Expert: Mike Evans - 10/23/2010

Question
QUESTION: Hi--

My Nissan sentra has been having a rough idle and has been taking a while to start and it also has been using a lot of gas, i have adjusted the idle speed checked about everything that there is, everything that would be a common check for this problem has been checked, I was told that this wire i have off, located by the master cylinder was a "spark plug boot" and it came disconnected and that is what was causing this, i am unable to locate where it plugs into, i am not sure if the mechanic i sent that picture of the wire to was thinking it was a spark plug wire??? I would just like your opinion, thanks

ANSWER:      Well, I'm sure you're aware of whether or not all four plug wires are connected.  A bad or disconnected plug wire would give symptoms just like you describe.  I would recommend that the cap, rotor, and wires be replaced with new before going any further.  Then I would look at the function of the idle speed motor and the coolant temperature sensor.  One or the other is probably at fault.

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QUESTION: How would a coolant temp. Sensor affect anything??? My car never even
gets hot

Answer
    The coolant temp sensor tells the ECM when the engine is cold (before you start it for the first time).  If it is bad, the engine will think it is cold all of the time.  Then the ECM applies a richer mixture, just as if you were applying the choke in an older, carburetted car.  It would be like trying to drive everywhere with the choke on full.