Carbon Build-up In E39 M5 Secondary Air Pump System (SAI)

Like any modern car, the E39 M5 utilizes an air pump as part of the emissions system. The air pump, and related air passages and tubing, is designed to pump fresh air into the exhaust system ahead of the catalytic converter. As the air is heated it helps to combust the un-burned fuel coming from the engine as well as bring the catalytic converters up to operating temperature sooner. The system only runs for 1-2 minutes on start-up, when the car is cold (temperature based on coolant temp).

The problem is that carbon builds up in the passageways for the air pump system. Carbon is a by-product of the engine's combustion. But this issue is different than the normal carbon build-up from inside the combustion chamber. Normally, the carbon exits the engine with the other exhaust gases through the manifolds, cats, and out the tailpipe. However, the backpressure in the exhaust forces some carbon back into the cylinder head and into the air pump system where it settles and hardens. There is a shut-off valve that is designed to prevent exhaust from re-entering the system but this valve can become faulty over time and with no warning. Eventually each passageway and air port becomes clogged and blocked, as much as 100%. The layout and design of the system, which includes several 90-degree turns, inadvertantly encourages the carbon to build-up. The carbon is especially heavy during the engine's warm-up period when the mixture is fuel-rich.

With the system blocked, air ceases to flow or is dramatically reduced. The M5's emissions system monitors the air flow and it will not like to see it reduced or stopped completely. It will interpret that as a failure of the air pump, tripping a fault code, and illuminating the SES/MIL light on the dash. Since the fault is emissions-related, it will likely prevent the car from passing a state emissions inspection test (either for the presence of the SES light or from the fault code itself). So even though the air pump may be functioning normally, it is being blamed for the reduced air flow when it's really due to the blocked air passages.

Even if the system did not monitor the air pump flow, there would still be the problem of excessive carbon build-up in the cylinder head. Removing the carbon is a tricky process and involves disassembling the top end of the engine, pulling the cylinder heads, drilling or breaking up the carbon in multiple places, cleaning (decking) the head, replacing all of the valves and the valvetrain, along with any other customary head machining. Repair bills can run into the thousands of dollars and weeks of down-time. Cleaning the carbon out will make the engine 'carbon free' but the problem is likely to re-occur. It may take several years, and many tens of thousands of miles, but the design flaw of the system guarantees that it will happen again.

As of now (September, 2011) there is no viable alternative or permanent fix for this issue. There is special software on the market that will tell the car's computers to ignore the flow of the air pump. This will keep the fault code and related SES light from appearing. These software tricks are not approved by BMW or the EPA and are not a guarantee that you will pass an emissions inspection test - but it will keep the code and the light away. Any modification to the emissions system is against federal and many state laws, which is probably preventing any hardware solutions to be implemented as well since any change would likely have to have BMW's approval, as well as be thoroughly tested by the federal EPA, CARB, and any other regulatory agency.

While carbon deposits are inevitable, there are several things you can do to delay the problem from becoming severe -
+ Use premium fuels (the best you can find) that will burn better and not create as much carbon.
+ Regular oil changes.
+ Monitoring the efficiency of your cooling system (including regular replacement of your coolant temp sensor, located in the lower radiator hose) (part number 13 62 1 433 077).
+ Cleaning or regular replacement of the HFMs (mass air meters, part number 13 62 1 433 566), CRC HFM Cleaner.
+ Regular replacement of the SAI Shut-Off Valve (supposed to prevent blowback of the exhaust) (part number 11 72 1 433 713).
+ Regular replacement of the front (pre-cat) oxygen sensors (part number 11 78 1 742 050).
+ Non-sluggish driving (regular WOT runs up to red line).
+ Removal and cleaning of the front aluminum secondary air pipe (major job but you might be able to clean out the carbon before it accumulates).

Images (courtesy of user mertm5 on m5board.com)

Front of cylinder head.	Rear of cylinder head.
Secondary air ports in the combustion chamber.	One of the system's 90-degree bends.