Hey folks,

I swapped out the engine harness on my car this last weekend and wanted to share my experience for anyone else that ends up having to do the same. The car is a 2011 with the M272 engine. This guide should more or less apply to everyone with this engine and probably the M273 to a large extent. I changed the harness because I was finding oil in an uncomfortable number of connectors, namely the ECM connector. A brand new harness itself was only about $500 before shipping/taxes from an online dealership, and the sensors I replaced were around 300. I decided to swap a number of sensors either because I was worried about them contributing to the oil saturation or probably being due anyway based on age.

Since the intake manifold needs to come out for this, consider replacing it while you're at it if needed. Mine was black inside from the PCV, and had a shocking quantity of oil sloshing around inside. I decided to go ahead and swap it out with a newer spare I had picked up at the pick n pull a while back since everything on it was newer and fresher. I was prepared to swap out the vacuum hoses on the front since they were crumbling pretty bad, but I knew that snapping off old plastic nipples was a risk.

I highly recommend changing the crank sensor. To get to it you need to remove the vacuum pump on the back of the driver's head. They get lazy and go bad with time and age. It's not fun to get to, but it's comforting knowing you'll probably never need to touch it again. Since the vacuum pump needs to come out, it's a good idea to change its gasket while you're at it (0219973645 for me).

There's a coolant valve (2722000031) on the back of the driver's head near the vacuum pump that you may want to replace. Its plastic body degrades over time and can break. Be warned: the last engine I pulled one out of broke off inside the engine and getting the bits out were not fun, even with the engine removed from the car. If you do that, you might as well replace the coolant sensor too since you'll have the coolant drained anyway. Food for thought.

Have a can of electronics cleaner handy. I used it to clean out any connectors that weren't getting replaced. I followed it up with an air compressor blast to make sure it was completely dry. The air compressor proved to be very useful as well. I used it to blow out as much of the dirt sitting around the intake manifold and followed it up with the vacuum cleaner for good measure.

Here's a list of what the harness connected to:

• Spark plug coils (plus a ground point on each cam cover)
• Intake manifold temp sensors (one on either side of the intake manifold in the back)
• Fuel injectors (half of them were swimming in oil)
• Two connectors going to the other harness connected to the other side of the ECM
• Passenger side oxygen sensors (behind the cylinder head, mounted to the transmission)
• Driver side oxygen sensors (runs down driver's side and connects on the bottom of the transmission)
• A coolant valve mounted on the rear of the driver's head
• Coolant sensor (near the valve and crank sensor)
• Crank sensor (mounted on the top of the transmission on the driver's side. Replaced this sensor)
• Knock sensors (mounted underneath the intake manifold, requires removing the manifold. I hear they last forever, but at $10 for 2 each I sprung for new ones)
• Throttle body
• Mass airflow sensor (replaced this, they also go bad with time, so why not. Not difficult to get to independently, though)
• Back to the front of the engine, the thermostat housing
• Blower fan on front of engine
• MAP sensor (mounted on the driver's side of the blower motor. This sensor was replaced, but not hard to get to)
• Intake manifold flap solenoids (next to MAP sensor)
• Camshaft sensors/adjuster magnets (replaced the sensors, left the magnets as they had been replaced recently)
• Secondary air injection (I think that's what it's called) solenoid (near the cam sensors/adjusters)

These next few are farther down the engine, more out of sight

• Alternator (small connector, I think it's the regulator)
• Oil level sensor (bottom of the oil pan)
• Power steering pump (had to loosen power steering fluid reservoir from head to provide enough clearance for this and the next two connectors)
• A/C compressor (these connectors were welded together and I ended up breaking the connector. I got it connected enough and taped them together for now. It looks like I can replace the small harness that's on the compressor body later)
• Starter motor (the activating solenoid on it, I believe)

Here are some other parts I changed while I was at it:

• Intake manifold (the old one looked rough and I had a newer spare)
• Intake manifold gaskets
• Gaskets on the air duct elbow between the throttle body and MAF sensor (2721410780 and 2721410880)
• Sacrificial camshaft solenoid pigtails ($10 each because the old ones were pretty wet 2711500156)
• Fuel injector o-rings (the old ones are not only old, but they'll very likely tear upon removing the fuel rail even if they're fairly new. Make sure you lube the new ones with oil before reinstalling the fuel rail 013997904564)

I hope this helps someone at some point. Happy trails!