BMW M5: Carbon build up done the right way

By August 7, 2017BMW, MAINTENANCE, SHOP

BMW M5: Air Pump Repair (Secondary Air Pump System Carbon Build Up Repair) – Done The Right Way!

We believe in taking the time to do repairs the right way and our BMW Service is no exception.

Chuck’s 2002 BMW E39 M5 has been suffering from the annoying CEL (Check engine light) caused by clogged SAI ports. There are short cuts like patching the software to trick the system and turning the CEL, or different injectors. However, software patch will fail federal emissions and fail the inspections. Aftermarket injectors or rerouting, eliminating SAI also not favorable options. Quite work, but sure is worth for one of the last true //M cars every produced.

“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, normal 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.” *Credit: Turner Motorsports

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