My Rennlist Write-up

SAI System Overview

The 993 was arguably the best air-cooled variation of the 911 that Porsche ever built. However, its greatest shortcoming has to be the Secondary Air Injection (SAI) system, an emission control system found only on United States' variants and not Rest of the World (ROW) models. Yet, the U.S. is always criticized for not giving a crap about the environment - SCREW YOU GUYS!!! But I digress...

In a nutshell, the SAI system consists of an electric air pump, a couple of valves, and a series of tubing and ports that direct fresh air directly aft of the exhaust valves. The fresh air mixes with the hot exhausted gases and assists in completing their combustion, thereby, leaning the exhaust. The reaction also brings the exhaust system up to operating temperature quickly which is necessary for clean emissions.

Layout of 993 SAI system

The system typically operates for about 2 minutes after starting a cold engine. If the engine is already warm, a thermostat inhibits the system from activating.

The problem with 1996 model year 993's and later was the introduction of On Board Diagnostics II (OBD II). This diagnostic system, largely meant to find flaws with the vehicle's emissions, monitors the SAI system on the 993 and illuminates the "Check Engine" light (CEL) if there is a problem. In many states, the car will not pass emissions testing with the CEL lit.

So, what faults in the SAI system trigger the CEL? Often times, the CEL will illuminate because the SAI ports in the cylinder heads have become clogged with carbon soot and fresh air is no longer able to reach the exhaust system. The overly "rich" emission is sensed by 4 oxygen sensors and a fault is recorded.

Except in extreme cases of fouling, when a top-end overhaul is required ($$$$), the ports can be cleaned and even unplugged with a little elbow grease and a LOT of solvent. The general idea is flush out the SAI system (tubes and ports) with some kind of carbon clearing fluid so air can flow unobstructed again. I've done this on 3 993's with positive results.

SAI System Parts

First, you're going to need some parts (about $150 dollars worth). Here they are with the proper Porsche part numbers (you're welcome):

  • (1) SAI Check Valve: 993-113-250-03 (Many believe this check valve fails and allows carbon from the exhaust to "back flow" into the SAI port and clog it.)
  • (12) Lock Nuts: 999-084-052-02 (These hold the heat exchangers/exhaust manifolds to your heads. Do NOT reuse the old nuts.)
  • (6) Sealing Rings: 993-111-195-00 (These rings fit between the head and the heat exchangers. Again, do NOT reuse the old ones.)
  • (2) Gaskets: 928-111-127-02 (Gillet) -OR- 930-111-192-06 (Bischoff) (These gaskets fit between the heat exchangers and the rest of the exhaust system. Do NOT reuse... OH, you know!!!!)

SAI System Flush Procedure

1. Remove the air box and detach the rubber hose from the top of check valve.

2. Get the old check valve out. This is often the toughest part because the part's threads corrode in its seat and seize it. Some people make a special tool to remove it. I made the special tool as well (shown below), but used it to install the new valve, not remove the old one. Instead, I sprayed some PB Blaster onto the threads and ground a small notch into the side of the check valve with my Dremel tool. A few whacks with a cold chisel and the thing was out. A friend of mine achieved similar success using an air chisel!!!
Using a chisel to remove valve
Try not to damage the delicate stainless steel tubes. Those things carry the fresh air to the ports.

3. Clean the threads where the check valve sits before you install the new one. I used a wire wheel in my Dremel tool.
Threads where check valve goes

4. Get underneath the car and detach the orange heater hoses near the sway bar. Also, detach the black heater hoses above the heat exchangers. Now, remove those 12 lock nuts (the last one above the sway bar is a bitch) and try not to drop the heat exchangers onto your head... they're heavy! At this point you can now look up and see your exhaust valves and the infamous SAI ports.
Photo of exhaust valve and port
Make sure you get all 6 old sealing rings out. If any are stuck to the head, they'll have to be pried out before you can reassemble the car.

5. Ok, you have now opened up the system for flushing. First, you'll need some way to route cleaning fluid into the air manifold (where the check valve screwed in). I used the following tip and cut open the OLD check valve and brazed an air compressor fitting into it so I could attach a hose.
Check valve cut open

Air fitting brazed into valve

6. There are as many different ways to run fluid through the system as there are people doing it. The first time, I filled the air manifold with solvent (Techron & Berryman's B-12) and then connected my air compressor to the fitting I made above and blew the fluid through the system (place drip pans below the engine before you turn your garage into a Superfund Clean-up site).
Some cleaning fluids
I did this repeatedly, but I noticed more air than solvent was getting pushed through the passages (the air manifold simply doesn't hold a lot of fluid). So the compressed air was simply pushing that small slug of fluid along the walls of the tubing and ports coating them more than cleaning them. So, I devised another trick.

7. I modified a common garden sprayer so I could force a LOT of solvent through the SAI system under positive pressure. This drawing is what I was thinking at the time.
Drawing of fluid canister
Then I used lacquer thinner. It's a powerful solvent (wear gloves and googles), available by the gallon, relatively inexpensive, and dries quickly.

8. In addition to forcing fluid "down" the system, I also shot some solvent "up". Using a spray can with extension straws coupled together, I inserted the straws as far up the SAI injector ports as I could and sprayed.
Spray can with long straw

Drawing of spray can in port
After spraying into one port, I noticed fluid coming out adjacent ports. Must be clear up there!

Note: One car I flushed actually had some clogged ports, i.e., fluid wouldn't flow through it. The owner and I chucked up a short length of steel cable (picture hanger wire) to a Dremel tool and, using it like a sewer snake, we Roto-Rootered the ports until they were clear.

9. Blow a few tanks of air through the system to dry it. Then let it air out overnight if you can.

10. Clean the exhaust studs and mating surfaces so the heat exchangers and heads fit together nicely.

11. Install the new check valve. I rubbed some anti-seize compound on the threads first. Then I used my special tool to tighten it 1/8 of a turn. I see NO reason to really torque this valve down. With the rubber hose attached to the top, it isn't going anywhere!
Special tool
I bought a relatively cheap 1-1/16" wrench since 27 mm wrenches aren't easy to come buy, at least not inexpensively.

12. While you have the anti-seize compound out, use it to "glue" your new sealing rings in place while you bolt the heat exchangers back in place (with your new lock nuts, remember?).

13. Reconnect the exhaust system. I used new stainless steel hardware, but you can use the old stuff here.

14. Reconnect the heater hoses. Don't forget the black ones on top.

15. The last step is tricky, reattaching the air box. The best advice I got was loosen the Mass Air Flow (MAF) hose clamp on top of the engine. You'll need a long shafted phillips screwdriver to reach the screw. After it’s loose, the entire MAF can be rotated. Reference the following pictures, because this worked like a charm. Fortunately, I have small girlie hands and can reach between the intake manifold and the compartment’s overhead. First, rotate the top of the MAF forward (towards the front of the car). Next, insert the air box intake port into the MAF. Finally, rotate the MAF aft locking it onto the air box. Geee, what a PITA… thanks Stuttgart!!!
Screw on MAF hose clamp

How to rotate the MAF

16. Replace the check valve every 15,000 miles and hopefully you'll never have to flush this thing again!!!

Punch it back to the top