Steve Weiner-Rennsport Systems

Hi:

I've not started a thread in quite some time, but this one deserves some additional information that will not get buried here. I've watched this subject for some time now.

Both of the related threads posted here have contained some good & useful information. I'll shed some additional light on what I see when doing valve jobs on these engines.

For sure, products like Techron & LubroMoly do little if anything to aleviate the root cause of this issue, and I'll share some observations and opinions about what I've seen after thinking about this for awhile.

One sees 2 kinds of carbon buildups in the heads and exhaust passages. One kind is found on the backs of intake valves & injector tips and the other kind is found on piston crowns and in the exhaust ports. The former is a byproduct of burning unleaded fuels and the latter is a byproduct of burning unleaded fuels and engine oil.

The heavy black stuff I see that clogs the air injection passages in the heads (triggering the CE light,) is from excessive oil consumption and its coming from loose/worn valve guides. I've seen worn guides in 993's with as little as 30K miles and this is consistent with the same issue that plagued 911's since the 3.2's and Turbo's.

Its NOT new; its just that the draconian OBD-II tolerances report on the problem and do not tolerate this at all compared to OBD-I cars and earlier. Frankly, if the older cars (pre-89) had something like OBD-II, we would have seen this years ago,...

To their credit, Porsche has quietly repaired some of these cars under warranty if they qualified but one should understand the big picture here. I cannot comment on whether Porsche should repair the affected cars or not, but I will mention that IMHO, the cure is really to install NON-FACTORY guides and making sure that they are fitted correctly (valve stem-to-guide clearance and honed, never reamed to fit).

I've also looked at the feasility of enlarging the air injection passages a little bit to help prevent this from recurring. For sure, Porsche will not repair these cars twice and IMHO, I'd be concerned that this will happen again if the same OEM parts are used for the initial repair.

Lastly, I've seen many creative & talented people here come up with inventive "solutions" to this like fooling the secondary sensors with some circuitry.

While this may extinguish the CE light for the time being, it does nothing to address the effects of excessive carbon inside the engine. This lowers the threshold of detonation and poorly cooled valves caused by loose/worn factory guides can separate with catastrophic results.

My advice (as always) is to fix something right the first time and not have to do something over again. In that vein, I'd suggest that anyone faced with this problem make certain that the heads are not only decarbonized, but better-than-factory guides be fitted to the heads and perhaps the passages be machined larger. This may require that someone other than a Dealer perform the head work.

I hope this is useful to someone here,...

Bill 993

Steve,

What would you consider "excessive oil consumption" for an 993? I get 1,000 miles per quart with only 24K mileage on the engine.

Thanks

Steve Weiner-Rennsport Systems

Hi Bill:

A 911 engine in good shape should use about 1 qt per 2K miles or more.

Porsche says 600 miles per quart is excessive and I'll say that with 24K on your motor, you are looking at worn guides with 1K/qt.

ksjcorpus

yikes steve ..... for your suggestions on the initial post that's a lot of money for a car that runs well.

Steve Weiner-Rennsport Systems

Hi Karim:

LOL,....Yes,...I am well aware of the implications here and I do not take this situation lightly.

My only point is for people with this problem is NOT to ignore the CE light. At some time, one needs to understand the "big picture" and deal with it at an appropriate time that the proper solution is affordable.

Porsche's "Achille's Heel" has been their less-than-satisfactory choice of valve guide materials and up until the advent of the OBD-II cars, we could simply ignore it. Now, we cannot even though the car runs very well,...

Even the new GT3's STILL use that crappy same crappy guide material. I cannot explain it.

Aside from any CE light issues, the guiding principle about when to replace valve guides has always been the rate of oil consumption and the 993 is no exception.

When one is adding a 1 quart of more of oil every 1K miles, its time for new guides and the opportunity to fix this, once and for all. The bottom end is VERY durable and goes a long long time.

Steve 96C4S

Thanks for that brilliant post, Steve. You've shed some scary (ie: $$$) light into the situation. So, are we talking $5k to fix the problem ... for good?

I'm printing out a copy of your "fix" and filing it away in my 993 faq file for my independent repair shop in case I get to this point one day. I find that he isn't always as knowlegeable about all the fixes to these cars as you people on Rennlist are and appreciates SOME of the information I get him from here.

Nervously waiting till the dreaded CEL comes on next. There goes our trip to Switzerland and Northern Italy next year if I need this expensive fix.

So much for driving my car hard, thinking this will "fix" any CEL related problem.

Steve

Randall G.

To bolster further what Steve has written...."Carbon build-up" is unheard of over on the 964 board. Similarly, no one over there has ever had their heads rebuilt because of carbon build-up. At least not that I'm aware of. The valve guides may be wearing out, but I can't remember reading of anyone that has rebuilt their heads because of excessive oil consumption (unless the oil is leaking out the heads, that is).

In my own case, the original engine lasted 100k miles, no (tangible) ill effects from worn valve guides. Engine seemed to run as strong as new, oil consumption was normal. The engine did spin a rod bearing at 100k miles, but that's another story.

My point? I'm not going to have my $heads rebuilt$ because of carbon build-up. If I get the check-engine light locked in because of SAI, I'll have the code written out by a super-tuner (money there, too). If oil consumption becomes high, then I'll worry about the valve guides and upgrading the material. This is my personal strategy, others may take another tack.

Finally, I received the SAI code once....last fall. Had another CE light a few months later, which I reset by disconnecting the battery (so, don't know the code). Haven't had a CE light since (fingers crossed).

914und993

Steve -

I'm curious about the valve guide material. Exactly what does Porsche use, and what is the superior material and who makes that part?

Thanks,
Chip

MRFLATSIX

I'm curious about valve guide material too. This seems to be the Achilies' heel for our cars. Am I wrong, or did the SC use a different material. They seem to be pretty long-lived. The shop that did my heads used Silicon Bronze guides. It's a puzzle to me that a company with such great engineering would use a crappy guide material year after year.

Edward

So Steve,

Am I reading you right when you say that the general rule of thumb is not milage, but oil consumption greater than 1 quart/1000 miles is the time for valve-guide concerns? The reason I ask is that I've got just under 80K (on a '95) and add a quart at about 4 or 5K (drive it daily to redline, and track it). TIA! and Thanks for your insight and experience here!

Edward

Jim Morton

As an attempt to add some value here... Porsche like several German car makers use a soft Phosphorus Bronze for their valve guides. The metallurgy at work here is not completely simple as German valve stems are also of a material and surface finish that lends itself to the use of a softer phosphorus bronze as a guide before we had things like emissions systems and ODB.

Why they continue to use the bronze they do is likely a matter of materials availability and cost to them in Germany. I would speculate that Porsche has not considered our U.S. "off-shore" (to them) sources of bronze for their use. Sometimes we forget how the U.S. has sources of materials not typically produced world wide. I say this as the German soft bronze shows up all over the German product base, not only in automotive, but machine tools and other mechanical systems as well.

For anyone who is interested in these sorts of materials curiosities, the German metals do not always lend themselves to an A.S.M. (American Society of Metals) or A.S.M.E. designation. As such, I have tried to cross reference the material from tests performed on some old guides I had. The closest A.S.M. bronze I have seen to the German bronze is a soft variant of a 54x series phosphor bronze. A.S.M. spec's 541 phosphorus is actually harder to the German guide material when tested with a Rockwell tester. However, this is not a completely relevant test to predicting guide wear as the material needs to perform many more functions than simple load strength. The use of the material along with the valve stem counter part plays a huge role in the acceptability of making a "sliding" style bearing.

If anyone wants to read more than you likely care to on this, folks should lookup and review the many articles available from S.A.E. on this very topic. There is a great guide for matching guide materials to valve stem material if you know all of the properties of the components involved.

The problem with our P-Cars is knowing the valve materials in question...

With all that is needed to be known with all of this, I am not looking forward to head rebuilds on my P-Car. I see many evenings requiring "antacids" to make it through the specs on the valves / guides.

Regards

Rick Lee

Speaking of oil consumption, I just added some yesterday. I keep a detailed log book of everything and it looks like I added .75 qt. yesterday after about 1150 miles to bring the level back up to the middle. So that would indicate about 1 qt. per 1533 miles. I drive my car like I stole it. Does this consumption spell imminent doom, impending doom or down the road doom? Or nothing?

ecobb993

Rick,

There's doom at the end of the tunnel.

JC

Steve Weiner-Rennsport Systems

Wow,......OK,..........

I'm glad that Jim stepped in with his post on materials.

I do not know for sure exactly what Porsche uses, but its VERY soft and tears out when drilling these old guides out. Porsche has used this same lousy material since 1978 and the variations in each car have as much to do with production tolerances as anything. In short, the reason why some cars use more oil than others, is due to less-than-optimal production fitments and variations in valve stem diameters.

We use our own Phosporus-Bronze material that is made here in the US and is FAR harder than Porsche's. We make these for all 911's including the new 996 TT's, GT-3's and GT-3 RS. We've not replaced any of our guides since '78 and I've not noted any unusual valve stem wear. I use the excellent factory Viton guide seals, too.

Once again, if your car uses little oil OR is not OBD-II, there is little to be concerned about.

Regardless of model year, if your car uses a quart per 1000 miles of oil, you should start budgeting for a good valve job that includes a better guide than the OEM items.

I hope this answers some questions and clarifies a few things here,...

john.rogers

Ugly !

What is the cost to repair this problem? Someone mentioned - hopefully tongue in cheek - $5,000. Say it ain't so.

I know it will be expensive - it is a Porsche.

But can anyone put a dollar on it from experience.

Thanks,