MAS vs. Rough rich start-up

[MR.G.]

My 2000 Boxter runs rough and rich for about 30-45 sec's. after start-up. I took it to the local foreign repair shop and they say it need a new MAS sensor. The car has 57k on it. That sounds more like an automatic choke or some other related sensor to me, otherwise wouldn't the car run crappy at all speeds if it was a MAS? After the initial start-up problem it smoothes out. I am new with Porsches so I admit that I could be way off on this one. Anyone want to give their two cents worth is welcome.
Tks.,
MR.G.

[Lil bastard]

The Boxster doesn't have an MAS, it has a MAF. It is a Bosch Hot Film type which sits in the airstream in the intake. It's not the most robust unit despite several redesigns. It passes an electrical current through this conductive film and heats it to a constant temperature. As air flows over it, this film cools and more power is required to keep it heated to the proper temperature resulting in slight variations to this constant current. The computer extrapolates the air mass from these variations. Because it sits directly in the airflow, it is subject to dirt, oil and such and can become dirty. This accumulation of dirt can result in current variances outside the normal, expected, range. The film can also tear. When these things happen, the computer is unable to select the proper map for the existing conditions and usually goes to a fixed default map.

If the MAF is just dirty, it can be removed and cleaned using a spray solvent to remove the dirt and oil. This is successful about 50% of the time. That would be the first thing to try. Use the search function for a How-To - it's been discussed here many times and there are good instructions.

But, a failed MAF doesn't create too many issues when idling. It usually manifests itself in reduced power and sputtering at speed.

From what you describe, it doesn't sound like much of a problem at all. At 57k, it just may need new plugs - the service interval is a nominal 60k mi. (though I would not go that long on a set of plugs).

Most cars run rich at startup because their computers are in Open Loop operation (relying on fixed, pre-programmed fuel/spark maps) until the O² sensors heat up enough to start supplying data to the computer. Extra fuel is pumped in the engine to keep it from stalling until it warms up, similar to what a choke did on old carburetted cars (though a choke enriched the mixture by reducing the amount of air supplied to the engine). Many engines even have an extra fuel injector (often called a Cold Start Valve) for this purpose, the Boxster does not.

Once the O² sensors warm up , it switches to Closed Loop operation, varying the spark and fuel in response to what the sensors are telling it - it becomes dynamic, measuring numerous data points from several sensors many times per second.

So, it's not unusual to run rich as you describe. If you think it's excessive, swapping plugs, or the O² sensors is likely to have a greater effect than cleaning or replacing the MAF. The new ones (O² sensors) will heat up quicker and cause a switchover to Closed Loop operation sooner, which would lean things out.

Personally, whenever I get a 'new' used car, the first thing I do is replace the filters, oil, plugs, brake fluid, coolant, etc. to insure everything is in good order. It isn't that expensive, especially if you do your own work. It establishes a baseline and removes a lot of the guesswork about the car's condition.