Theroy Secondary Air injector cloging
08-09-2013, 01:21 PM
#16
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Hi Mgianzero,
You bring a very good point to the discussion concerning Techron compatibility with the catalytic converter.
I shot the following over the Chevron for a little cross validation of your very important concern.
"Hi, The Cheveron product "Techron" is a fuel additive that cleans the internals of the engine. The product is also useful for cleaning carbon deposits from the secondary air injection ports as they become clogged with carbon deposits. My question is, if I introduce liquid Techron into an exhaust system ahead of the catalytic converter will it damage the converter as it passes through it? Please advise.
Lets see what they come back with!
Andy
You bring a very good point to the discussion concerning Techron compatibility with the catalytic converter.
I shot the following over the Chevron for a little cross validation of your very important concern.
"Hi, The Cheveron product "Techron" is a fuel additive that cleans the internals of the engine. The product is also useful for cleaning carbon deposits from the secondary air injection ports as they become clogged with carbon deposits. My question is, if I introduce liquid Techron into an exhaust system ahead of the catalytic converter will it damage the converter as it passes through it? Please advise.
Lets see what they come back with!
Andy
08-09-2013, 02:29 PM
#17
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.
I'm with you on the root cause, but...
Based upon this theory, how do we account for the 50k vs 200k mile top end rebuilds? Soft vs hard valve guide material used by Porsche? Multiple vendors supplying valve guides? Driving habits/conditions? Ambient conditions?
Traffic conditions? Seems to be a veritable smorgasboard of variable contributing factors, yes?
I'm not so sure these factors can cover the 150k spread in the top end rebuilds. Feedback from the contributors please?
In the mean time and while I'm thinking of it; excuse me while I order an SAI check valve from Sunset...
08-09-2013, 02:50 PM
#18
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There will be a lot of opinions on why there is variance on valve guide wear, and the list list will get pretty long with no certain solution ...
- Owners tolerance for oil consumption
- City vs highway
- ambient temperatures (including engine covers)
- who was drinking beer on the assembly line that day causing a worse or better fit
- metallurgical differences in manufacturing
- quality of valve guide seals
- Oil weight/type/brand
- driving style
- brand of fuel (well, who knows...)
and so on. Its clear though that rebuilding the heads with a good guide material, and proper fitting of the valves will make the engine use very little oil, and greatly reduce if not eliminate the incidence of SAI faults.
An interesting observation is the seemingly less incidence of SAI issues on the Turbos. It could be that they run hotter, or on the average have less mileage, but observationally the incident rate appears lower. I doubt if we can mathematically conclude anything though, the sample sizes are pretty low with lots of variables.
There appears to be several ways to abate the SAI issue detection on the ODBII cars:
- periodic deep cleaning of the passages (using mechanical and chemical processes) and replacement of the SAI one-way valve
- bypass the passages using a mechanical system (i.e. pipe in bypasses to the exhaust ports)
- electronic solutions (of mixed results depending on the model year)
- top end rebuild to reduce oil coking
Cheers,
Mike
08-09-2013, 05:10 PM
#19
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Hi Mike J.,
And don't forget the salesmanship of the service advisor selling the need for very profitable rebuild procedure.
Andy
And don't forget the salesmanship of the service advisor selling the need for very profitable rebuild procedure.
Andy
08-09-2013, 07:31 PM
#20
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Paul,
Yes, it's hard to explain why some engine valve guides wear out sooner than others. Like Mike says, there are so many parameters that it's really hard to say why the mileage span for this needed repair on OBDII cars is so wide.
But, that's not my primary point here. My basic point goes back to what Andy was asking (I think) in his original question which is titled "the Theory of Secondary Air Clogging." I was answering that there is very definitive evidence that the primary CAUSE of the SAI clogging has to due with oil leakage thru the exhaust valve guides on these 993 motors. This excessive oil leads to clogging of the ports. I think most every 993 specialist (note that Steve Weiner already commented on my points above) will agree with me on this. Now, there are factors that can accelerate the problem, but once the heads are rebuilt with more durable guides (not sure what the material differences are b/w old and new) the clogging problem goes away. Not to mention, the oil consumption goes dramatically down once these valve guides are replaced.
Correct me if I'm off on this, but I've heard that replacing a liter of oil for less than 1,000 miles is kinda the cutoff for needing a top-end repair. Not to mention the inability to pass smog in states that require them with OBDII models. If you can refer to someone who had a top-engine rebuild who had a re-occurring SAI clog (with or without a CEL code) please tell us. Because I am not aware of any. Perhaps Steve Weiner can chime in on this again. I really value his expertise as we all know he does these repairs frequent enough to be able to comment on this with good reason.
Yes, it's hard to explain why some engine valve guides wear out sooner than others. Like Mike says, there are so many parameters that it's really hard to say why the mileage span for this needed repair on OBDII cars is so wide.
But, that's not my primary point here. My basic point goes back to what Andy was asking (I think) in his original question which is titled "the Theory of Secondary Air Clogging." I was answering that there is very definitive evidence that the primary CAUSE of the SAI clogging has to due with oil leakage thru the exhaust valve guides on these 993 motors. This excessive oil leads to clogging of the ports. I think most every 993 specialist (note that Steve Weiner already commented on my points above) will agree with me on this. Now, there are factors that can accelerate the problem, but once the heads are rebuilt with more durable guides (not sure what the material differences are b/w old and new) the clogging problem goes away. Not to mention, the oil consumption goes dramatically down once these valve guides are replaced.
Correct me if I'm off on this, but I've heard that replacing a liter of oil for less than 1,000 miles is kinda the cutoff for needing a top-end repair. Not to mention the inability to pass smog in states that require them with OBDII models. If you can refer to someone who had a top-engine rebuild who had a re-occurring SAI clog (with or without a CEL code) please tell us. Because I am not aware of any. Perhaps Steve Weiner can chime in on this again. I really value his expertise as we all know he does these repairs frequent enough to be able to comment on this with good reason.
08-10-2013, 12:09 AM
#21
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There are so many variables it is pretty tough to draw many conclusions. If you are buying, just because it had a rebuild how do you know what guides they used?
08-10-2013, 12:32 AM
#22
Paul,
Yes, it's hard to explain why some engine valve guides wear out sooner than others. Like Mike says, there are so many parameters that it's really hard to say why the mileage span for this needed repair on OBDII cars is so wide.
But, that's not my primary point here. My basic point goes back to what Andy was asking (I think) in his original question which is titled "the Theory of Secondary Air Clogging." I was answering that there is very definitive evidence that the primary CAUSE of the SAI clogging has to due with oil leakage thru the exhaust valve guides on these 993 motors. This excessive oil leads to clogging of the ports. I think most every 993 specialist (note that Steve Weiner already commented on my points above) will agree with me on this. Now, there are factors that can accelerate the problem, but once the heads are rebuilt with more durable guides (not sure what the material differences are b/w old and new) the clogging problem goes away. Not to mention, the oil consumption goes dramatically down once these valve guides are replaced.
Correct me if I'm off on this, but I've heard that replacing a liter of oil for less than 1,000 miles is kinda the cutoff for needing a top-end repair. Not to mention the inability to pass smog in states that require them with OBDII models. If you can refer to someone who had a top-engine rebuild who had a re-occurring SAI clog (with or without a CEL code) please tell us. Because I am not aware of any. Perhaps Steve Weiner can chime in on this again. I really value his expertise as we all know he does these repairs frequent enough to be able to comment on this with good reason.
Yes, it's hard to explain why some engine valve guides wear out sooner than others. Like Mike says, there are so many parameters that it's really hard to say why the mileage span for this needed repair on OBDII cars is so wide.
But, that's not my primary point here. My basic point goes back to what Andy was asking (I think) in his original question which is titled "the Theory of Secondary Air Clogging." I was answering that there is very definitive evidence that the primary CAUSE of the SAI clogging has to due with oil leakage thru the exhaust valve guides on these 993 motors. This excessive oil leads to clogging of the ports. I think most every 993 specialist (note that Steve Weiner already commented on my points above) will agree with me on this. Now, there are factors that can accelerate the problem, but once the heads are rebuilt with more durable guides (not sure what the material differences are b/w old and new) the clogging problem goes away. Not to mention, the oil consumption goes dramatically down once these valve guides are replaced.
Correct me if I'm off on this, but I've heard that replacing a liter of oil for less than 1,000 miles is kinda the cutoff for needing a top-end repair. Not to mention the inability to pass smog in states that require them with OBDII models. If you can refer to someone who had a top-engine rebuild who had a re-occurring SAI clog (with or without a CEL code) please tell us. Because I am not aware of any. Perhaps Steve Weiner can chime in on this again. I really value his expertise as we all know he does these repairs frequent enough to be able to comment on this with good reason.
I am not contradicting the root cause, I'm simply trying to point out that possible contributing causes are many and varied. I'm suggesting this is evident by the range (50k-200k mi.) at which top end rebuilds which are predicated upon clogged SAI passages.
I would additionally venture that if you plotted the mileage on each vehicle as each top end is done; the mean of the bell curve is probably well under 100k mi.
It seems bizarre that Porsche would continue to use substandard valve guide material from 84-98. Are they really that bullheaded and stupid as to refute a known single point of failure, and risk the reputation of their flagship? Or does it all boil down to admission and the potential cost of liability?
If the latter is true, that was a hell of a gamble, especially in the time frame when Japanese auto makers were king of the hill.
08-10-2013, 02:39 AM
#23
RL Technical Advisor
Remember, they are not very concerned about what happens beyond the warranty & emissions certification periods, only when they have some liability during the time they must support their products according to Federal laws.
We've found major discrepancies and inconsistencies in guide-to-stem clearances with many cars being delivered well outside of their own factory wear specifications, right from new. Some engines are better (tighter) than others which accounts for why some cars need guides sooner (and consume more oil) than later. I could tell you stories about what I've seen, just in 964/993 engines since 1990,.............................
This situation required a solid, permanent solution which we have found by using custom guides made for us and spending a LOT of time in the installation and fitment. This assures proper guide-to-stem contact for cooling, much reduced oil consumption, and far better durability. For OBD-II cars, this assures that the SAI passages will not become clogged for a very long time.
08-10-2013, 05:42 PM
#24
Steve,
Thanks for clarifying the causes...I would have thought a hand built Porsche would have been built to and inspected to more exacting standards. You make Porsche sound like an "American" auto maker. <sigh>
Thanks for clarifying the causes...I would have thought a hand built Porsche would have been built to and inspected to more exacting standards. You make Porsche sound like an "American" auto maker. <sigh>
08-10-2013, 06:30 PM
#25
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In the VW the air injector seems to be in one place in the exhaust manifold upstream of the catalytic converter.
Could an alternate permanent fix for the 993 be to tap a steel line into each of the exhaust manifolds, rout the air pump output to each and bag the entire casting passage issue? If such external lines became clogged they could easily be removed and replaced low cost.
Your thoughts please>>>
Could an alternate permanent fix for the 993 be to tap a steel line into each of the exhaust manifolds, rout the air pump output to each and bag the entire casting passage issue? If such external lines became clogged they could easily be removed and replaced low cost.
Your thoughts please>>>
08-11-2013, 03:58 AM
#26
RL Technical Advisor
Engines are built on a production line before being mated to a car and its not possible to take that amount of time in a production environment to fit things inside factory tolerances in every case. Mahle P/C assemblies, OTOH, are made off-site and provided to the factory so their QC is better.
The best independent shops do not operate under such strict time constraints so they are better suited for "blueprinting" engine assembly and paying very close attention to tolerances & fitment.
Overall, the factory did a pretty good job on these street engines so we regard this as an opportunity to "fine-tune" their efforts so they last a very long time.
08-11-2013, 12:55 PM
#27
06-17-2014, 05:19 PM
#28
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Help………I am having my engine 97 993 C2s top end rebuild. At one time there was a list of about 4 or 5 seals/gaskets that the 993 community says you should also have replaced. Does anyone remember this topic or offer a opinion. I just remember it was 4 or 5 things to do when the engine is out.
rlsammax
rlsammax
06-17-2014, 07:02 PM
#29
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If you are getting it done (I assume you are not doing it yourself) would not your mechanic know which ones should be done, based on condition and how deep he went.
Normally you use a top end gasket set (use Wrightwood racing gaskets - better material than the stock rubber) which covers almost all the gaskets you will replace doing a top end. If he pulls the pistons/cylinders, you need to replace the base gaskets (or should anyways). If the pistons/cylinders are pulled, then you might as well look at the through bolt gaskets, doing them one by one.
Cheers,
Mike
Normally you use a top end gasket set (use Wrightwood racing gaskets - better material than the stock rubber) which covers almost all the gaskets you will replace doing a top end. If he pulls the pistons/cylinders, you need to replace the base gaskets (or should anyways). If the pistons/cylinders are pulled, then you might as well look at the through bolt gaskets, doing them one by one.
Cheers,
Mike
06-17-2014, 08:21 PM
#30
Nordschleife Master
I bought the complete gasket set, it is part number 993 100 902 00.
got a power steering belt too, as that is one of those, while you are in there things.
got a power steering belt too, as that is one of those, while you are in there things.
