpipes perfect...

[Mandrake]

All the welds look fine on mine but there's certainly plenty of sharp edges on the tail pipes, flanges etc that need a bit of work with a file... But as you say what can expect for £72 ? As long as it is solid, fits, and works I don't mind if I have to tidy it up a bit with a file...

[citronut]

i spoz those tail pipes will straighten them selfs out Simon once fitted and you floor the loud pedal

[RichardW]

File, shmile. We'll let Mr A Grinder do the talking He has a much more 'direct' approach

[Mandrake]

I like your style Richard.

Did I mention I hack-sawed the bottom drop-link bolt off one of my drop-links by hand when I did the drop-links last year ? It sure would have been handy to have a grinder then... I'm not even going to say how how long it took...

One of the first things I'm going to buy if we move house to somewhere with a garage is some power tools including an angle grinder.

[Northern_Mike]

I like your style Richard.

Did I mention I hack-sawed the bottom drop-link bolt off one of my drop-links by hand when I did the drop-links last year ? It sure would have been handy to have a grinder then...

http://www.amazon.co.uk/Dremel-Lithium- ... B00067AZV6" onclick="window.open(this.href);return false;

Hope that helps

Brilliant tools, took me all of about 10 mins to change all 4 drop links on the MX5 with one of them.

[Mandrake]

I like your style Richard.

Did I mention I hack-sawed the bottom drop-link bolt off one of my drop-links by hand when I did the drop-links last year ? It sure would have been handy to have a grinder then...

http://www.amazon.co.uk/Dremel-Lithium- ... B00067AZV6" onclick="window.open(this.href);return false;

Hope that helps

Brilliant tools, took me all of about 10 mins to change all 4 drop links on the MX5 with one of them.

Out of stock and no price listed...

How much was it ? And how much grunt did it have ? I don't have a lot of success with battery powered power tools, my drill loves to give out on me just when I'm 9/10 of the way through a job...

[Mandrake]

In the words of Hanibal from the A-Team, "I love it when a plan comes together"

My back pressure adaptor kit arrived today in time for the weekend and it looks ideally suited to the job:





I won't be using the weld-in threaded bung and blanking bolt this time around, (I'll test through the oxygen sensor port if I can) but its nice to know I have it if I need to use it.

A new air filter arrived today too, the old one was certainly pretty dirty:



I'm not sure about the colour of the new one though...at least its hidden It doesn't seem to have made any noticeable difference in performance, but it was due to be replaced and needed to be ruled out anyway.

All I'm waiting on now is hydraulic pump o-rings, they've got some of them in but are still waiting for the 6 green ones which should be here tomorrow. Looking forward to some nice weather this weekend. AHEM!

[CitroJim]

Looking forward to some nice weather this weekend. AHEM!

Indeed we are for the X Rally

That filter looks very manky Simon...

The new one is a bit pink around the edges

Looking forward to news of the back pressure tests..

[lexi]

Fwiw guys a simple mercury vacuum gauge as used on my 61 Landy will show a blocked exhaust on the needle movement.

[Mandrake]

I've done some testing with a vacuum gauge already lexi, (way back somewhere in this monster thread ) but the results were inconclusive. What signs do you look for in the vacuum reading when the exhaust is blocked ?

Lower than normal vacuum at idle ? At higher RPM with no load ? At higher RPM under load ? Vacuum that steadily drops when the revs are held high ? Unusually quick vacuum drop when snapping the throttle open ?

Trouble is there is a lot of conflicting and contradictory advise out there on what vacuum symptoms a blocked exhaust will cause. I'm still confused after all the research I've done. Even then unless you have an identical motor to compare it to as a known good I don't think you can make a diagnosis based on the vacuum reading alone as the normal vacuum reading of different engines under a given load varies so much...

A source of information whose opinion I trust says he doesn't use or recommend vacuum as a diagnosis tool for blocked exhausts as he's not found it to be reliable - only a really badly blocked exhaust will cause an obvious problem with the vacuum reading under workshop (non driving) test conditions, and that many partial exhaust restrictions that are bad enough to affect the running of the engine, cause power loss and which show up clearly on a back pressure test don't cause an obvious problem with the vacuum reading.

I will say that I find the vacuum at idle to be lower than I expected - 15" - 17" Hg, rising to around 20" at 1000rpm, but without a known good reading to compare I can't tell if there is anything wrong with this. While back pressure at a known RPM is only ever going to be caused by an exhaust restriction, vacuum is also affected by tons of other stuff including valve sealing, intake manifold leaks etc etc...

So I really do think the back pressure test is the only unambiguous way to diagnose a restricted exhaust. There is some variation in the "acceptable" figure quoted by different sources but the one I'm going with is no more than 2 psi at a constant 3000rpm. All going well the question should be answered this weekend!

[lexi]

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;

[addo]

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.