Porsche 993’s “Achilles heel” THE S.A.I. SYSTEM& how it relates to the "CHECK ENGINE" warning light
FIRST SOME HARD FACTS:
S.A.I. is an acronym for ” secondary air injection”
This air injection system is part of the vehicles emission control compliance. It does not negatively effect the vehicles performance when correctly operating. This system does significantly lower harmful tail pipe emissions.Any vehicle manufactured with such a system, driven in the U.S. must have the system fully operational within designed limits or it will cause said vehicle to exceed maximum allowable emissions. When certain situations arise to cause an actual failure, or, conditions exist that the vehicles on board computer diagnostics predict such an occurrence is likely to cause elevated levels of emissions, it will set a fault code in the ECU (electronic control unit) and turn on the “check engine” light. Any vehicle with a check engine light on will not pass state mandated annual inspections.
MOST FREQUENTLY ASKED QUESTIONS:
Q: How do I know what is the cause of my “check engine light”
A: Excellent question, the answer is, you don’t. In fact no one will know which one of, or combination of, the 144 possible fault codes that can cause the dreaded “check engine” light to illuminate on your dash. Until, a scanner and qualified tech pulls the code or codes and analyzes the data. Codes PO410 and PO411 indicate air injection problems on one or both cylinder banks. Other fault codes for mixture trim out of range and a myriad of oxygen sensor codes may accompany the SAI codes if the system is not repaired and the car is driven with the light left on.
Q: My check engine light is not on, what preventative maintenance can I do?
A: The system has no maintenance intervals, cleaning the lower ports is not advisable or recommended unless or until the problem occurs
Q: Is it inevitable that all 96-98 993’s will experience this failure?
A: Eventually, probably yes
Q: Is the likely hood of failure higher with age and mileage?
A: We found patterns and driving habits that seem to effect this situation. Among them are track driven cars seem to rarely have this problem, or it happens at very high miles. 993’s driven around town short trips, a/c on high all the time, seem to be more frequent. We have done an alarming amount of cars from Florida and Texas. We recently have done 4 cars with less than 50,000 miles on the odometer, two of them used a quart of oil every 1,000 miles. This oil consumption was thru the valve guides, our simple external cleaning would not work. The passages had to be drilled out because the carbon was so dense.
Q: What are my chances of solving my problem with Bodymotions’ “engine in car” port cleaning?
A: Actually very good. We have repaired roughly 100 of these SAI problem cars. Some have been driving for 5 or six years with out a return problem. We test the car and can give you and idea because we flow test the system before and after and can predict by the results how long the artery cleaning may last. the permanent solution is to replace worn valve guides and or rings if necessary and ream all the ports with the engine apart.
Q: How much does it cost?
A: The external, leave the engine in cleanup testing and reset is $1295
Q: And if it is too far gone?
A: Engine removal ,reinstallation, disassembly of upper cam housings, gaskets and reaming is around $4,000
add $1500 if the valve guides need replacement
Q: What if I ignore the light, its been on for a while and the car runs great?
A: See above comments regarding repair chances of success, the longer the car is driven with the check light on, the worse the compaction of carbon becomes, eventually requiring total overhaul vs. less expensive in- car option
Q: How long will Bodymotions’ clean up cure last?
A: Depends on all the variables above, but we have ,to our credit, gotten many cars not only thru inspection, but extended by years, the need for engine teardown.
TECH BABBLE AND HISTORY OF SECONDARY AIR INJECTION
The above situation is accurate for all cars produced from 1996 and newer. The manufacturers have, over the years devised different ways to achieve lower emissions. There are many systems that interact to achieve that common goal. That goal , of course is less carbon monoxide, carbon dioxide, hydrocarbons, and oxides of nitrogen. These oxides are the real green house killers and complete burning of the spent exhaust gases and filtering thru the catalytic converter are key to today’s drastically lower emissions.
Back to the S.A.I. system in Porsche. This air injection system has been in use since the 1970’s. The early belt driven air compressors took horsepower from the engine to drive them. So, you guessed it, most were removed in the interest of performance. Today the pumps are 12 volt D.C. driven. Very powerful, they deliver, on command from the ecu, pressurized atmospheric air, to the exhaust system, upstream of the catalytic converter(s). The air, atmospheric, same as we breath with 21+% oxygen content, mixes with the red hot exhaust gases and causes an additional burning. This additional combustion, in conjunction with the three way catalytic converter completes the job. Almost breathable at the tailpipe. Kind of like a sewerage treatment plant that boasts drinkable water on the outlet pipe. Maybe its true, but I’ll pass on being the test dummy.
In years past the delivery system included a one way valve, also called a diverter or gulp valve. This one way valve did just that, allow fresh air to flow in, but kept back hot exhaust gases from back flowing and damaging the pump and hoses. Also used in the past were external piping. This plumbing system, usually made of stainless steel tubing, delivered the air to the external exhaust manifolds. Due to the harsh environment these plumbing systems quickly corroded, collapsed and caused under hood exhaust leaks.
Most manufactures, including Porsche, now integrate the plumbing system into the engine block, or cylinder head casting. This eliminates the external plumbing and associated problems, but creates other glitches. This is the area of concern in the 1996-1998 Porsche 993 3.6 liter non-turbo engines. 1999 and newer liquid cooled cars have the same system, but, the delivery is thru tunnels and portals cast into the one piece cylinder heads. These ports are huge and are not subject to the clogging in the earlier air cooled models. The air cooled 993 has small passages (5 and 6 mm diameter) in the camshaft housings. These are the capillaries that connect the fresh air source to the cylinder heads. these passages are not accessible with the engine assembled. The connecting passages into the cylinder heads are accessible from the exhaust ports once the entire exhaust system is removed. Once the clogged passages backup into the cam housings the clean up is very difficult or plain impossible.
If you have followed along so far, or if you skipped the disorte’ above, we have arrived at the problem. the clogging of the arteries. Porsche did not use one piece cylinder heads until the liquid cooled engines arrived in 1999. Therefore the job of plumbing injected air utilized various dilled passages thru the camshaft housings first, then threw portals in each cylinder head that lead to the exhaust port behind each exhaust valve. It is because these passages go thru both the cylinder head and the cam housing they follow routes that have angles causing carbon to lodge and built up. This build up of carbon eventually restricts airflow to each cylinder. When one or both banks are restricted enough to a point that causes the exhaust gases to emit, or potentially emit harmful emissions, as monitored by the secondary oxygen sensors, it triggers the check light. Once this fault circuit is energized the car will not pass emissions certification, and will be rejected at state inspection. Since Bodymotion has been both N.J. State inspection Facility and, licensed Emissions repair facility and we repair and service Porsche’s for 25 years, we see these problems frequently. In fact Bodymotion Inc has calculated that over the past 10 years we have now repaired around 100 of these S.A.I. problem cars.Our technicians have developed both mechanical and chemical remedies for all but the worst cases. These worse cases are usually those cars that have been driven while the check light has been on and ignored. Also those driven locally, extended curb idle time and always those with high oil consumption and heavily worn valve guides. Once cleaned, the ports are flow tested and compared to known flow rates, it is at this time we can predict the success of our repair in times of immediate results and long term health of the system.
993 cylinder head, this is what you see when the cam tower is removed, the passage that the pointer is indicating is 6 mm diameter and goes directly to the exhaust portbehind the valve
Underside of camshaft housing, this teardrop port is what mates to the cylinder head below it.
Two different views of the camshaft tower, at the top is the threaded hole that feeds the fresh air to the top of each tower. The large passage that runs inside fore and aft is of sufficient diameter and does not clog, the 6mm channels from that supply log down to the head are the areas of heavy and inaccesible carbon blockage