I have a 1996 2.5ltr subaru car, it idols at less then 1000 rpm .?

I have a 1996 2.5ltr subaru car, it idols at less then 1000 rpm ! with rough idol , it has new plugs wires coil pakage ! any ideas? could a bad knock sensor cause this ? Thank you .

1 Answer

  • 9 years ago
    Best Answer

    a knock sensor is like a microphone attached to the engine. it detects the signature vibration caused by engine preignition. pre-ignition is a condition in which gasoline detonates inside the cylinder rather than burns smoothly. in most engines (including your 2.5 dohc) it is undesireable because it wastes energy. when your knock sensor detects a pre-ignition it sends that information to the Engine Control Computer. the ECM then retards the spark to compensate for the faster fuel burn therby maximizing power and fuel efficiency. failure of the a knock sensor will normally cause a fault code as indicated by a check engine light.

    an idle ofapprox 800 rpm is normal, a rough idle is not. if its ignition related its usually aggrivated by ambient high humidity. ignition wires, coils, plugs tend to leak voltage in wet conditions.(all conditions usually develop gradually, worse on rainy days). you can test this by spraying water on the wires and coil and noting the difference in rpm.

    the next most common cause of misfire is un-stoiciometric fuel mixture, improper air to fuel ratio. commonly caused by a)a malfunction mass airflow meter, b)air leak downstream of the MAF, c)malfunctionioning or obstructed fuel injector, d)leak upstream of the oxygen sensor or air/ fuel ratio sensor. (a) usually sudden onset. (b,c,d) can be either sudden or gradual onset.

    the least common cause would be compression. caused by a, a)improper valve timing, b)broken valve spring, c)burned valve, d)worn compression rings. (a) usually after a major service, (a,b) after overrevving engine due to a sudden change in engine speed ie. downshifting to an innapropriate gear, sudden onset. (c,d) very gradual onset, or related to improper timing or deficcient lubrication.

    first step, get the mil code. if its a single cylinder misfire look towards ignition, injector,or much less likely, valve

    2 cyl on the same bank, look at compression (head gasket), fuel rails. anyhing shared by the 2 cylinders involved

    opposite cylinders ft or rear- suspect the coil

    3 or more cylinders (usually wont idle), suspect MAF, or other sensor, possible air leak downtream of MAF or upstream of O2 sensor/ A?F sensor. (FYI...computer failures are usually all or nothing)

    btw if you dont have an obd scanner, you can identify the missing cylinders by disconnecting and reconnecting the ignition wires one at a time and noting the difference in rpm.(do this with an insulated plug wire puller, 50k volts hurt!!!)

    sensor failure: runs smoothly but can cause a noticible loss of power (MAF,O2,A/F), loss of fuel efficiency and less power (MAF/O2,A/F), long cranking before engine start (CRPos, CAPos), No start condition (ECM/ECU failure). No noticable symptons, no MIL; (Knock sensor)

    hope this helps.

    Source(s): .
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