earossi

Thinking about the 993's Achille's heel of the SAI circuit failing due to carbonized oil fouling of the injected air ports, I wonder if anyone has attempted to replace the engine's valve stem seals in conjunction with the fix of clearing the injection ports at the exhaust valves. Though I have not replaced valve stem seals on a Porsche engine, I have done it on other designs without the need to remove the heads. So, if replacing these seals could be done at the same time that one cleans out the air passages in the heads, would this be a mechanical solution that would be good for another 40k to 50k miles? Has anyone done this?

Mike J

Fixes that I know about:

1 - Rebuild the heads, new guides, clean out the ports
2 - Regularly clean out the ports, use solvents and motorcycle brake cables
3 - Pipe in another path for the SAI pump to inject gases into the exhaust stream, possibly using the test ports before the cats
4 - SAI bypass circuit

I am not sure, when the valves guides are worn, how long the seals last before they too are beaten up given the movement of the valve stems. The wobbling of the stem in the guide can cause the seal to become looser - but I have not seen anyone do this. It would involved using compressed air to hold the valve tightly closed, removing the springs, and then redoing the guide.

Cheers,

Mike

FisterD

Not sure what you mean by regularly. Mine had the clean out 30K miles ago and has been trouble free for the last 7 years. Oil consumption is not an issue on my car, so I am not looking at valve guides yet.

Steve Weiner-Rennsport Systems

Ernie,

JMHO, based on doing a lot of heads here in-house since 1978,......

The root of the issue is worn & loosely fitted guides at the factory so simply changing guide seals will not change or improve anything.

The real and proper fix is to replace the guides (hopefully, with better ones) and fit them properly. Anything else is a band-aid and delaying the inevitable.

One thing to remember is that when oil consumption goes up, the engine's threshold of detonation goes lower due to the low octane rating of engine oil and excess carbon deposits.

FisterD

In my case, my car had 42K and the SAI issue when I purchased it back in '04. After monitoring the oil consumption, or lack there-of, I deducted that the valve guides were probably in decent shape. Sure, a top end rebuild with improved valve guides at that time would not hurt the longevity of the engine and it would solve my SAI problem, but I did the clean-out as a viable option and saved many thousands of dollars.
At 7 years and 30K miles so far, that is one hell of a band aid.

Now if your valve guides are truly in bad shape, then yes, I too would consider the clean out to be a band aid at that point. But in my case it was the way to go.

You know I have major respect for you Steve, as you have probably forgotten more about Porsches than I will ever know, I just don't think the SAI clean out is always a band aid fix. Especially not in my case.

Mike J

I agree Darin, I think the SAI cleaning is a good preventive maintenance, and you can get a lot of miles even if the guides have some wear. I have seen clogged SAI ports with car with very low consumption, and the same with oil burners.

I thought the biggest deal with worn guides was the heat transmission from the valve stem to the head, so the valves run hotter, and the possible hammering of the valve head on the seat, but that would be with excessively worn guides.

Oh, but regularly I would say every 40-50K on cars that are in good shape, but its mostly driven from the owners - they like it cleaned more often.

Darin, what have you seen? You must get cars in the shop that have SAI cleaning as part of the owners ask?

Cheers,

Mike

earossi

I am assuming that the source of oil that cokes off the SAI ports is oil that is sucked into the annulus between the valve guides and the valve stems. This can only happen if you have a failure of the valve stem seal (my assumption). Since an air cooled motor sees a lot more temperature in the heads then water cooled, I can assume that the valve stem seal materials degrade fairly quickly with time and heat cycles on the motor. So, I would not be surprised to see the failure of the elastomer O-ring seals early in the life of most engines. Of course, prematurely worn valve guides can also be a reason for premature seal failure. But, on engines that see coking, but are not oil burners, could one of the causes be oil leaking past a failure valve seal and not a worn valve guide?

I have never changed valve stem seals on a Porsche engine, but have done it on American engines, and it is fairly straight forward. The key would be getting access to the valve springs to compress them with the motor in the car. If this is not feasible or practical, then changing out valve stem seals is a bad idea. But, if one can compress the springs with the motor installed, then changing out the seals should be relatively easy and I suspect requiring about the same effort as to change out hydraulic lifters on a 993 motor.

And, though I have not researched it, I would hope that the seal materials available today are superior to the elastomers that were installed in the 993 14 to 18 years ago.

If this procedure were practical and cost effective, it would be very easy, once the valve spring is removed from the valve, to determine how much slop exists between the valve stem and its guide.

Just my 2 cents.

Mike J

Well, if the seal is too good, then the guide will be starved for oil, so the seal has to let in some oil. The tops of the valves are not that easy to access when the cam carriers are still in place, so that adds some complexity.

The oil that gets to the valve head is due the normal oil being applied to the top of the stem, but because the clearances are larger than new (given wear), the oil can move from the top of the stem to the valve head easily. I do not think this be fixed with a new seal, but it would be interesting to try.

Cheers,

Mike

Steve Weiner-Rennsport Systems

Hi Ernie,

Porsche uses the same Viton seals that were found in the 3.0 litre and later engines. These simply grow hard and brittle from heat & age so we replace them when we recondition the heads. In my experience, changing guide seals to arrest an oil consumption problem has been fruitless and to date, I've never seen that fix the problem.

One can measure valve guide side play to get a "Kentucky windage" idea of guide wear & taper, but that won't show you how much the valve may be moving around on the seat. We really like to measure the taper and ID of the guides and valve stems as well as inspect and measure the seats for concentricity.

I suppose this is the time I should say that I have no "dog in this hunt"; I'm not trying to sell anyone anything. My only purpose for even posting here is to offer unbiased information gleaned from doing Porsche heads here for 30+ years. I'd suggest speaking with other Porsche machine shops that do 911 heads for their experience, too.

I'll conclude by saying that when these heads are all done up properly, oil consumption is no longer an issue unless there are worn piston ring lands and rings. I'd also recommend having the SAI passages enlarged in the heads and cam housings to help prevent any recurrence of this problem.

Hope this helps,

earossi

Thanks Steve,

The reason I posted this "idea" was to prompt input from those of you that have worked on these issues. Out of curiosity, what diameter are you opening the SAI passages up to? And, have you been able to go back into those engines to see if the larger ports gather any carbon?

Incidentally, based upon your comments and others, it appears that my "idea" was probably not that good of one! Can't fault me for trying.

jimbo3

My understanding is that a 993 with a clogged SAI system but low oil consumption indicates a bad SAI check valve, which will allow normal exhaust particles to blow back up the SAI passages and eventually clog them.

The previous owner of my car had the passages cleared at 36,000 miles and replaced the SAI check valve. At 46,000, there is no sign of a recurring issue and oil consumption remains at ~1/2 qt every 2000 miles, which seems to indicate that there isn't a valve guide issue.

Lorenfb

My comments years ago:

http://www.systemsc.com/pictures.htm

race911

Who else, but from Steve (or maybe me when I'm in a pissy mood) will you hear discussion about intake stem taper and ring land wear. I've experienced about .00001% of what Steve has in rebuilding these things, but these are a couple of the most critical things to note as wear items on an air cooled engine.

Plenty of engines in these cars run fine. Doesn't mean they're in fine working order against the desired specs.

(Personal worst valve guides I ever saw were a factory 15K miler on a thermal reactor equipped 2.7L car that puked a tensioner. We laughed. I've had a handful of 30-45K virgins apart, that had also puked tensioners. To think we'd have reused any good guides was laughable.)