A plugged Secondary Air Injection Port on a Mercedes Benz M112 V-6

So the air pump is not accomplishing it’s task of heating up the three way catalyst. How does DME (Digital Motor Electronics) know any way?

Programmed into the computer is a self test of the air pump. Each time the car is started DME at some point in the drive cycle will turn the air pump on to see if it’s working.  How it sees is interesting.

Part of the fuel management system are oxygen sensors or on newer cars AF (air/fuel) sensors or broad band oxygen sensors report back to DME.  By sensing the amount of oxygen in the exhaust gases these sensors allow DME to make adjustments to the air fuel mixture going into the engine for the most efficient performance under all conditions.

The O2 sensors also tell DME if the air from the air pump is reaching the catalyst. If the secondary air injection system is working properly DME should see a drop in voltage in a conventional O2 sensor.  On the newer A/F sensors the technician will need to access fuel trim data to see if the fuel system is trying to adjust when the air pump is artificially triggered.

So the air pump pumps air but there are some mechanical controls in the way of the air actually getting to the exhaust side of the equation. There is a vacuum switchover valve (VSV) and a vacuum operated “Dump Valve” that when vacuum is applied by  way of the VSV allows air to enter the exhaust through a check valve which keeps exhaust gasses from returning through the “Dump Valve” damaging it and the pump.

So the long and short of it is, the DME turns on the Air pump, the sensors sense no change indicating A Secondary Air Injection malfunction or low flow and DME turns on the Check Engine Light. Now what?

Dig in.

My next topic came to me some time ago in dramatic form. The shelf life of tires was featured on an ABC News info drama. I choose to leave the fearful drama aside but think it’s a topic worth noting. The video can be found on ABC’s website.

I broach the topic for the Bellingham Business Journal with the help of our ever present inquisitive customer.

Q: My father in law asked me how old my new tires are? I just bought them they are new, right?

A: Not necessarily. Like milk, eggs or corn chips, tires have a “best if used by” date printed on them. Only outdated tires could be a lot more hazardous than tainted treats. On the side wall of every tire is a sequence of letters and numbers beginning with the letters DOT.  For example: DOT B97R FW6X 2202.

The numbers at the end of our example are the week of the year the tires were manufactured. In our example the tires were manufactured in the twenty second week of two thousand two. Tires older than six years may be unsafe and should not be used.

A reputable tire store will readily make this date available for you. I recently bought a set of tires for my wife’s Toyota. The dates of manufacture; 0309, the third week of 2009 were conveniently printed on the invoice.

MMMM fresh tires.

Thanks for visiting our Blog Site. We are still under construction. What you may expect in the future is:

1. A regular blog on motor sport, SCCA, Formula 1, WRX and endurance racing. Probably won’t be anything of interest here for NASCAR  and little for CART fans.

2. A tech site highlighting Technical Service Bulletins that effect Mercedes Benz, Toyota, BMW and Volkswagen/Audi car owners.

3. Some excellent technical and performance car videos.

4. Tech tips with photos, how to helps for the folks who love their cars and love to tinker.

If you have and suggestions on possible topics or comments e-mail me Bob Taylor bob@masterautotechllc.com