Hey everyone!
have my car for 3 weeks and one thing i noticed: in when letting the car roll out (without pressing throttle) from any RPM untill ca.1100 to 1200 RPM, at 1200 RPM there is slight jerking.Same happens whenbraking, with the rpms passing 1200. The lower the gear, the more you notice it.
Now i went to the dealer, and went for a ride with the tech, who had no idea what to do. Then we went for a spin in the NON S Cayman, which was standing at the dealer, and <u>this car does the same</u> (although it has a different motor, and therefor a different motor management). So the tech said : they all do that.
what do you say?
Do you get this: In netrual with idle, depress accel very slightly. It starts to pulse RPMs up and down every second. Remove foot or press harder and it goes away. What the?
havent tried, but will do......what about the issue i adressed?
Not sure, it maybe the same thing. Try the above test.
I believe the behaviour you experience is caused by the engine management system oscillating between fuel on-fuel off.
When the throttle is closed and engine is above a threshold speed of around 1100 rpm, the engine management system is designed to shut off the fuel entirely in order to improve the emissions and fuel consumption performance. Typically this happens on the over-run (rolling in gear with the throttle closed).
Necessarily, when the engine speed drops below the threshold speed the fuel is turned back on to enable idling. You can experience the lurch of the engine being turned back on by driving in say 2nd gear at 2000 rpm, getting off the throttle, and not on the brakes, but remaining in gear, and letting the car coast back down towards idle. When the engine reaches about 1100 or so you'll feel the lurch of the fuel being turned back on.
The behaviour you experience is caused by the engine management system oscillating between fuel on-fuel off. This happens when, despite the throttle being closed, the momentum of the car or a slight downhill holds the engine at the threshold speed. As the car/engine speed drifts down below the threshold, the fuel turns on, and the car/engine speed increase just enough to get above the threshold. But then since the throttle is closed, the fuel shuts off, and the engine and road speed drift downward again; thus the cycle repeats, and thus the lurching.
This behaviour used to be more pronounced in earlier electronically controled fuel injection systems. I am surprised to see it in a modern Porsche. The BMWs I've had recently have designed out this issue.
Rob
Yep its the same thing with 2 different way to observe it. Just a bit more software in the ECU would be able to fix it. Can you believe they (Porsche)never noticed it, I dont think so. I bet they figured we never would! ha ha. Egg on their face.
Thanks for explaining, makes sense.... but what do we do?
anyone tried (maybe because of other problems) resetting the ECU?
No that wont work. Mine did it from day one. Guess we have to put up with the minor issue.
