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Wednesday, October 4, 2023

Doug's Domain

Doug Vetter, ATP/CFI

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October 2002

Well, with almost five years and 68000 miles, it was bound to happen...yes, even to a BMW. I had my first unexpected maintenance problem this month. What do I define as "unexpected", you ask? Well...I know that certain stuff tends to wear out like tires, shocks, struts, and brakes. What I don't expect to wear out by a certain interval qualifies as "unexpected maintenance".

On one of the first cold mornings of the season I went out to the car and started it. I instantly heard a loud whining noise coming from the right rear portion of the engine bay. The whine faded in and out, and then turned into a grumbling noise. Sure enough, the air injection pump was on its last legs. Sounded like a classic case of bad bearings.

(Image: Air injection pump)What's an air injection pump? Pretty simple...a motor with a fan on its shaft that takes air from the engine bay and injects it into the exhaust to reburn any unspent fuel that makes it out of the combustion chamber due to a richer-than-required mixture. This process is required because when the car is cold, the computer runs the engine with a richer than normal mixture to maintain driveability until everything comes up to temperature. If you increase the volume of fuel and don't increase the volume of air, some degree of unburned fuel will wind up in the exhaust, and thanks to the EPA, this is unacceptable, so auto manufacturers are required to ensure that the excess fuel is combined with additional air and "afterburned" in the exhaust.

The typical domestic car solution has been to install an air pump on the car driven by a belt...that runs continuously, whether it's needed or not. The more elegant BMW solution provides an electrically-driven air pump (black in color, shown in the top left of the picture) that runs only for the first couple of minutes of operation (my technician says until the exhaust gas temperatures exceed the level required for the oxygen sensors to go into closed loop, or a maximum of 90 seconds). My experience has corroborated that and I have also noticed that the pump only runs following cold starts.

The pump is one of the more easily replaced devices under the hood, so I figured I'd try to replace it myself, but I decided to talk with my mechanic about it just to be sure. He told me that the pumps typically don't fail themselves...they usually go bad because a nearby check valve, which prevents exhaust from entering the pump when it is idle, has failed first. When the valve fails it lets exhaust into the pump, which destroys it. He also mentioned that sometimes the rubber isolation mounts fail, causing an imbalance that generates a sound like I was describing, but after a brief inspection verified the mounts were fine. He then suggested I have two options:

  1. Since it wasn't so bad as to trip the check-engine light, I could drive it like that. (uh, yea, I want my car whining like a jet engine every morning).
  2. Replace the pump and the check valve just to make sure a faulty check valve doesn't force me to go through another pump prematurely. Needless to say, I chose option #2.

Damage? $200 for the pump, $95 for the check valve, $5 for a gasket, and $140 in labor. The interesting thing is this pump is far quieter during normal operation than the original pump was, so I'm not convinced it wasn't a simple case of bad (indeed, defective) bearings.

Of course, the invoice wasn't the worst news of the day....I was forced to drive an POS Olds for a day, since they had no BMW loaners available for the first time in my long history with this dealer. The circumstances, while perhaps justifiable to them, were unacceptable to me, and I found myself bitching at the very people I've come to know on a friendly, first-name basis. I was particularly miffed at the fact that they quietly changed their policy in the last year sometime and no longer take requests for BMW loaners, so everything is on a first-come, first-served basis. In my case, they said someone didn't return a car on time, but I realize that's just PR/Dealer-speak for "we're out of cars, and you're out of luck...here are the keys to your deathtrap".

Now, I grant you, this loaner arrangement is still a better deal than at a nearby dealer where I was forced to drive a 3-cylinder death trap, but I see where this is going. Errrr...guess I'll finally have something interesting to say when the BMW survey people call for a follow-up.

Mileage: 67662