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Erratic Idle and hot start issues V70-XC70

posted by  metzger XC90 V8 on  Sun Jul 7 13:16 CST 2013
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last visit: Sat Oct 4 17:44 CST 2014
[RELATED]

My son's 1996 850 has developed a slight rpm surge at idle. It goes from 900 rpm to 1500rpm and back, even when warmed-up (although it does this less when warm).
At the same time, it developed a problem starting hot. It starts fine cold, but drive it 15 minutes, shut it off, and it won't restart. It cranks, and acts like it's trying to start. But, it won't actually start unless the engine is cool. Vacuum lines were replaced 20k miles ago.

Will the engine coolant sensor cause these problems? What are the OHM readings for a good ECT?

What will unplugging the air mass sensor tell me?

Thanks!!








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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    The ECT also drives the instrument gauge. If the gauge is always cold, it could be a broken thermostat as well as a bad ECT sensor.

    The ECT sensor is linier, so when the coolant is hot, the resistance is around 240 Ohms. 500 Ohms for one at 90F.

    Unplugging the AMM is not wanted so long as the engine runs well at higher rpms. That will trigger a CEL - and never unplug or plug it back in while the engine is running.

    If you have a NA engine, check the hot air feed at the air box. If the vlave is always closed to fresh air, the MAF/AMM will cook.
    --
    Klausc, a Volvoholic



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      posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

      Klaus:

      I saw an earlier post of yours that said the resistance should be 150 ohms, or less at operating temp.

      I just ran the car. Temp gauge works, but don't know if it's correct. Got the car to operating temp (by the gauge), and the ECT read 314 ohms. Car runs perfectly under acceleration, but misses some at idle. Also think it's running rich.....

      MAF and air valve have been replaced.

      Change the ECT?

      thanks,

      Bill


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        posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

        Here are the correct Ohm values:

        100C (212F) - 150
        80C (176F) - 300
        40C (104F) - 1200
        20C (68F) - 2800
        0C (32F) - 7300

        So, it appears that your sensor is a bit off.
        --
        Klausc, a Volvoholic



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          posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

          Replaced ECT and it did seem to help. But, still stumbles at idle sometimes. Runs perfectly when accelerating. I wonder if the plugs got a little fouled by the rich mixture from the bad ECT?

          Thanks,

          Bill



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            posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

            At least it is running. Now is the time to check the vacuum line(s) to the fuel pressure regulator, obviously check the spark plugs, and look for other air leaks into the intake manifold. Any air leak will cause a rich idle.
            --
            Klausc, a Volvoholic



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              posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

              The idle doesn't seem rich now that the ECT was changed. All vacuum lines have been changed. The odd thing was that it started stumbling when it got warmed-up. Then, after about five minutes of idling, it quit stumbling completely. Then after another five minutes of idling perfectly, it stumbled so bad it almost quit.

              Very strange.



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                posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

                How old are your plug wires and rotor? Good wires will have a manufacture date on them. Is the MAF resonably clean.

                If this is a non-turbo, check the bottom of the air filter box and see if the flap to the warm air supply is wide open. I completely disabled this function on my 1996, so I only get cold air.
                --
                Klausc, a Volvoholic



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                  posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

                  MAF one year old. Plugs, cap and rotor two years old. Not sure about the wires.

                  Idle air valve was judged to be defective, and replaced with a used on last summer. Maybe it is defective now.....



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