Bruce7 -

Thanks for the write up and the socket modification removal mouse trap. I'll be certain to have the socket strategy on hand for the next SAI valve removal.
Now... where will my 27mm crowfoot wrench ground down by 1/2 find use? Go figure.

A 30/40k mile interval SAI R&R has served the 993 owner well. IIRC Porsche AG had been silent on the subject - Isn't 80K miles/ 8 years the required service life of emission systems in North America? Any failed component would be the on distributors/manufactures nickle, no? And to that end, many SAI valve's or SAI bore passages can and do operate well beyond 80K mile threshold.

 

Peter, Function of the SAI valve is to keep the hot, carbon rich exhaust from traversing up the SAI passages versus out through the heat exchanger below the engine. If the one way valve (which works for 99.999% of your driving time since the SAI pump only runs for 2 minutes on a cold engine) fails, the carbon deposits will eventually fully occlude the sai passages which will cause a CEL via the O2 sensor in the catalytic converter. In hindsight, an overly engineered troublesome "emissions" solution. As long as the one way valve works, the passages should stay open since ALL the carbon rich exhaust that enters that area when the exhaust valve opens will go down the heat exchanger.

--Brian

 

Brian, but doesn't that mean if the valve functioned correctly we would not get blocked passages and therefore the cause of all blocked passages could be attributed to the faulty valve? I don't think that is the case. What am I missing?

 

I'm no expert, but I do believe that to be the case. A bad SAI check valve allowed exhaust gases to flow backwards up the SAI passages clogging them with carbon deposits. 50,000 miles on my car. good sai check valve and my passages were clean as a whistle. changing sai valve as a routine item so as not to worry about it for the next decade.

 

Isn't there also some feeling that the faulty or worn valve guides in our cars may be permitting oil to get into those passages and clog them?

True? False?

 

Yes.
Engine oil of the excessive kind getting past worn, sloppy fitting valve guides can contribute to a clogged SAI passage at the cylinder head.

 

I am going to say something crazy, something that some of you are actually leaning towards. I say SAI passage clogging is caused mainly or soley by a non-functioning or leaky SAI check valve. NOT by worn valve guides as urban legend has it. I say routine replacement of this valve will prevent clogging of the passages regardless of miles on the engine.

Even though the check valve is well upstream of the passages in the head, if the valve is holding tightly with no leakage, no exhaust gasses should enter the passages. Much like if you block the end of a straw with your finger and dip the straw into a glass of water, no water enters the straw.

Bruce cut his valve open and it is corroded, surely not able to close tightly, thereby allowing exhaust gases to back flow up into the passages. Now what causes the valve to corrode is the question. A functioning, well sealing valve should not allow moisture laden exhaust gasses to reach it and corrode it. Perhaps moisture is entering from the pump end. Or eventually the valve gets hung up and then corrosive exhaust gasses are allowed to reach it.

I do NOT believe worn valve guides is the source of carbon blockage of the passages. Even if you had heavily worn guides, the amount of oil entering the exhaust port is miniscule compared to the amount of burnt fuel that passes through. Also, the SAI hole in the exhaust port is upstream of the valve guide. It is unlikely any oil leakage from the guide could end up in the SAI port.
Just throwing it out there. Wolfgang

 

Interesting theory. I hope some others chip in on this one.

I wonder if there is a way to analyze those deposits or if anyone has done that. Seems that would prove disprove your theory pretty definitively to see if was partially burnt fuel or oil deposits

 

You have two theorys there. The oil one and the straw one.

Thinking about the straw theory I think it is a little different since there are THREE holes at the bottom ie 3 ports correct? And the pressure on the 3 ports is very likely different at all times since the valves would be opening and shutting at different times which COULD let flow in one port and out another? No?

I think the sai tubing from the point where the first split is (at the entrance to the cam housing?) up to the check valve would follow your idea though.

Hmm. actually the split of the sai line is after the check valve, no? So technically pressure could even leak out to the other side of the engine although unlikely but that just complicates the straw more - there's six holes connected...

 

Hmm, good point. Possibly exhaust gasses flowing through the ports from one cylinder to another. I have not worked my system on my 993 yet, so I do not know the SAI flow paths.

However, I still believe it is not related to oil getting past the valve guides, because the port is upstream the valve guide.

 

OK I think I'm convinced. I had talked myself out of the SAI valve change hassle by rationalizing that the valve guides were going to get me in any case so why bother? That air filter box and mass flow sensor were a BITCH to get back on when I changed the fuel filter!!

My car just past 30K miles and so I think I'll do the change this next summer. Seems there are a lot of good reasons for doing it and very few for not doing it.

 

I edited the original post to include a check for the vacuum line connections
between the shut-off valve and the change-over valve.

Also added one picture to the album showing the vacuum line connections
that are largely hidden.

-bruce

 

I updated the photo album with a SAI system components diagram
and added notes to the original post explaining how the system operates.

-bruce

 

What tool would a Porsche mechanic at the dealership use to remove the SAI Check valve?

 

I see an obvious product idea here. I think there'd be an aftermarket demand for modified SAI check valves that have had their outer lips machined with notches to allow a large 12 point socket to grab it. No more need for a special modified tool that is STILL a tremendous hassle to use in removing the check valve. It'd become as easy to remove as an oil filter with just a bit of machining.

I know I'd pay more for a modified SAI check valve with this convenience built into it. Fix the underlying problem and make the job easier, and the preventative maintenance can happen easier/more often (and less likely to be deferred).