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Subject: PSA: Clean your IAC periodically and replace your oxygen sensors every 100,000 Miles!
submitted by /u/TheImprovedMan
Hey y'all! I wanted to share what finally fixed my idling issue on my 2001 BMW 525i Touring with 180,000 miles! This will be similar to any car with the M54 engine since they share most of these parts between the different displacements, so this can apply to your 3 series, X3, etc. I wanted to do a little write up here, and explain why you should complete these procedures, and also how to do them.
Just a quick explanation of these two critical parts. The IAC is referring to the Idle Air Control valve. It's a servo that, when your vehicle is idling, it will modulate an opening, with a servo, to allow a changing amount of air into the intake manifold. This allows the car to keep the idle steady under different conditions.
The oxygen sensor, or commonly referred to as the O2 sensor, is responsible for two things. The upstream sensors are used to adjust your air/fuel mixture, while your downstream sensors are used to verify your catalytic converters are doing their job. It does this by measuring the oxygen content of your exhaust. The important sensors are the upstream ones. When your car is running lean, you will see left over oxygen in the exhaust since all the fuel was used up, but the air was not. When your car is running rich, the oxygen sensor doesn't pick up oxygen in the exhaust. Our vehicles are built to "switch" between slightly rich and slightly lean 2-3 times a second to achieve an average stoichiometric fuel mixture. A lazy sensor might switch once every 4-5 seconds, and that's exactly what my sensors were doing, so it becomes really hard to run the most efficient air/fuel mixture.
Now, one more things. I understand that I didn't use the "BMW" oxygen sensors. I did some research, and from that, NTK has made sensors for Bosch, who then sells them to BMW to sell at a mark-up. You can save yourself a serious penny if you pick up an NTK or Bosch sensor. As long as you do your research and pick a reputable parts company (NTK is NGK for those who are unaware) you're going to be just fine. These sensors are the NTK 25569 Zirconia sensors, and I got them for around $55 each.
Anyhow, into the repairs!
This is what the IAC looks like on our engines! It comes mounted on this rubber bushings, and the metal bracket is mounted directly underneath your DISA valve. That's the big box with a vacuum actuator mounted on the side of the intake manifold. Two Torx bolts, and a nut to hold on the engine harness and it's off. Before you clean it, use a screwdriver to actuate the valve inside. It should go right back to it's original position. Mine did not, so it definitely needed cleaned. It was also black from carbon on the inside.
Here's the inside all cleaned up. I just used some mass air flow sensor cleaner. Using the screwdriver, I just moved the valve, and it goes right back to the normal position. When reinstalling, just hit the connectors with some electrical connector cleaner (it should be safe on plastics!) and it should be perfect.
On to the Oxygen sensors!
The oxygen sensors we're replacing are mounted on the top of your exhaust manifold. Make sure you mark which sensors go where, since there are two of them. You don't want to mix them up! Using a 22mm wrench, you have just enough room to loosen them, and spin them off by hand. Here they are sitting on my workbench.
These are the old ones versus the new ones. You can see some contamination on the old oxygen sensors, while the new ones are nice and clean. The old sensors still worked, and I didn't have any check engine lights, but it was still worth it to change them.
Here they are installed. You can see just how easy the sensors are to get to. They came pre-lubed with some anti-seize so they won't be hard to take off in the future. The connectors are easy to get to, and they clip right into the size of the valve cover.
Doing both of these procedures changed how this car feels. The car runs much smoother, and throttle application is much more responsive. The engine doesn't "bog down" on hills as much, although it was originally a lot worse (Change your VANOS seals people! I'll do a write up on that next).
Let me know if you have any questions, I'll answer them as best as I can.
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