Back to where I started...
- Thread starter hotfrog1
- Start date
It has been a real thorn in my side with this car, big dissapointment to say the least.
last run was about 9 miles, and hot enough to do an oil level check which was perfect, this engine does not use oil.
Today, just done 190 miles, be interesting to see what happens tomorrow morning.
Such a shame, the car is great otherwise.
thinking of having a full engine check done, leakdown etc to see if a bore is in trouble.
last run was about 9 miles, and hot enough to do an oil level check which was perfect, this engine does not use oil.
Today, just done 190 miles, be interesting to see what happens tomorrow morning.
Such a shame, the car is great otherwise.
thinking of having a full engine check done, leakdown etc to see if a bore is in trouble.
Boxster, gen 2 2009, 2.9, after a year of fitting my OPC supplied aos, the car is starting to white smoke at times on cold start.
as many will remember, I have been though the mill over this topic.
when the aos was new, the smoke went for months and about 4000 miles, car running perfectly.
over the last month is has crept back, sometime very light, sometimes more, but not yet huge.
last big smoke was after the car had sat level after a good hot run same place on the drive as usual.
The chance of another cold state smoke seems less the more frequently I use it, i.e. 3 day intervals rather than 7 days.
Do these aos units have a performance life, and if so it must be more than 12 months.
Thinking of replacing it yet again, it would be my 3rd in 25k miles.
Growingly, a Nissan 370 Z is looking good.
as many will remember, I have been though the mill over this topic.
when the aos was new, the smoke went for months and about 4000 miles, car running perfectly.
over the last month is has crept back, sometime very light, sometimes more, but not yet huge.
last big smoke was after the car had sat level after a good hot run same place on the drive as usual.
The chance of another cold state smoke seems less the more frequently I use it, i.e. 3 day intervals rather than 7 days.
Do these aos units have a performance life, and if so it must be more than 12 months.
Thinking of replacing it yet again, it would be my 3rd in 25k miles.
Growingly, a Nissan 370 Z is looking good.
Afraid not so...
Bought the car from PCT and had this issue then.
pct had it back for correction over a week.
Said they changed a temperature sensor.
Problem persisted, got no further, so over to me.
Took it to Wolverhampton OPC for a service action to the ecu as the cars history showed it had missed this action.
They did this for £125, a discounted fee and suggested an aos change too for a further £300.....
bought the aos aos with the intent to diy fit, but has the indie Zuffenhaus fit it ( good people) and issue went away for about 5 months, crept back.
bought another aos from the OPC and installed myself ( pig of a job).
this was about 12 months ago or less.
Thus I have no way to get compensation from OPC.
These engines should not do this, and I find it really irritating.
Maybe the other theory is true, oil residue leaking past one piton's rings into the bore and that is a whole new task.
Bought the car from PCT and had this issue then.
pct had it back for correction over a week.
Said they changed a temperature sensor.
Problem persisted, got no further, so over to me.
Took it to Wolverhampton OPC for a service action to the ecu as the cars history showed it had missed this action.
They did this for £125, a discounted fee and suggested an aos change too for a further £300.....
bought the aos aos with the intent to diy fit, but has the indie Zuffenhaus fit it ( good people) and issue went away for about 5 months, crept back.
bought another aos from the OPC and installed myself ( pig of a job).
this was about 12 months ago or less.
Thus I have no way to get compensation from OPC.
These engines should not do this, and I find it really irritating.
Maybe the other theory is true, oil residue leaking past one piton's rings into the bore and that is a whole new task.
Briggy
PCGB Member
I completely remember your previous issues with this.
I also get smoke on cold startup (987.2 Cayman S), but only in very specific conditions. Where I used to park for work, 10 miles from home, I used to blast to the bottom of the car park, sharp right, double back, then drive 50m to a parking spot. When I returned and started the car, I'd get smoke. No smoke in any other circumstances. Like yours, mine uses no oil whatsoever.
Do you only get this at home or any location you start from cold? If the former, I wonder if your journey home is similar to the journey to work that I described?
Cheers,
Bryan
I also get smoke on cold startup (987.2 Cayman S), but only in very specific conditions. Where I used to park for work, 10 miles from home, I used to blast to the bottom of the car park, sharp right, double back, then drive 50m to a parking spot. When I returned and started the car, I'd get smoke. No smoke in any other circumstances. Like yours, mine uses no oil whatsoever.
Do you only get this at home or any location you start from cold? If the former, I wonder if your journey home is similar to the journey to work that I described?
Cheers,
Bryan
It's random Bryan, mostly at home as the car is not my daily.
It can happen after a short or much longer run.
It can happen after a short or much longer run.
Totally accept the following may be way off target, and it's pure "lateral thinking" / guesswork - and I know you have been around the loop with this before.....
But.... White smoke is water not oil, so thinking condensation in exhaust or coolant ( worst case ) - perhaps you have a slight leak somewhere, making it prone to building up condensation? Back in the day, with bikes, silencers used to have drain holes in them - never heard of it in a car exhaust but just thinking out loud.....
Also - does it smell particularly "rich" or and excess fuel smell - have seen a stuck injector throw white smoke, but it was accompanied by fuel smell.
But.... White smoke is water not oil, so thinking condensation in exhaust or coolant ( worst case ) - perhaps you have a slight leak somewhere, making it prone to building up condensation? Back in the day, with bikes, silencers used to have drain holes in them - never heard of it in a car exhaust but just thinking out loud.....
Also - does it smell particularly "rich" or and excess fuel smell - have seen a stuck injector throw white smoke, but it was accompanied by fuel smell.
Very much appreciate everyone's thoughts.
The white smoke smells fuel/oil and can be like the vids on You Tube.
This morning, cold start after a long run to North Wales, started perfectly. No smoke, no bother.
You can 'sense' when it is going to smoke by the sound of the start, far more laboured, starts after 2 secs (normal) and after3 secs out it will belch.
Drove the car, running perfectly, for 2 mins and stopped by the beach, level ground.
Returned 1 hour later and started to get a small belch, but I mean small.
Drove home with several brief stops and perfect.
It IS using small amounts of coolant, cannot find a wet spot in the usual places (car still with it's original front cross tubes), no grunge under the oil or coolant caps.
Indulge me:
The AOS separates the oil from the air in the crankcase in a sealed manner, any opil mist is then taken to the air plenum./inlet manifold and during running gets burnt off, saving the planet. The pulsing air from the operation of the engine is 'taken' by the 70mm or so rubber diaphragm in the top of the AOS device and the larger oil globules are trapped by the small helical traps in the AOS housing which eventually run down by gravity to the crank case.
Now, how does the finer oil mist get to the inlet hose that runs from the AOS to the inlet? It is popular theory that the oil in the tube eventually soils the tube and the neat oil trickles to the inlet valve and during starting the oil is washed with injected petrol into the chamber and it fires but badly producing the smoke. It does not need much to produce a lot of smoke.
I'm missing something here in the operation of the passive AOS except for the panting bellows. The last AOS I removed I opened up (clips together on the Gen2 device) and the diaphragm was good, but there was oil on both sides of the diaphragm.
Beam me up Scottie from this issue. The engineer in me must find the answer!
The white smoke smells fuel/oil and can be like the vids on You Tube.
This morning, cold start after a long run to North Wales, started perfectly. No smoke, no bother.
You can 'sense' when it is going to smoke by the sound of the start, far more laboured, starts after 2 secs (normal) and after3 secs out it will belch.
Drove the car, running perfectly, for 2 mins and stopped by the beach, level ground.
Returned 1 hour later and started to get a small belch, but I mean small.
Drove home with several brief stops and perfect.
It IS using small amounts of coolant, cannot find a wet spot in the usual places (car still with it's original front cross tubes), no grunge under the oil or coolant caps.
Indulge me:
The AOS separates the oil from the air in the crankcase in a sealed manner, any opil mist is then taken to the air plenum./inlet manifold and during running gets burnt off, saving the planet. The pulsing air from the operation of the engine is 'taken' by the 70mm or so rubber diaphragm in the top of the AOS device and the larger oil globules are trapped by the small helical traps in the AOS housing which eventually run down by gravity to the crank case.
Now, how does the finer oil mist get to the inlet hose that runs from the AOS to the inlet? It is popular theory that the oil in the tube eventually soils the tube and the neat oil trickles to the inlet valve and during starting the oil is washed with injected petrol into the chamber and it fires but badly producing the smoke. It does not need much to produce a lot of smoke.
I'm missing something here in the operation of the passive AOS except for the panting bellows. The last AOS I removed I opened up (clips together on the Gen2 device) and the diaphragm was good, but there was oil on both sides of the diaphragm.
Beam me up Scottie from this issue. The engineer in me must find the answer!
Oh dear Graham! Back to square one?
Thankfully I've not been in the same boat with my 987.2 CS, and from your sectioned unit it's not clear to me how the AOS functions. Does the diaphragm draw the oil mist through the AOS for the oil then to separate out by gravity to drain back into the engine (crankcase or cam box?).
The only thing that occurred to me which may or not be relevant is that the Gen 2 3.4L engines are DFI, so unlike IDI engines there's no dilution of the oil in the intake duct which might possibly lead to problems ... unfortunately I noted that your car is the IDI 2.9L engine, so that theory was shot down..!
Unless you've been very unlucky I can't believe that you've got yet another AOS failure unless there's something amiss with your engine which accelerates the failure, but I've no idea what that could be. If you're unable to just live with it on the basis that the engine is running well and not using oil, short of stripping the engine I think the only option is to do compression and leakage tests and conduct a borescope examination to see if that throws-up any potential issues. The only problem with engine smoking is that potentially it's contaminating the cats, which isn't a good thing.
Jeff
Thankfully I've not been in the same boat with my 987.2 CS, and from your sectioned unit it's not clear to me how the AOS functions. Does the diaphragm draw the oil mist through the AOS for the oil then to separate out by gravity to drain back into the engine (crankcase or cam box?).
The only thing that occurred to me which may or not be relevant is that the Gen 2 3.4L engines are DFI, so unlike IDI engines there's no dilution of the oil in the intake duct which might possibly lead to problems ... unfortunately I noted that your car is the IDI 2.9L engine, so that theory was shot down..!
Unless you've been very unlucky I can't believe that you've got yet another AOS failure unless there's something amiss with your engine which accelerates the failure, but I've no idea what that could be. If you're unable to just live with it on the basis that the engine is running well and not using oil, short of stripping the engine I think the only option is to do compression and leakage tests and conduct a borescope examination to see if that throws-up any potential issues. The only problem with engine smoking is that potentially it's contaminating the cats, which isn't a good thing.
Jeff
There seems to be a crude cycle to this.
Put a new aos in. No smoke for ages/months then slowly it happens randomly and the frequency increases.
Good news I take from that is if the engine bores etc were in trouble, the smoke would be these quickly after fitting the aos.
The AOS has a pipe from the top of the diaphragm chamber but on the dry side of the rubber diaphragm, the wet side works as you say, heavy drops of oil mist 'condense' on the diaphragm etc and dribbles back to the engine sump.
The diaphragm has a weak spring so tensions the disc to one side and the hard plastic disc in the picture against the AOS body, a bit of a valve I guess.
The 10mm hose goes to the engine so is designed to carry air to the engine, and when contaminated, oil to the valve head.
The oil down this small tube must deliver oil all the time the engine is running, but at times over a long non-running period, enough oil at times gets to puddle at the valve head and hence smoke in profusion.
Question is, what makes oil get to the tube, and what is the purpose of the tube?
Put a new aos in. No smoke for ages/months then slowly it happens randomly and the frequency increases.
Good news I take from that is if the engine bores etc were in trouble, the smoke would be these quickly after fitting the aos.
The AOS has a pipe from the top of the diaphragm chamber but on the dry side of the rubber diaphragm, the wet side works as you say, heavy drops of oil mist 'condense' on the diaphragm etc and dribbles back to the engine sump.
The diaphragm has a weak spring so tensions the disc to one side and the hard plastic disc in the picture against the AOS body, a bit of a valve I guess.
The 10mm hose goes to the engine so is designed to carry air to the engine, and when contaminated, oil to the valve head.
The oil down this small tube must deliver oil all the time the engine is running, but at times over a long non-running period, enough oil at times gets to puddle at the valve head and hence smoke in profusion.
Question is, what makes oil get to the tube, and what is the purpose of the tube?
Graham,
I agree with your comment that if there's bore scoring [or maybe a leaky valve stem?] you'd expect smoking even after AOS replacement. The fact that you're getting a temporary respite seems to indicate that the bores, etc. are OK.
There doesn't appear to be any info about the AOS operation online other than how to change it, but at least this link gives an indication of where the various connections go, assuming your AOS is the same as that in Pic1:
https://www.pelicanparts.com/techarticles/Boxster_Tech/09-ENGINE-Air_Oil_Separator/09-ENGINE-Air_Oil_Separator.htm
From Pic2 it seems that the tube connected to the diaphragm leads to the intake. Pic3 says that the smaller tube at the base of the unit is the quick-disconnect vacuum line that leads to the opposite [left side?] camshaft housing while the other flexible hose connects the separator to the right side camshaft housing. I assume that the pipe at the bottom of the AOS with the brown multiple O-ring seal fits in the crankcase?
It's still not clear to me exactly how oil mist and fluid oil flow in the arrangement but surely the only way for oil to get into the top tube is if the diaphragm is perforated? If you remove the top tube and find oil in there I presume that will tell you that the diaphragm is leaking and the AOS needs replacing once again, but as to what mechanism is causing premature diaphragm failure is a mystery to me.
One thing which is suggested and which you could try as a temporary measure is to remove the top tube at the AOS and blank it off to see if you still get the smoking, but you'd need to have a continuously smoking condition to check that out and I suppose that it would only confirm what you know already.
Jeff
I agree with your comment that if there's bore scoring [or maybe a leaky valve stem?] you'd expect smoking even after AOS replacement. The fact that you're getting a temporary respite seems to indicate that the bores, etc. are OK.
There doesn't appear to be any info about the AOS operation online other than how to change it, but at least this link gives an indication of where the various connections go, assuming your AOS is the same as that in Pic1:
https://www.pelicanparts.com/techarticles/Boxster_Tech/09-ENGINE-Air_Oil_Separator/09-ENGINE-Air_Oil_Separator.htm
From Pic2 it seems that the tube connected to the diaphragm leads to the intake. Pic3 says that the smaller tube at the base of the unit is the quick-disconnect vacuum line that leads to the opposite [left side?] camshaft housing while the other flexible hose connects the separator to the right side camshaft housing. I assume that the pipe at the bottom of the AOS with the brown multiple O-ring seal fits in the crankcase?
It's still not clear to me exactly how oil mist and fluid oil flow in the arrangement but surely the only way for oil to get into the top tube is if the diaphragm is perforated? If you remove the top tube and find oil in there I presume that will tell you that the diaphragm is leaking and the AOS needs replacing once again, but as to what mechanism is causing premature diaphragm failure is a mystery to me.
One thing which is suggested and which you could try as a temporary measure is to remove the top tube at the AOS and blank it off to see if you still get the smoking, but you'd need to have a continuously smoking condition to check that out and I suppose that it would only confirm what you know already.
Jeff
Thank you Jeff, you are referring to the different 986/987 gen 1 AOS, the 987 gen 2 is simpler and a lot easier to fit.
This design was Porsche's fix to the 986 AOS device.
There is indeed no info on how the Gen2 device works, and I think the diaphragm has to split on either design for oil to be flooding the inlet system.
On the Gen2 design, the diaphragm is simply clamped/trapped between the mouldings by the assembly clips so some leakage could occur.
However, you make a good point; get to the AOS and detach the top pipe and see if it is wet with oil, and if so the AOS has failed.
I'll do that first.
This design was Porsche's fix to the 986 AOS device.
There is indeed no info on how the Gen2 device works, and I think the diaphragm has to split on either design for oil to be flooding the inlet system.
On the Gen2 design, the diaphragm is simply clamped/trapped between the mouldings by the assembly clips so some leakage could occur.
However, you make a good point; get to the AOS and detach the top pipe and see if it is wet with oil, and if so the AOS has failed.
I'll do that first.
Alistair,
That may be the case, but just to point out that the problem relates to a 987.2 car with a different, extensively updated engine design compared with the 986. The problem went away for a period after the AOS was replaced which, as Graham has pointed out, suggests that bore scoring or leaking valve guides aren't the issue, nor I suspect is a cracked head. Apparently the smoke has a fuel/oil smell rather than being water-based, which would also discount the cracked head theory.
Jeff
That may be the case, but just to point out that the problem relates to a 987.2 car with a different, extensively updated engine design compared with the 986. The problem went away for a period after the AOS was replaced which, as Graham has pointed out, suggests that bore scoring or leaking valve guides aren't the issue, nor I suspect is a cracked head. Apparently the smoke has a fuel/oil smell rather than being water-based, which would also discount the cracked head theory.
Jeff
I have no experience of your problem but I am intrigued and have been reading the trail. I too would like to understand how the AOS works and found this link with some similarities to your issue. Wonder if you have seen it? Well down the page there is an entry by Porschetech3 about spark plug tubes. Probably a red herring but the link is an interesting read all the way through. I have owned my 2001 S from new. It has done 65K miles and the AOS is original and untouched. I rarely do any short journeys and go out my way to make sure that the car is fully warmed before putting it away. I hope that you get a resolution soon.
https://www.6speedonline.com/forums/996/399650-2001-996-aos-bad-again-already.html
https://www.6speedonline.com/forums/996/399650-2001-996-aos-bad-again-already.html
Thank you for that link, thought I had read EVERYTHING on this topic.
It is about the early type of AOS I think, the Gen 2 AOS is a simpler installation (thank goodness), but interesting to read.
This is my take on the device:
The AOS simply gets the oil droplets to the internal surface of the device and as they accumulate, they dribble down to the sump and this cycle happens every time the engine runs, for a short or for a long time, cold or warm.
On cold start, the engine is given a rich mix of fuel/air to fire and after some time eases back to normal mix as engine temp rises to 180C ish.
That alone on a cold day will show with some very light smoke, but not like this issue. All petrol engine have this to some extent.
The AOS has the rubber diaphragm to allow the pulses of the engine to be accommodated in this sealed system and it is dampened by a light spring. These pulses come thick and fast depending on engine speed. The oil droplets are on the wet side hence the accumulated oil dribbling down to the sump.
On the dry side of the diaphragm is the small hose at the top of the AOS that leads to the inlet. As the diaphragm pulses, so does the air in the tube.
If the dry side become wet with oil the oil will get eventually into the pipe and then to the inlet, then to the inlet valve(s) and will accumulate say overnight and on start-up the fuel washes the oil to the chamber and the smoke is produced. More oil enters, more smoke is produced.
If the diaphragm is not broken (the usual failure mode is it tears) then oil can only get to the dry side (small pipe) past the periphery of the diaphragm disc to the 2 halves of the AOS body.
Whew, in danger of over-thinking this lot.
I need to check the small pipe to see if it is wet.
Wife get annoyed when I try to fix this, a problem that could be simply ignored, but the car should not do this!
To add to this if anyone is reading anymore, the car has never since I've had it tick over smoothly, esp when hot. It's got a full service history, plugs changes to schedule etc but runs fabulously when moving from 1000 rpm, amazingly flexible.
My 140K miles 911 aircooled 3.2 hums like a sewing machine.....
It is about the early type of AOS I think, the Gen 2 AOS is a simpler installation (thank goodness), but interesting to read.
This is my take on the device:
The AOS simply gets the oil droplets to the internal surface of the device and as they accumulate, they dribble down to the sump and this cycle happens every time the engine runs, for a short or for a long time, cold or warm.
On cold start, the engine is given a rich mix of fuel/air to fire and after some time eases back to normal mix as engine temp rises to 180C ish.
That alone on a cold day will show with some very light smoke, but not like this issue. All petrol engine have this to some extent.
The AOS has the rubber diaphragm to allow the pulses of the engine to be accommodated in this sealed system and it is dampened by a light spring. These pulses come thick and fast depending on engine speed. The oil droplets are on the wet side hence the accumulated oil dribbling down to the sump.
On the dry side of the diaphragm is the small hose at the top of the AOS that leads to the inlet. As the diaphragm pulses, so does the air in the tube.
If the dry side become wet with oil the oil will get eventually into the pipe and then to the inlet, then to the inlet valve(s) and will accumulate say overnight and on start-up the fuel washes the oil to the chamber and the smoke is produced. More oil enters, more smoke is produced.
If the diaphragm is not broken (the usual failure mode is it tears) then oil can only get to the dry side (small pipe) past the periphery of the diaphragm disc to the 2 halves of the AOS body.
Whew, in danger of over-thinking this lot.
I need to check the small pipe to see if it is wet.
Wife get annoyed when I try to fix this, a problem that could be simply ignored, but the car should not do this!
To add to this if anyone is reading anymore, the car has never since I've had it tick over smoothly, esp when hot. It's got a full service history, plugs changes to schedule etc but runs fabulously when moving from 1000 rpm, amazingly flexible.
My 140K miles 911 aircooled 3.2 hums like a sewing machine.....
Thanks for all that as it is very interesting and is a new subject for me. I have been tinkering with cars all ,my life and the satisfaction of fixing a difficult issue that the dealer failed on, can be immense. Of course time is on my side so it is an unfair contest.
Regarding the idling, the Boxster should idle like a sewing machine. My car has a very slight deviation if say I stop from motorway speed at a traffic jam. Then idling is little bit lumpy but after a couple of minutes it is smooth again. I am going to have to investigate this but it may be that the long term fuel trim gets set for 70mph and needs to adjust to deal with the sudden idling. I put an OBD2 scanner on it and found that idling at home, bank 2 LTFT was steady at -2.3% indicating a rich mixture correction on one side. Bank 1 LTFT was zero. Could be a leaky injector but not sure. I think that your idling issue may be down to a mixture deviation, possibly. Looking at the LTFT should give you an insight about what corrections the electronics are making. If you take the oil filler cap off whilst idling, thereby partially bypassing the MAF, you can see the response in the fuel trims. Taking the oil cap off should also exercise the AOS as it will have to cope with extra air flow. Please note that I am not a car pro. and never have been.
I have a pdf Porsche training manual called "Advanced Engine Management Systems". It is not a repair manual but an extremely detailed and readable description of the systems involved. If you send your email to dene99@gmail.com I will forward it to you.
Regarding the idling, the Boxster should idle like a sewing machine. My car has a very slight deviation if say I stop from motorway speed at a traffic jam. Then idling is little bit lumpy but after a couple of minutes it is smooth again. I am going to have to investigate this but it may be that the long term fuel trim gets set for 70mph and needs to adjust to deal with the sudden idling. I put an OBD2 scanner on it and found that idling at home, bank 2 LTFT was steady at -2.3% indicating a rich mixture correction on one side. Bank 1 LTFT was zero. Could be a leaky injector but not sure. I think that your idling issue may be down to a mixture deviation, possibly. Looking at the LTFT should give you an insight about what corrections the electronics are making. If you take the oil filler cap off whilst idling, thereby partially bypassing the MAF, you can see the response in the fuel trims. Taking the oil cap off should also exercise the AOS as it will have to cope with extra air flow. Please note that I am not a car pro. and never have been.
I have a pdf Porsche training manual called "Advanced Engine Management Systems". It is not a repair manual but an extremely detailed and readable description of the systems involved. If you send your email to dene99@gmail.com I will forward it to you.
Graham,
In this long-running saga I'd either missed or forgotten the rough idle issue you describe, which may or may not be relevant to the smoking issue. I'm sure you're aware that usually it's ignition related - plugs, coil packs, crank sensor - but it could also be a problem injector, a vacuum leak or perhaps a MAF sensor issue, and I'm assuming that diagnostics hasn't thrown-up any issues around the Variocam Plus system.
All I can suggest is that when you have the engine cover off to look for the presence of oil in the AOS vacuum tube you check for vacuum leaks and MAF sensor contamination (I know that the sensor is upstream of the AOS vacuum tube, but I suppose 'blow-back' is a remote possibility). I think I'm correct in saying that the engine will run in a default F/A ratio mode if you disconnect the MAF, and if that clears the rough idle it would indicate a problem there. I don't know what fuel you use but it might be worth trying a fuel additive at the next fill-up to clean out the fuel system, the injectors included, to see if that helps.
I hope that you make some progress. Like you the dogged engineer in me wouldn't be happy until I'd at least found out what's causing the problem(s) even if I couldn't do much about it without stripping-down the engine, which is somewhere you don't want or need to go.
Jeff
In this long-running saga I'd either missed or forgotten the rough idle issue you describe, which may or may not be relevant to the smoking issue. I'm sure you're aware that usually it's ignition related - plugs, coil packs, crank sensor - but it could also be a problem injector, a vacuum leak or perhaps a MAF sensor issue, and I'm assuming that diagnostics hasn't thrown-up any issues around the Variocam Plus system.
All I can suggest is that when you have the engine cover off to look for the presence of oil in the AOS vacuum tube you check for vacuum leaks and MAF sensor contamination (I know that the sensor is upstream of the AOS vacuum tube, but I suppose 'blow-back' is a remote possibility). I think I'm correct in saying that the engine will run in a default F/A ratio mode if you disconnect the MAF, and if that clears the rough idle it would indicate a problem there. I don't know what fuel you use but it might be worth trying a fuel additive at the next fill-up to clean out the fuel system, the injectors included, to see if that helps.
I hope that you make some progress. Like you the dogged engineer in me wouldn't be happy until I'd at least found out what's causing the problem(s) even if I couldn't do much about it without stripping-down the engine, which is somewhere you don't want or need to go.
Jeff
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