Expert: Roland Finston - 1/16/2012

Question
QUESTION: Im having some difficulty with the idle speed on my '11 200.  Stopped at lights the idle drops at times and even stalls out other times revving to save itself. No error lights come on and when I take it in to service, they can't duplicate the problem. Average idle has it at 500

ANSWER: Hi Lisa,
An '11 model is a bit too new for me to be up to speed on the details. The chances are that the engine control computer will identify the problem and log a fault code in its memory. Usually the check engine (or similarly named) light will come on and stay on, or at least stay on long enough for you to notice it. That is the point when you want to find out the 4-digit fault code number. Try the ignition key:" on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the odometer window to see if the mileage reading changes to show such a 4-digit number preceded by P. Let me know what it is and we'll go from there. If there is no indication of any kind on the mileage window, then a nationwide autoparts store will often do a free readout, or an independent shop will do it for $40 or less by means of a plug-in code reader that mates to a socket under the dash that is connected to the digital computer system on-board.
I would not try 'throwing parts' at the problem. Wait until you get a clue.
Roland

---------- FOLLOW-UP ----------

QUESTION: Hi Roland,
Thanks for your respose. No check engine light and no error code. No trails of anything
Ing wrong.

Answer
Hi Lisa,
Which size engine (L) is in the car? It might be that the exhaust gas recirculation valve (egr) is not closing tightly when you slow down to a stop. That can happen without being picked up as a fault code. But I need to check out whether you have such a valve on the engine, so let me know the size.
An intermittent vacuum leak from any of the several vacuum hoses that are connected to the intake manifold will also cause a loss of rpm/stall perhaps without setting a code. So those two sorts of problems which momentarily thin the mixture would be suspects. Those would be pretty quick and easy to check out so ask the dealer to consider those possible causes, under warranty or course. The engine senses the leak and responds by lengthening the fuel injector pulse width, and that is why it revs back up, possibly. With no codes yet, these would be my choice as to what to do until a code appears.
Roland
PS If you would take the time to rate my answer I would be most appreciative.