1997 BMW Z3 2.8L - "check engine" light (P1423)

tcj, 4/14/04 (scroll down for photos!)

79,000 miles.

Check engine light has been on for a few weeks. Borrowed an autoXray code reader from Schucks to extract the OBD II (On-Board-Diagnostics II) code. Results: P1423. Cleared the "check engine light". A few days later it came back on. Borrowed the autoXray again - same code, P1423.

The P0xxx codes are generic, but the P1xxx codes are manufacturer-specific. For a BMW Z3, P1423 is defined as: "P1423: Secondary Air Injection (Bank 1) Flow Too Low"

The secondary air injection pump is emissions equipment, and comes into play when the engine is cold. The pump runs when you first start the car, until the engine warms up. When the engine is cold, the fuel mixture has to be richer than normal, because fuel vaporizes poorly when cold.

The air pump's job is to inject air into the exhaust manifold, to help lower emissions by providing oxygen to combust the unburned hydrocarbons that result from running rich.

If the air pump isn't working, then the catalytic converter has to work much harder. Cats are very expensive to replace (due to platinum lining). Much better to pay $250 for a new air pump, vs. $1250 for a new cat.

Before replacing the air pump, I checked hoses and wires which feed into and from it. No cracks or defects found. Externally the pump itself looked fine as well, no signs of cracks or moisture.

Then I disconnected the air pump for closer examination. The connectors were totally corroded. How'd that happen? There's no signs of moisture anywhere else in that area of the engine compartment.

The next clue was when I tilted the air pump, and saw that the connector housing filled up with water right before my eyes. WTF? Apparently the pump itself contained water.

I then popped off the rear housing of the pump and found... the most corroded disgusting rust slurry I've ever seen! How'd it get in this state? Well, it turns that if you look at the combustion equation for gasoline, there is approx. a pound of water (vapor) emitted for each pound of gasoline burned. Seems impossible, until you realize that the air:fuel ratio is 14.7:1. Do the chemistry, it actually works out... Anyway, the problem is that the one-way valve in the exhaust manifold is no longer working properly. So in addition to ordering a new pump, a new one-way valve is also required.

Cleaned up pump as best as possible, but the main body is a sealed unit (rivets), thus most of the corrosion remains. Reinstalled old unit for now, and ordered a new replacement unit from BMW parts dept.

(Notes continued on next page... tcj, 5/10/04)

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