Theroy Secondary Air injector cloging
08-08-2013, 06:21 PM
#1
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Theroy Secondary Air injector cloging
It is a well discussed issue that over time the passageways to the exhaust air injection points clog on many 993 engines. There are several antidotal theories floating around as to why.
I had a new theory I formulated while observing how the air injection system works on my daughter's New Beetle. On her car the valve that connects the exhaust manifold to the air pump is vacuum actuated and closes after the brief warm up period. When it is closed it is a positive seal of the plumbing that leads to the exhaust system entry port. Because of this valve there can be no backflow into the secondary air injection plumbing after the injection cycle ends. WV has been using this same design since the early 90's.
I believe the valve that serves the same function in the 993 is a passive one way valve that during transients between exhaust pressure pulses may be allowing for exhaust gases to backflow into the air injector system. The passive design inherently does not provide as positive a seal as the VW design.
This small amount of backflow over time could be carrying incremental carbon particulates into the injection passages. As the valve ages I would think greater amounts of exhaust gasses slip past the valve and greater amounts of particulates make there way into the passages.
If someone was to make an aftermarket kit with a VW actuator controlled air injection valve and a threaded adapter for the 993 engine valve mount would it eliminate this costly service item? I am sure there is a way to control such a valve using a VW vacuum control solenoid valve in conjunction with a vacuum line to the intake manifold and a related electrical ECU device control line.
Please your thoughts?
Andy
I had a new theory I formulated while observing how the air injection system works on my daughter's New Beetle. On her car the valve that connects the exhaust manifold to the air pump is vacuum actuated and closes after the brief warm up period. When it is closed it is a positive seal of the plumbing that leads to the exhaust system entry port. Because of this valve there can be no backflow into the secondary air injection plumbing after the injection cycle ends. WV has been using this same design since the early 90's.
I believe the valve that serves the same function in the 993 is a passive one way valve that during transients between exhaust pressure pulses may be allowing for exhaust gases to backflow into the air injector system. The passive design inherently does not provide as positive a seal as the VW design.
This small amount of backflow over time could be carrying incremental carbon particulates into the injection passages. As the valve ages I would think greater amounts of exhaust gasses slip past the valve and greater amounts of particulates make there way into the passages.
If someone was to make an aftermarket kit with a VW actuator controlled air injection valve and a threaded adapter for the 993 engine valve mount would it eliminate this costly service item? I am sure there is a way to control such a valve using a VW vacuum control solenoid valve in conjunction with a vacuum line to the intake manifold and a related electrical ECU device control line.
Please your thoughts?
Andy
08-08-2013, 06:45 PM
#2
Rennlist Member
Yes, this is exaclty what causes the carbon build up.
The SAI ports are open all of the way through from the exhaust side of the head, through the cam towers to the SAI check valve
The root of the problem is increased oil consumption due to the "soft" valve guide material Porsche uses. Guides wear, stem seals fail. Oil is burned...
The SAI ports are open all of the way through from the exhaust side of the head, through the cam towers to the SAI check valve
The root of the problem is increased oil consumption due to the "soft" valve guide material Porsche uses. Guides wear, stem seals fail. Oil is burned...
08-08-2013, 06:55 PM
#3
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Hi Mark,
Thank you for your thoughtful reply.
Does your view suggest that the regular replacement of the check valve may increase service life before clogging becomes a problem?
The reason I proposed the VW valve is because it looks rather small and self contained however I have never looked at one up close out of the vehicle to see if it would be feasible to attach a section to it for mounting on a 993.
Andy
Thank you for your thoughtful reply.
Does your view suggest that the regular replacement of the check valve may increase service life before clogging becomes a problem?
The reason I proposed the VW valve is because it looks rather small and self contained however I have never looked at one up close out of the vehicle to see if it would be feasible to attach a section to it for mounting on a 993.
Andy
08-08-2013, 07:03 PM
#4
Rennlist Member
Yes, chaning the SAI check valve as preventative maintenence will help...
Trouble is, the main clogging begins so close the the port in the head that exhaust gasses gather regardless.
The primary trouble spot is right where the head meets the cam tower. The port's path has quite a sharp bend in it... just asking for trouble when oil consumption increases
Your logic is spot on though
Trouble is, the main clogging begins so close the the port in the head that exhaust gasses gather regardless.
The primary trouble spot is right where the head meets the cam tower. The port's path has quite a sharp bend in it... just asking for trouble when oil consumption increases
Your logic is spot on though
08-08-2013, 07:14 PM
#5
Rennlist Member
It is a well discussed issue that over time the passageways to the exhaust air injection points clog on many 993 engines. There are several antidotal theories floating around as to why.
I had a new theory I formulated while observing how the air injection system works on my daughter's New Beetle. On her car the valve that connects the exhaust manifold to the air pump is vacuum actuated and closes after the brief warm up period. When it is closed it is a positive seal of the plumbing that leads to the exhaust system entry port. Because of this valve there can be no backflow into the secondary air injection plumbing after the injection cycle ends. WV has been using this same design since the early 90's.
I believe the valve that serves the same function in the 993 is a passive one way valve that during transients between exhaust pressure pulses may be allowing for exhaust gases to backflow into the air injector system. The passive design inherently does not provide as positive a seal as the VW design.
This small amount of backflow over time could be carrying incremental carbon particulates into the injection passages. As the valve ages I would think greater amounts of exhaust gasses slip past the valve and greater amounts of particulates make there way into the passages.
If someone was to make an aftermarket kit with a VW actuator controlled air injection valve and a threaded adapter for the 993 engine valve mount would it eliminate this costly service item? I am sure there is a way to control such a valve using a VW vacuum control solenoid valve in conjunction with a vacuum line to the intake manifold and a related electrical ECU device control line.
Please your thoughts?
Andy
I had a new theory I formulated while observing how the air injection system works on my daughter's New Beetle. On her car the valve that connects the exhaust manifold to the air pump is vacuum actuated and closes after the brief warm up period. When it is closed it is a positive seal of the plumbing that leads to the exhaust system entry port. Because of this valve there can be no backflow into the secondary air injection plumbing after the injection cycle ends. WV has been using this same design since the early 90's.
I believe the valve that serves the same function in the 993 is a passive one way valve that during transients between exhaust pressure pulses may be allowing for exhaust gases to backflow into the air injector system. The passive design inherently does not provide as positive a seal as the VW design.
This small amount of backflow over time could be carrying incremental carbon particulates into the injection passages. As the valve ages I would think greater amounts of exhaust gasses slip past the valve and greater amounts of particulates make there way into the passages.
If someone was to make an aftermarket kit with a VW actuator controlled air injection valve and a threaded adapter for the 993 engine valve mount would it eliminate this costly service item? I am sure there is a way to control such a valve using a VW vacuum control solenoid valve in conjunction with a vacuum line to the intake manifold and a related electrical ECU device control line.
Please your thoughts?
Andy
08-08-2013, 07:46 PM
#6
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Thanks for the info Hoggel.
Do you know if there is a diagram laying out the air injector system and its components?
Andy
Do you know if there is a diagram laying out the air injector system and its components?
Andy
08-08-2013, 08:11 PM
#7
Rennlist Member
Yes. I can't remember if it is in the repair manual or the OBD manual. It is called something non-intuitive, auxiliary pump or auxiliary air I think. I'll find it and let you know.
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08-08-2013, 11:54 PM
#8
993 Workshop Manual
The workshop manual has some information on this in Chapter 24, DME Diagnosis.
If you have the .pdf version of the workshop manual downloaded from the internet, open the document and search for "24 - 9". Be sure to enter a ' ' (space) on each side of the '-' when you are searching.
This will take you relatively quickly to page 9 of chapter 24.
Hoggel - If the OBDII Manual has any information, would it be possible to post it here?
If you have the .pdf version of the workshop manual downloaded from the internet, open the document and search for "24 - 9". Be sure to enter a ' ' (space) on each side of the '-' when you are searching.
This will take you relatively quickly to page 9 of chapter 24.
Hoggel - If the OBDII Manual has any information, would it be possible to post it here?
08-09-2013, 01:15 AM
#9
Rennlist Member
I believe the general understanding of why these ports clog really has little to do with having a passive or active seal of the secondary air injection system, or even the convoluted and sometimes sharp bends the air takes through these cam towers. These design weaknesses may possibly accelerate the carbon buildup in these lines, but they are not the source of the problem.
The real source of the problem appears to have to do with the oil seepage thru the softer valve guides that Porsche manufactured on these 993 engines. When the valve guides wear (particularly the exhaust valve guides due to higher heat) they cause a higher carbon flow from excessive oil which flows backwards into the cam towers under extreme heat. Even with minimal flow (by shutting off the SAI system with a check valve), the ends of the ports will still eventually clog due to increased carbon and high engine heat.
One very strong piece of evidence to support this is the fact that once the valve guides are replaced with a top-end rebuild, the re-clogging of these ports appears to be minimal. I know of several shops who do this service on a regular basis and they have yet to see a rebuilt top-end return for the same problem. When my friend had his 993 top-end rebuild done last year, he was really insistent that they install some sort of clean-out in case his ports clog again. The shop stated that they fixed the source of the problem and he would not have this issue again.
The complete clogging of the SAI ports (and an eventual CEL code on OBDII models) also does not seem to be directly correlated to the mileage of these cars, although increased mileage will accelerate the wearing of these guides. I know of a few RL'ers who had top-end rebuilds at 50,000 miles, and then there are many with over 100,000 miles and no problems. Our previous PCA president drove her 993 over 200,000 before she needed a top-end rebuild.
The real source of the problem appears to have to do with the oil seepage thru the softer valve guides that Porsche manufactured on these 993 engines. When the valve guides wear (particularly the exhaust valve guides due to higher heat) they cause a higher carbon flow from excessive oil which flows backwards into the cam towers under extreme heat. Even with minimal flow (by shutting off the SAI system with a check valve), the ends of the ports will still eventually clog due to increased carbon and high engine heat.
One very strong piece of evidence to support this is the fact that once the valve guides are replaced with a top-end rebuild, the re-clogging of these ports appears to be minimal. I know of several shops who do this service on a regular basis and they have yet to see a rebuilt top-end return for the same problem. When my friend had his 993 top-end rebuild done last year, he was really insistent that they install some sort of clean-out in case his ports clog again. The shop stated that they fixed the source of the problem and he would not have this issue again.
The complete clogging of the SAI ports (and an eventual CEL code on OBDII models) also does not seem to be directly correlated to the mileage of these cars, although increased mileage will accelerate the wearing of these guides. I know of a few RL'ers who had top-end rebuilds at 50,000 miles, and then there are many with over 100,000 miles and no problems. Our previous PCA president drove her 993 over 200,000 before she needed a top-end rebuild.
08-09-2013, 02:06 AM
#10
Rennlist Member
The workshop manual has some information on this in Chapter 24, DME Diagnosis.
If you have the .pdf version of the workshop manual downloaded from the internet, open the document and search for "24 - 9". Be sure to enter a ' ' (space) on each side of the '-' when you are searching.
This will take you relatively quickly to page 9 of chapter 24.
Hoggel - If the OBDII Manual has any information, would it be possible to post it here?
If you have the .pdf version of the workshop manual downloaded from the internet, open the document and search for "24 - 9". Be sure to enter a ' ' (space) on each side of the '-' when you are searching.
This will take you relatively quickly to page 9 of chapter 24.
Hoggel - If the OBDII Manual has any information, would it be possible to post it here?
9. Secondary Air Injection Pump
8. Pneumatic Secondary Air Injection Switching Valve
7. Check Valve
6. Electro-pneumatic Secondary Injection Switching Valve
As I described above, the SAI operates when the ECU engages a relay that turns on the pump 9. and opens the vacuum valve 6.. The switched vacuum from valve 6. opens the valve 8.. The air from the pump 9. flows through the valve 8. and then through the one-way check valve 7. into the heads.
08-09-2013, 02:43 AM
#11
RL Technical Advisor
Mark is spot-on,...
Based on observations with these OBD-II cars since 1996, I do recommend replacing the SAI valve every 60K due to corrosion issues.
Beyond that, replacing the soft (and poorly fitted) factory valve guides with a better material and close attention to fitment, the SAI issues do not return. We soda blast the heads to remove all carbon and includes scouring out the SAI passages from the cam housings to the exhaust ports.
Based on observations with these OBD-II cars since 1996, I do recommend replacing the SAI valve every 60K due to corrosion issues.
Beyond that, replacing the soft (and poorly fitted) factory valve guides with a better material and close attention to fitment, the SAI issues do not return. We soda blast the heads to remove all carbon and includes scouring out the SAI passages from the cam housings to the exhaust ports.
08-09-2013, 02:44 AM
#12
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Hmm - jump over to pcarworkshop and its all there -- haha
Main link:
http://www.pcarworkshop.com/index.ph...e_SAI_passages
Some pictures of the passages:
Comparison of the head angle vs the cam carrier angles
Cheers,
Mike
Main link:
http://www.pcarworkshop.com/index.ph...e_SAI_passages
Some pictures of the passages:
Comparison of the head angle vs the cam carrier angles
Cheers,
Mike
08-09-2013, 10:10 AM
#14
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Another DIY idea!
Wow! Thanks for all the useful info and pictures.
I have another question. If a T pipe was put in the hose just on the pump side of the valve #7 in Hoggels drawing above and it led to a 1/2 inch ID vertical pipe with a threaded cap, could such a pipe act as a reservoir and be filled with a little Techron.
The idea being that it would let the solvent work its way through the passages using a combination of gravity and the air pump's action flushing out some of the carbon incrementally over time.
Would this reasonably forestall the passages clogging?
Something where one would fill the pipe with Techron say three or four times a year as a flush. The cap might need a little breather in the form of a very small hole or a one way diaphragm vent valve of the type used inline on the vacuum hoses on the VW's system.
Constructing such a system could easily be a DIY project with parts from the local hardware store on maybe one little valve from the VW dealership.
Your thoughts please!
Andy
I have another question. If a T pipe was put in the hose just on the pump side of the valve #7 in Hoggels drawing above and it led to a 1/2 inch ID vertical pipe with a threaded cap, could such a pipe act as a reservoir and be filled with a little Techron.
The idea being that it would let the solvent work its way through the passages using a combination of gravity and the air pump's action flushing out some of the carbon incrementally over time.
Would this reasonably forestall the passages clogging?
Something where one would fill the pipe with Techron say three or four times a year as a flush. The cap might need a little breather in the form of a very small hole or a one way diaphragm vent valve of the type used inline on the vacuum hoses on the VW's system.
Constructing such a system could easily be a DIY project with parts from the local hardware store on maybe one little valve from the VW dealership.
Your thoughts please!
Andy
Last edited by pp000830; 08-09-2013 at 01:02 PM.
08-09-2013, 11:51 AM
#15
Rennlist Member
Andy,
Your thoughts of using Techron can indeed help to breakdown the carbon deposits.
However, you do NOT want to flush Techron through your exhaust when the system is completely assembled for the the single reason that you do NOT want that stuff to go into your catalytic converter. It will quickly shorten the life of your cat.
If you take a look at any of the SAI flushing procedures that are on Rennlist (Mike has a very good website that includes this - pcarworkshop.com) you will notice that it is a fairly involved process. Most people choose to remove the heat exchangers so they get direct access to the SAI exit ports which is just inside the exhaust port where the heat exchanger was located. When you do this, you essentially blow the carbon buildup into the air and NOT into the exhaust piping system. Carbon (and oil and flush) that gets into your exhaust will burn off inside your exhaust (heat exchanger, mufflers, and cats) and is not healthy for your car.
Your thoughts of using Techron can indeed help to breakdown the carbon deposits.
However, you do NOT want to flush Techron through your exhaust when the system is completely assembled for the the single reason that you do NOT want that stuff to go into your catalytic converter. It will quickly shorten the life of your cat.
If you take a look at any of the SAI flushing procedures that are on Rennlist (Mike has a very good website that includes this - pcarworkshop.com) you will notice that it is a fairly involved process. Most people choose to remove the heat exchangers so they get direct access to the SAI exit ports which is just inside the exhaust port where the heat exchanger was located. When you do this, you essentially blow the carbon buildup into the air and NOT into the exhaust piping system. Carbon (and oil and flush) that gets into your exhaust will burn off inside your exhaust (heat exchanger, mufflers, and cats) and is not healthy for your car.