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Wednesday, December 6, 2023

Doug's Domain

Doug Vetter, ATP/CFI

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March 2, 2004

Ordered shocks, scheduled maintenance for the following week, (unrelated) notes on oxygen sensors and catalytic converters.

The inspection light came on a few days ago, and that means it's time for some routine service. But, this time I won't be doing the routine Inspection I. I talked things over with my mechanic, and he and I agreed that it makes little sense to pay for an inspection, given that he's seen the car a lot recently, he's done a lot of work that would be recommended or performed at the same time as the Inspection, and he always gives the car a good look when the car is in for any reason...even a simple mid-cycle oil service.

So, this time around, I'll be dispensing with the formal "Inspection I" and instead doing the few "concrete" things that are performed at the inspection instead...specifically:

The difference between the Inspection I and this "a la carte" plan? About $400.

I have absolutely no problem paying people to do a job, but in this case there simply wouldn't be much of a job to do, so I just couldn't justify spending almost six bills on the inspection when I had other work planned...which brings me to the reason for this report.

While riding over bumps the ABS will activate when the REAR tires pass over the bump. I've noticed this only in the last 20-30K miles or so, and, I guess because of all the frost heaving in the roads around here, I've noticed it a lot more most recently. Since the job of a shock is to dampen oscillations resulting from the compression/extension cycle of the spring and keep the tire in contact with the pavement, I took this to mean that it was time to do the shocks.

When I went to order the parts, I found there are two types of shocks listed for my specific model and VIN. One is called "Motorsport", and those are made by Bilstein. The second is called "MSport", and those are made by Boge. The question of the day was "well, which one is it?". My car has the "Sports" suspension, but NOT the M suspension. If you'll recall, when I had the struts done, they initially ordered the wrong (M) struts for the car, so I began to think maybe BMW got their part numbers crossed for various suspension components. The parts guy called the BMW tech line, and they said the "MSport" shocks are correct for this application.

Naturally, I was still a bit skeptical. When I told my mechanic the situation, he offered to put the car up on the lift to pull the numbers directly from the existing shocks, just to eliminate any confusion. Five minutes later we had the number "1090831" off one of the shocks. That so happened to be the "MSport" (Boge) number, so the tech line was right after all.

The shock mounts in this car are purposefully made soft, and they're a "hot button", so we'll be replacing those as well. There's also a heavy paper gasket that goes between the body and the shock mount, which prevents moisture from getting up into the body. They are replaced by default, and at a mere $0.50 each, they should be. All parts are on order and will be here in plenty of time for my appointment next Thursday.

Side Note: Oxygen Sensors and Catalytic Converters

My mechanic and I briefly talked about another car in the shop he was working on. It had an oxygen sensor problem, and since I've received more than a few emails about ox sensors, I asked him a couple of questions related to it.

First, I asked, "If the oxygen sensor fails, will that produce a code (and turn on the check engine light)?" He said that the ox sensor is one of the things most likely to produce a code. The point being, that if the ox sensor fails, you'll know it....so don't go replacing the ox sensor for fun. Replace it "on condition" as they say in the aviation biz -- in other words, when it fails.

Second, "Is it safe to drive the car with a known-failed ox sensor?" Turns out (contrary to my original belief) that on the E36, it IS okay, simply because if the ox sensor fails, the computer knows this and uses default "mid-range" values for mixture control, so it is unlikely to damage the catalytic converters. The car may not run well...particularly at altitude or temperature extremes, but it will run. Prior to the E36 series (pre 1991) the computers lacked any kind of "default" values, and would refuse to run if the ox sensor failed.

Lastly, "If there is a misfire, will that cause damage to the catalytic converter due to a rich mixture and an "afterburning" in the exhaust?" Turns out "no", simply because the computer knows what cylinder is misfiring and will turn off the fuel injector for that cylinder. The coil (spark plug) will remain operational, however, to ensure that anything in the cylinder is burned off safely. I know from experience that a hard miss makes the car run like trash, but it's nice to know that in an emergency situation (dark road out in the sticks on a rainy night), you can drive it to the service location without fear of destroying $2500 worth of exhaust components.

Oh, and speaking of exhaust components, I asked him how long the exhaust systems typically last in these cars. His answer: "forever". Knock on wood.