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High idle?

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  • High idle?

    Start car - idles fine. Short time after (1 to 2 minutes), idle increases a lot, though tach shows 500 to 1,000 rpm the whole time, varying only slightly, but clearly way off from actual engine revs. When accelerator is depressed a bit, engine balks; when depressed a lot, engine revs a lot, as one would expect (didn't notice the tach at that time unfortunately). Any ideas? I figure some sensor, wire, or connector must be sending a faulty signal to the ECU to cause this behavior, but where to look first, etc.?

    In addition to flaky tach reading, water temp. gauge registers nothing, and oil pressure gauge is pegged to the right, but these issues may be unrelated to the idle issue (?).

    Any help greatly appreciated... :)

    edit --- 2.3 Duratec, MBE ECU
    Last edited by Sean; March 14, 2009, 09:20 PM. Reason: I thought engine & ECU type might be relevant info.
    | | Sean

  • #2
    Not knowing what kind of ecu and other stuff you got....the incorrect gauge reading may be a sign of a ground problem (assuming the typical VDO 1-wire senders). If so, it may also screw up your ecu?


    Edit: Ah, now I see MBE ecu. Well, no experience with that one....


    • #3

      I'm not familer with the MBE, Magnus worked on one at the dyno day but I don't know how well he knows them.

      My ECU has a 1 minute start cycle after which the map changes depending on coolant temp and other factors. The ECU also has a default value if the sensor is disconnected. Do your oil pressure and temperature senders also send info to the ECU? I have sensors for the gauges and separate sensors for the ECU.

      Can you hook the MBE up to your laptop and see what is going on?



      • #4
        There's some information about the ECU & tchometer here (note that the web site doesn't pass urls - click on the 'MBE ECUs & Easimap Software' link) but based on your description I'm not sure the ECU has anything to do w/ the symptoms you're seeing. This smells like electrics.
        A day you don't go a hundred is a day wasted


        • #5
          Repeated procedure from yesterday, with same result - starts fine, idle goes quite high once warm. So I let it warm up fully, idling high for a couple of minutes, and then mashed the throttle to the floor for a half-second, and it settled into a right fine normal idle. Tach, OP, and WT gauges still... wonky? Went for a very short drive (silencer not fully mounted yet), and engine behaved as normal. So, primary issue apparently sorted, at least until it happens next time. :)

          Moral of the story: When in doubt, gassit!

          Thanks all!
          | | Sean


          • #6
            Far fetched speculation:

            I would guess it's a combination of an cold start cycle and a somewhat unstable idle mapping.

            The cold start cycle adds a bit of fuel during the first 60 seconds or so after engine start when the water is cold. This cycle usually eases of gradually as the 60 seconds come to a close (and the water starts to heat up a little bit).

            What can happen is that the cold cycle gradually drops off the engine into a part of the map that doesn't let it drop any further. Think of it as an enriched/advanced timing island in the map that is normally never hit by the engine, except for when the cold start cycle drops it off there.

            When you blip the throttle the engine exits the island and then misses it as it falls back into idle.

            My suggestion is that you hook up a laptop and have a look at the cold start cycle and the parts of the map that gets used during its advance. Once the engine ends up at high idle, play around with that part of the map and see if you can bring the idle down.

            You cannot break the engine if you just modify that part of the map since too little fuel is involved to do any damage. Save a backup before you start modifying.

            Post screen dumps from the mapping software so that we can have a look.

            /Magnus F.


            • #7
              Thanks Magnus, sounds logical, but the high idle has been banished to the land of wind and ghosts since the 15th.
              | | Sean