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Thursday, February 29, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Wednesday, January 31, 2007

Emissions Woes

Last weekend I was on my way back from lunch when the Check Engine light illuminated again. I didn't need to connect the GT1 to know why. This car has had a long history of storing codes about the bank 2 (cylinders 4,5 and 6) catalyst efficiency, but it wasn't until last year that it started to trip the check engine light (CEL) on a steadily increasing basis. Last weekend's event was a mere three weeks from the last event, and I wasn't interested in dealing with the aggravation of running to my dealer and bugging my technician to clear the code every five minutes, so after my tech cleared the codes yet again Monday morning I committed myself to come up with a permanent solution for this problem -- a solution that would require a lot of research.

The post-1996 (OBD II) 328is exhaust system utilizes the tubular stainless steel headers and mid-section from the M3 to provide a dual exhaust configuration. Dual exhaust is great from a performance perspective because it lets the engine breathe, but it hurts the wallet when it comes time to replace emissions components because there's two of everything. Each pipe has an oxygen sensor positioned in the header that controls the fuel mixture, a catalytic converter that helps reduce harmful emissions, a second oxygen sensor to verify how well the catalyst is doing its job, and a resonator to reduce noise and tune the exhaust note. Only at the muffler are the exhaust gases recombined to reduce noise to conforming levels.

The catalyst efficiency code results when the second / downstream oxygen sensor reports the emissions are out of the acceptable range and thus the cat isn't doing its job. Failure of an oxygen sensor or its integral heater element triggers its own code, so I'm about 90% sure at this point that I need to replace the cats. They come as a unit with the mid-section, so I'm really looking at replacing a major exhaust component, the retail price for which is $2000. Fortunately, Jason at Tischer BMW quoted a far more reasonable price, but $1500 and change is a huge nut to crack.

There is a small chance that I may not need to replace the cats, however. The cat efficiency warning can be triggered if the engine is running rich enough that the cats can't keep up. I was once told that oxygen sensors either work or they don't, but further research revealed that as oxygen sensors age they effectively report a leaner mixture than is actually occurring. Since the BMW DME relies on the output of the pre-cat oxygen sensor, my theory is that by replacing the pre-cat oxygen sensors I can get the mixture back to spec again and reduce the amount of combustion byproducts that are plugging up the cats and causing the decrease in efficiency. It may be too little, too late, but it's worth a try since they're already 38K miles past their recommended service interval and need to be replaced anyway.

Oxygen Sensor Supply Chain

(Image: New BMW oxygen sensor)So when I began to look for new oxygen sensors I checked realoem.com for a part number and price. It turns out that until a build date of 4/98 (shortly after my car was produced) all four oxygen sensors are the same: BMW part number 11781427884. This part has the notation "SIEMENS/L=990" in the description. Siemens, of course, is the manufacturer of the part and research revealed that L means the length of the lead. 990mm is 38 inches, or a bit over three feet, which is clearly the length needed to reach from the top of the engine, near the fuel rail where the pre-cat sensor plugs are, to the headers. The obvious downside to the BMW part was the price -- $195 on a somewhat outdated realoem price book (the actual price is closer to $225 now), so I went in search for aftermarket sensors. This was an educational experience, let me tell you.

The first thing I learned is that there are two types of oxygen sensors available in the aftermarket. The type required by the Bosch DME is the titania sensor. So, if you do decide to buy aftermarket sensors or are intent on trying to fit a universal sensor to this car (that practice is actually discouraged by many DIYers), no matter what you buy, you must make sure they're of this type. If you don't, at the very least the car will run like crap and you'll be out the cost of the sensor.

The second thing I learned is that while Siemens or Bosch are technically listed as the parts supplier on BMW's ETK, the sensors are actually made by NTK -- a Japanese company. This is no surprise, really. Siemens and Bosch are smart enough to realize that they can't build everything themselves and they source parts from other manufacturers just like BMW does. The problem comes in when you add one or more tiers of profit margin onto an otherwise reasonably priced sensor -- it gets expensive very quickly.

NTK actually makes the sensor (NTK part number 25013) and sells it for about $80 in the aftermarket (courtesy of bimmerparts.com). They also sell the sensor to Siemens and Bosch (Bosch part number 13844), who then turn around and retail it for about $110 under their respective names (courtesy of bavauto.com). BMW contracts with Bosch to provide the BMW OE part with their logo stamped on it, but as all automotive companies do, they tack on about 100% margin. This results in the $225 retail price that you or I would pay at the dealer's parts desk.

At this point I wondered if I could really trust the NTK sensor to be identical in function to the Siemens / OE sensor so I went in search of user feedback. Sure enough, I found several reports of the NTK 25013 parts failing prematurely. Now, I know full well that the supply chain works in such a way that the parts that meet OE specs are sold to the OE, while parts that don't necessarily meet that spec but are otherwise marketable are sold in the aftermarket, so it wouldn't surprise me that NTK plays this game as well. But since a failed pre-cat sensor causes the engine to run rough and can damage the cats, while failed post-cat sensors simply trip codes, I didn't want to take a chance here. I ultimately decided to buy the OE sensors from Tischer (at $155 each) and see if they were stamped with NTK as some people said they were. The sensors arrived today and I hastily opened the box to find my new sensor that had the familar BMW logo stamped on it...right next to those of Siemens and NTK.

So, I think it's safe to say at this point that NTK is the sensor OEM and Siemens just reboxes them for BMW, but I'm not 100% confident that they are the same exact sensor made to the same quality standards. If you want to save $60 a sensor, you're welcome to test that theory, but for me, I'll stick with the OE sensor for the pre-cat application so I don't give BMW any excuses to deny warranty coverage for the new cat when or if it's installed. I am, however, more than likely to go for the NTK or Bosch branded parts for the post-cat units. I'll be pissed if they fail prematurely, that's for sure, but at least I won't be pissed that they destroyed my $1600 mid-section or left me on the side of the road with a rough running engine.

Mileage: 138800