Gen 2 not intended as a scare story and is just my Car

GREAT that you have your Porsche back Phil, I look forward to reading of your increased enjoyment of it..

Call me dumb, even very dumb...... but I am still struggling to understand how elements of a damaged cat ended up inside the engine... Any idea as to the route of entry...?

Have read of negative exhaust pressure perhaps playing a part..? However I wonder when that might occur given there are two other cylinders feeding into the same exhaust pipe..? Sure firing order will play a part.. however surely when the exhaust valve is opened there are hot gasses flowing out pressurised by the piston rising..? I guess if the valve timing is off or the cam has a degree of overlap... BUT... I am still struggling to understand the entry point of the detritus..?

Any ideas folks.. :?:

I don't understand the exact mechanism of it, but it is certainly the case that this does happen. A quick search on the internet will reveal cases of this happening on all sorts of engines. I know of at least one other person on here who had it happen on their GT car after catalyst issues.

MC

I'm with MC here .. i don't fully understand the mechanics of it but what i do know ..

The cat is a solid object , any parts that break off in the Header side will stay there .. depending on driving conditions then on over run there could be a negative pressure which can draw material back into a cylinder via the valves ..

The inlet .. or airflow into the engine is almost shut off in these conditions .

modern cars often switch off the injectors on over run so there is no compression cycle and there for much lower pressure from the cylinders / pistons .

people often fit aftermarket exhaust systems in the hope of a higher BHP gain .. this upsets the dme map and can actually cause a lowering of the power .. a dme remap is required to compensate .

i can in a sense understand the mechanics of this and realise its possible .. but getting my head around it is challenging .. not something we are taught im afraid .

Link here might be of interest ..

https://www.dieselnet.com/tech/diesel_exh_pres.php

Very many thanks for posting the link Iain. I have little to no experience of working with modern engines or knowledge of their control stratedgy.

The text in the link is even more verbose than I, and seems not to be aimed at those seeking a basic practical understanding, though perhaps more suited to those who may be seeking to create an answer via a formulaic process.... which is outside my comfort zone... (-:

I picked up from the text that primarily modern exhaust systems have greater restrictions to exhaust gas flow built into them in order to comply with EU vehicle construction and use regulations. I guess the statement that was an eye opener for me was that in some circumstances during the exhaust cycle a cylinder may not fully evacuate all the exhaust gasses created at the end of the ignition/power stroke, due to those exhaust system restrictions...!

The mention of turbo`s loosing efficiency due to those same restrictions seems entirely logical given the speed of the turbo is determined by the rate of the flow of the exhaust gasses driving it`s vanes, and logically directly affecting the boost available.

I note that a turbo diesel is mentioned and wonder if the author has ever been near a BIG diesel engine running out of control when it`s turbo oil seal fails leaking the engines lubricating oil into the intake which is then burned as fuel thus it`s fuelling is beyond control.... not unusual for engines to self destruct once they have burned the majority of their lubricant..Ouch..! In that situation formula may be less handy than a pair of overalls jammed into the intake to choke it to death before it starts throwing bits of rods etc about the place..(-:

Sorry I digressed..

I can understand the logic of the situation where.... if the restriction in the exhaust is considerably increased perhaps by a cat breaking up and partially blocking the passage of gasses trying to exit the system, and that the number of cylinders feeding into that cat will be increasing the "back-pressure" as the engine continues to run...?

Using nothing other than basic mechanical logic.... IF that pressure could ever be higher at any time during the exhaust cycle of any one cylinder feeding into that exhaust system, say at the end of that cylinders exhaust stroke, where it`s exhaust valve would be opened to some extent, in that event I guess there could be bits of cat circulating in the disturbed gas flow within the exhaust system, and that exhaust system gas being at a higher pressure than within the cylinder with the not yet closed exhaust valve, then it seems possible if not probable that some of the bits of cat will end up being forced into that cylinder..????

As the over-run is mentioned as playing a part, I guess the volume of air in the cylinder will have been reduced due to the air intake (throttle plate) being closed, creating a vacuum in the inlet manifold the fuel being shut off also will likely reduce the volume of air in the cylinder waiting to be expelled during the exhaust stroke, perhaps providing more risk of the pressure in the exhaust system exceeding that produced in the cylinder near the end of it`s exhaust stroke...???

Of course my logic could be entirely flawed, and I am more than happy to be corrected...

Just thinking in type..

Thanks again Iain. and MisterCorn.. :thumb:

Luddite said:

Very many thanks for posting the link Iain. I have little to no experience of working with modern engines or knowledge of their control stratedgy.

The text in the link is even more verbose than I, and seems not to be aimed at those seeking a basic practical understanding, though perhaps more suited to those who may be seeking to create an answer via a formulaic process.... which is outside my comfort zone... (-:

I picked up from the text that primarily modern exhaust systems have greater restrictions to exhaust gas flow built into them in order to comply with EU vehicle construction and use regulations. I guess the statement that was an eye opener for me was that in some circumstances during the exhaust cycle a cylinder may not fully evacuate all the exhaust gasses created at the end of the ignition/power stroke, due to those exhaust system restrictions...!

The mention of turbo`s loosing efficiency due to those same restrictions seems entirely logical given the speed of the turbo is determined by the rate of the flow of the exhaust gasses driving it`s vanes, and logically directly affecting the boost available.

I note that a turbo diesel is mentioned and wonder if the author has ever been near a BIG diesel engine running out of control when it`s turbo oil seal fails leaking the engines lubricating oil into the intake which is then burned as fuel thus it`s fuelling is beyond control.... not unusual for engines to self destruct once they have burned the majority of their lubricant..Ouch..! In that situation formula may be less handy than a pair of overalls jammed into the intake to choke it to death before it starts throwing bits of rods etc about the place..(-:

Sorry I digressed..

I can understand the logic of the situation where.... if the restriction in the exhaust is considerably increased perhaps by a cat breaking up and partially blocking the passage of gasses trying to exit the system, and that the number of cylinders feeding into that cat will be increasing the "back-pressure" as the engine continues to run...?

Using nothing other than basic mechanical logic.... IF that pressure could ever be higher at any time during the exhaust cycle of any one cylinder feeding into that exhaust system, say at the end of that cylinders exhaust stroke, where it`s exhaust valve would be opened to some extent, in that event I guess there could be bits of cat circulating in the disturbed gas flow within the exhaust system, and that exhaust system gas being at a higher pressure than within the cylinder with the not yet closed exhaust valve, then it seems possible if not probable that some of the bits of cat will end up being forced into that cylinder..????

As the over-run is mentioned as playing a part, I guess the volume of air in the cylinder will have been reduced due to the air intake (throttle plate) being closed, creating a vacuum in the inlet manifold the fuel being shut off also will likely reduce the volume of air in the cylinder waiting to be expelled during the exhaust stroke, perhaps providing more risk of the pressure in the exhaust system exceeding that produced in the cylinder near the end of it`s exhaust stroke...???

Of course my logic could be entirely flawed, and I am more than happy to be corrected...

Just thinking in type..

Thanks again Iain. and MisterCorn.. :thumb:

Click to expand...

I note that a turbo diesel is mentioned and wonder if the author has ever been near a BIG diesel engine running out of control when it`s turbo oil seal fails leaking the engines lubricating oil into the intake which is then burned as fuel thus it`s fuelling is beyond control.... not unusual for engines to self destruct once they have burned the majority of their lubricant..Ouch..! In that situation formula may be less handy than a pair of overalls jammed into the intake to choke it to death before it starts throwing bits of rods etc about the place..(-:

This is a frightening scene , I have seen it a number of times over my working life ... and its not a location to be in when it has any chance of happening .... Chalwin valves and other types of exhaust dampers were incorporated for this very issue ! (and they have to work properly of course ! )

Great read this thread ...... good coffee break stuff !

T

When trying to answer various unusual issues on forums - I do not usually start out knowing the answers nor having previously understood the causes and often I have to stop and try and think out the process and what could be involved.


I understood there was back pressure in the exhaust on the over-run but never really stopped to think out why - however with the throttle closed the inlet valve still opens so the inlet stroke "sucks" at a blocked inlet pipe creating a vacuum (and you can feel this as engine braking). OK the piston then goes up (on the compression stroke although there will be nothing inside the cylinder to compress) and when the pistons falls on what would normally be the power stroke the vacuum (or some of it) would return.


Even if the fuel injectors still delivered a tiny amount of fuel - I don't think there would be any significant expansion of gas on the power stroke because fuel will not burn quickly enough if it is not sufficiently compressed.


I guess that some vacuum will still be present when the exhaust valves open creating a back pressure in the exhaust and drawing air towards the cylinder to fill the vacuum from there and with it any tiny slivers of metal (that form the Cat matrix) that have broken off can flow towards the engine.


The evidence was perfectly clear that this is what happened in this case to Phil's engine as the internals were full of the material that was also missing from the cat.

I had not seen it before and I must admit it took a bit of finding. The first evidence was the small slivers of metal in the engine inlet and cylinders. The next problem was to establish where they came from.

After exhausting all the inlet system we put historical knowledge that there can be a vacuum on the over-run to look at the cats and there the explanation was staring us in the face - cause solved.

I think that all makes sense (hope so anyway).


Baz

As ever Baz your input is highly valued by folk like me seeking to improve our understanding, and you have a better way with words than I .. (-:

I type as I think and that can be a bit confusing for me let alone others trying to make sense of it...(-:

Undoubtedly inlet manifold depression (vacuum) is increased on the over-run for the reasons stated and logically (?) that will extend into the cylinder of the bore creating the vacuum. though when the inlet valve closes and the piston begins to compress whatever volume of air is in the cylinder, again it seems logical (?) that the vacuum will be overcome by the rising compression within the cylinder to become a positive force...

With little or no fuel present due to the ECU shutting down the electrical supply to the injector on over-run, there will be little or no explosive force applied to the piston when the spark plug fires... only assuming that the plug still fires and that the ECU does not also seek to conserve electrical energy by shutting that off too..?

Which takes us on the the inevitable exhaust stroke where the exhaust valve begins to open as the piston rises within the bore, again creating a positive airflow in the bore which will increase as the piston rises (?)

The problem area would seem to be that the pressure in the cylinder that is involved in it`s exhaust process may be exposed to a greater pressure than it is creating at some stage of it`s exhaust stroke, and perhaps nearer the end of that stroke (?)

Regardless, it seems with more than one cylinder feeding into the exhaust system that may be partially restricted by a self destructing cat, the exhaust system will be acting a bit like the air receiver in a compressor.. sure the exhaust system will be passing air but not free flowing enough to pass it all quickly enough to avoid the build up of what is being termed as termed as "back-pressure" within the system...??

If this seems a feasible assessment then it seems logical (?) that cat detritus could be forced into the cylinder bore by the higher pressure in the exhaust system than exists in the cylinder attempting it`s exhaust stroke...?

As ever just thinking in type.. and more than happy to be corrected..

2 most likely idiotic ideas come to mind,
1. a backfire causing a particularly large pressure wave
2. I have seen before very poor use of high pressure power washers on exhausts that have caused water in the bores, and could alos be responsible for pushing material from a flaking cat cell back into a bore if it had stopped with the valve open

feel free to rip them apart

Heartsdriver.... I suspect the idiotic questions are the ones we may be too fearful to ask.

Backfire seems unlikely to occur in modern EFI fed engines given I think it is caused by unburned fuel igniting within the exhaust system and from that which I have read here, fuel is automatically shut off in an over-run situation...Apparently over-run is sited as playing a part in the reconstruction of the failure process..? However, who is to say that such systems do not fail in a variety of ways... and yes it seems entirely logical (?) that a backfire would increase the pressure within the exhaust, and how that might effect a cat or worse still a an already self destructing cat.... who knows... Though don`t stand behind the exh pipe outlet lest you become impregnated with cat particles... (-:

As for pressure washing the exhaust system internals and managing to get water and cat parts into a cylinder bore... that would take some doing, but then I have come across some crazy causes of faults when trying to understand "how could THIS happen"... Every day can be a school day.. :?:

I know a lot of modern turbo engine VAG cars are specifically mapped to 'pop and crackle' on the over run, but Im not aware of any non turbos that could (or would want) to achieve this. Its particularly annoying when the local chavs have all got Golf R's with this rather noisy 'mod' and the dsg gear change that seems to go hand in hand with it.... sounds like a machine gun...
If the previous owner of the car has played around with mapping then its possible the failsafes were removed enabling a large backfire in certain circumstances

Hertsdriver said:

I know a lot of modern turbo engine VAG cars are specifically mapped to 'pop and crackle' on the over run, but Im not aware of any non turbos that could (or would want) to achieve this. Its particularly annoying when the local chavs have all got Golf R's with this rather noisy 'mod' and the dsg gear change that seems to go hand in hand with it.... sounds like a machine gun...
If the previous owner of the car has played around with mapping then its possible the failsafes were removed enabling a large backfire in certain circumstances

Click to expand...

There is a lot of fake trickery that goes on with NA cars too... offenders being the 981GTS for example;
https://www.youtube.com/watch?v=2F30Ij3oyAE
Jaguar F Type too, though that is supercharged rather than turbo charged.

I think you missed an important stroke (Luddite) because after the compression stroke (when the minute amount of air in the cylinder rises slightly in pressure) there is the power stroke when it expands again to a much lower pressure.

Bearing in mind that long before 45 degrees of fall from TDC the piston is usually falling faster than the burning fuel can keep up with - under normal full throttle driving - it seems logical to me that with hardly any air or fuel and no fast burn at such pressures - the falling piston on the original power stroke would result in a lower than atmospheric pressure in the cylinder at BDC and when the exhaust valve opens. Remeber also that in over-run situations the engine is usually at high revs as you change down to lower gears.


The amount of Cat debris was extremely small and no where near enough to have any affect on exhaust flow - but it doesn't take many small pieces of thin metal to jam a piston against a cylinder wall.

It also always amazes me when we strip an engine that has say dropped a valve head only to find the bits in all the cylinders and the opposte side inlet system as well - so there is clearly a lot of pressure wave positive and negative pressures going on in there to move often quite heavy bits in the opposite direction to that which makes any sense under normal considerations.

Anyway - that was what was in the engine and the only way it could get there that I can work out is with a reverse pressure wave.

Baz

Lets just say .. :agree:

I have a mental image in my head of what's going on .. but putting it into words is i feel .. beyond me .

i had a job once .. a bad one for me ..

997 GT3RS .. misfire ..

Found a plug with a missing electrode .. plugs had recently been changed .

boss told me to drop the engine and inspect for the missing part ..

i wanted to just replace the plug as it was only a misfire but did as told .

Engine dropped , fitted to a stand , head removed on that side to inspect the pistons / bores .. manifold also removed to check if the part was in there .

Evidence of slight damage ( marks in the piston ) but no sign of the electrode .

refitted head and engine , started up the car and a horrendous rattle .. rather scared i turned it off .. then started it again .. same thing ..

Lets just say at this point the workshop falls silent .. they all know something bad is happening .. it's a lonely place in times like this as everyone avoids you .

Convinced i had done something wrong i dropped the engine again and removed the same head .. i could find nothing wrong .

i refitted the engine .. again and the same thing happened ..

Kinda crapping myself at this point .. it's a £20k engine after all i dropped it again .

On turning over the engine by hand i could feel a tight spot ... something very wrong .

We Had NO borescope at that point .. important point to make in hindsight .

I took a gamble and removed the other head as i could find nothing with what i had done so far .... on removal there was a small object that fell out ...

It was the plug electrode !!!

What had happened ..

The electrode had gone back into the inlet manifold .. the first time i tilted the engine on the stand to remove the head it dropped into the other bank .

Lets just say 2 weeks of hell thinking not only was i out of a job but going to be sued by the garage .



lesson learned ..... parts in an engine or gases or anything can and will work its way into other cyls ..

I've seen it on mercs as well but that's another story .

Thanks for trying to advance my understanding guys. Heartsdriver, I have heard Evo`s and the like popping and banging which I doubt was pat of their initial design, but then as one who drives an old carb fed pre cat sports car that has straight through twin exhausts which roar a bit if I stand on the loud pedal.. who am I to be critical, and yes it has a map though normally kept in the door pocket...(-:

Robertb, thanks for the snap crackle and pop link.... (-:

Baz thanks for spending some of your valuable time to share your specialist knowledge.... I think I understand that during the normal power stroke given there will be little or no fuel present when in a state of over-run, and instead of the normal explosive forces being present and driving the piston downward.... the forces present in the cylinder are being created by forward momentum of the car ... this being the case, the force being generated within the cylinder at that time is likely to be vacuum... and dependant on the exhaust valve timing relative to distance past BDC with the piston rising, and when the exh valve begins to open, there may be less pressure in the cylinder than exists within the exhaust system for a very short period of time...?

deMort, thanks to you for sharing that story, at least you worked out how and why the errant electrode came to be other than where expected... a very uncomfortable situation until you resolved it... Had a few of those in my time..

I find this all very interesting and educational and prefer if possible to try to understand how and why machinery fails as opposed to defering to myth legend or any other beliefs...

Going down memory lane I recall that a single cylinder 4 stroke motorcycle engine with a hand held near the single exhaust pipe - will - (when the engine is revved up and down with the throttle first open and then closed) - suck the hand onto and off the pipe on the over-run. I think this is where I got that practical knowledge from in a totally tangible way. It can also do this with some multi cylinder car engines.

The pressure of unburnt air in an engine is largely proportional to the change in volume as the piston rises and falls - so forgetting valve overlap just for a moment - if the inlet is closed - on the inlet stroke the cylinder experiences the opposite of the compression ratio as a prressure drop (say 11/1) and this means that IF it started out with atmospheric pressure in the inlet before the throttle was closed (say 14.7 psi) - then it would pull a pressure drop to around 1.3 psi absolute (13.4 psi below atmospheric pressure).

With the inlet valve closed and the piston on the compression stroke it would return the pressure at TDC to atmospheric around which point EVEN IF both fuel and sparks were still present - the fuel present would be much too rich for the volume of air present and the compression ratio so low that hardly any burn pressure rise would result, it would be too slow to react as quickly as the cylinder is moving (and you might smell the unburnt fuel at the exhaust).


When the piston falls to BDC (on what should be the power stroke) if the 11/1 volume increase exceeds the pressure rise at TDC after this weak burn - the result will still be lower than atmospheric pressure (which we call a vacuum).

When the exhaust valve then opens - this lower pressure than atmospheric in the cylinder at BDC will result in a back pressure wave from the exhaust towards the cylinder - and remember this description is what would happen IF the inlet was at atmospheric when the inlet stroke started. But after several of these cycles, because no more air is going into the inlet manifold and due to that overlap of exhaust and inlet valves - the vacuum developed in the exhaust side will gradually over a few cycles (or revs) create a lower than atmospheric pressure in the inlet side as well so now when the cycle repeats itself the vacuum is even stronger on each stroke in both the inlet and exhaust.


We must never forget that a "Vacuum" or "suck" is not a real thing (just a convention we use to describe a pressure lower than atmospheric) - all there ever is - is a pressure difference between one area and another and that air/gas/vapour etc will simply flow from the higher to the lower pressure. It always amazes me therefore that enough air can travel into the engine from a pressure drop that can never be more than 14.7 psi in 0.0085 (8.5 thousandths) of a second (@7000 rpm) to fill the cylinder with at (or these days often just over) 100% volumetric efficiency (unless it is force charged) - amazing what resonances and charge effects can do to maximise that cylinder filling.


D-Mort - lovely story - but can I just add that because we often see a new customer for the first time when we are asked to rebuild their racing engine that has failed (for various reasons - they are at their limit when raced), without rotating the engine at all from dropping it - we often find pieces of metal that eminated from one side of the engine - in the opposite cylinders, inlets or even exhausts, furthermore it would be quite diffiuclt to rotate the engine so that the electrode fell back up past the valve, over the inlet system and back down into the other side - just when all the valves happen to be open so it ends up in the opther cylinder.

In our experience we would expect to find it on the other side as a result of these pressure pulses and just like you - years ago - would have spent time looking for debris on the failed side first but now always look in both sides (just as you will now) - although it still amazes me that such pulse strengths are there to move various bits and pieces from one side to the other.

This is actually quite a problem when we just repair a cylinder block for other specialists to subsequently rebuild the engine - or rebuild the engine for them to fit all the auxiliaries onto and get the car running again - because they often just lift off the inlet system and replace it whole without appreciating that debris from the damaged side has gone back up into it and often over to the opposite side.

As a result there have been a couple of previous occasions when the finished car has had a knocking noise which they naturally deny responsibility for and to resolve that disagreement the car comes back to us when we take the inlets off to find them absolutely full of debris and then both heads off to find a piece of metal stuck on the top of the piston and hitting the head at TDC. Despite making this point as clear as possible to them - many ignore it (because they make more money just re-fitting the manifolds as a whole) and we have been left to work on a car for nothing that we have done wrong - although we do sympathise because we too found it hard to belive such movements were possible until we found out the hard way for ourselves.


Since those experiences years ago now - when we have the whole car to rebuild - the whole inlet system is stripped right down, cleaned and inspected before the rebuild. It all adds a lot of time and cost but results in the reliability we are well known for - experience counts.

In fact similarly - I was on holiday abroad when informed that there was a missfire on an engine that we had built but a well known and respected specialist had re-fitted - again (as usual) it couldn't be their fault and the whole car was delivered to us. But because I had found a particular error before (that an injector plug can also fit another component in the engine management system) - i just advised on the phone that they will have swapped 2 plugs on the loom and to swap them back.

But you often have to experience things for yourself to understand them and who is going to believe a semi- retired boss who is sat in the Sun drinking Sangria? and so some time was spent looking at other possibilities before I received a lovely text saying "you've still got it - you old Bas**d - 2 plugs misplaced just as you said". Made me feel great in my dotage - but seriously - it doesn't matter who you are or how much you have done - there are always new things to learn and the people that learn the most (and eventually become the best engineers) are those (like De-Mort) who don't mind admitting it - because it shows they are honest, dedicated and willing to learn and pass on their knowledge rather than feel the need to try and wriggle out of things to protect their reputation.


So - a warning also for everyone to remember that there are various bits and pieces of carbon (and possibly other debris that highly educated and technical qualified people refer to at a very high level as "sh*t") and that changing down with engine braking on the over-run can draw it into the cylinders with resulting cylinder bore damage and with any engine failure bits of material can get anywhere in the engine and that everything needs stripping and inspecting on rebuild.


Baz