OBD-II CEL - Air injection Carbon Issues

[Embs]

I have read every bit of information about this that I can find. Could it be that the exhaust valve is over cooled by the air being injected and because of that it runs "sloppy" in the guide never reaching it's intended "warm" tolerance with respect to the guides?


I have seen the oil consumption in several US engines but not the Euro engines. I know that many say that oil consumption causes the issue, I am wondering if the air injection itself ultimately causes the oil consumption problem.

[Steve@Rennsport]

Todd:

The root causes for this problem lies with the crappy material that Porsche uses for valves guides (excessive wear), and the byproducts of using unleaded gasolines.

The former is fixable at valve job time, but we can do nothing about the latter. Its not related to the air injection principle or system.

[Embs]

Thanks Steve and Happy 2004!

[Lorenfb]

Check out the writeups on the Rennlist website. It's NOT as simple
as just bad guide material. It's much more complex.

[Lorenfb]

My post from Rennlist

ISSUE:

Whether the 993 has abnormal oil consumption compared to older
Porsches and if so what is the cause, since secondary air injection
is a major problem, i.e. which results from carbon buildup from oil
burning, for some '96 and later Porsche 993s.

Facts:

1. The 993 is the most powerful recent Porsche air cooled engine,
and as such runs the hottest, i.e. assuming that the air cooled
engine efficiency is unchanged from the 3.2 and 964, which can
lead to accelerated engine wear.
2. The valve guide/guide material is different for all three air
cooled engines (3.2, 964, 993), and Porsche has not updated
the 993 valve guide. The valve stem on the 993 is 8mm versus
9mm on the older engines.
3. Porsche recommends an oil change interval on the 3.2, 964 and
993 of 15,000 miles.
4. Both the 964 and the 993 use a lower body pan affecting air
flow to/from the engine.
5. Stop-and-go driving in heavy traffic causes an air cooled
engine to run very hot.
6. The quality of the machining, it appears, (initially and at
valve job time) is more critical on the 993 versus the 964
and 3.2.
7. The very small air injection passages on the 993 are prone
to fairly easy carbon buildup restricting air injection flow.
8. All Porsches engines can tend to burn some oil early on versus
other types based on many factors, e.g. the driving conditions,
the valve guide material/seals, and its basic design (high
reving).
9. There are non-OEM valve guides being used by some machine
shops when valve jobs are done on the 993 and other Porsches.
10. The late 993s use a light weight synthetic oil versus the 3.2
and 964 based on Porsche's recommendation.
11. The secondary air injection at the exhaust valve guide
may result in higher temperatures (lean condition) for
the exhaust valve guide versus no air injection.
12. Continued high engine rpms accelerates engine wear, i.e
Energy loss (heat/wear) = rpm x friction
13. Infrequent oil changes can cause ring sealing problems,
i.e. ring-lands fill with carbon and slug, resulting in
poor oil sealing at the oil ring causing excessive oil
burning.
14. Many 993s with 50K or less mileage are requiring valve jobs.


Conclusion:

There doesn't appear be any total consensus as to what is happening
either from dealer or independent techs. One key fact does standout;
valve jobs are being done at a much lower mileage for the 993 than
for the 3.2 and 964 engine. Excessive valve guide wear does appear
to be a major contributor to the oil burning on the 993, but its
cause is at issue.

Some techs feel that the oil burning for the 993 is about the same
as for the early engines, but the secondary air injection problem
magnifies it requiring a valve job much sooner. Other techs feel
that the 993 was pushed to the max causing excessive heat resulting
in increased valve guide wear. Still others feel that infrequent oil
changes (15K plus) have caused excessive oil burning.

One of my customers is doing two 993 valve jobs. One engine has
107K on it and has had two previous valve jobs (He did neither).
The other engine had a valve job done by a dealer 2K miles ago
and now has valve seats coming loose. This was, more than likely,
a machine shop quality problem.

There's not a clear cut answer to this problem without more data.
Some facts mentioned above may contribute little to not at all by
themselves. Maybe a number of them, when integrated, cause major
problems. Even if a new valve guide material was used, there's no
way without major testing to determine its long term positive or
negative effect on the 993 engine, i.e given that it's the single
cause of the problem.


Each 993 owner must assess his own driving and maintenance habits.
Without more data, the only answer I have is based on how I would
drive and maintain a 993 if I owned one, which is basically how I
drive and maintain my 3.2:

1. I would change the oil & filter at 7500 miles (Porsche's 930 oil
service).
2. I'd probably remove the lower body pan if I did a lot of stop
and go driving in hot weather.
3. I'd TRY and avoid using the A/C in stop and go hot weather,
as this heats the engine more.
4. I'd minimize that constant reving above 5K. I wind my 3.2
above 5K often but not from every light nor on every twisty.
5. I'd monitor my oil consumption. Greater than 1.5K to 2K per
quart would not indicate excessive oil burning. If the secondary
air problem does occur, then it's NOT a valve guide problem
but only an inherent air injection design problem.
6. I'd use the heaviest weight synthetic oil (equiv. to 20/50 in
a standard oil).
7. If a valve job is required, I'd assure that good referrals are
used to select a dealer or independent shop.

Good Luck
Loren

[Lorenfb]

Also, go to this url (www.systemsc.com/pictures.htm) for pictures
of a problem 993 SAI system.

[Tyson Schmidt]

FWIW, certain years of Honda's that used a similar system, had the exact same problem. There was a fix-it kit available from the dealer for those, since it is an emissions-control device, and had to be warranted for a certain mileage. (50,000, I think.)

I really don't think it is a good system, and they should've simply stuck with the old style injector nozzles in the exhaust port. At least those could be externally replaced/serviced.