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  1. #1
    DanlP
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    idle surge

    Have a dramatic RPM surge going on at specific engine temps as the car warms up.

    COLD ENGINE: no problem; idle correct, smooth

    WARM ENGINE: ICV will suddenly (and i mean SUDDENLY!) open so that the idle in neutral jumps from 500 to 1300. If in D, stopped at a light, risk of suddenly jumping into the car in front. Idle will stay high for a minute or 2, then slowly go back down to 500. Problem may repeat as engine warms further.

    HOT ENGINE: No problem; idle correct, smooth.

    Checked MAF connector for voltage: OK. Checked it for resistance: OK.
    Checked ICV connector for 12Vdc: OK. Checked ICV resistances: OK

    I have no idea what sensors (or the computer) could be sending a bad ICV signal to cause it to open. Again - this ONLY happens as the is warming; not when either cold or hot.

  2. #2
    DanlP
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    Re: idle surge

    Followup:

    Did an electrical test of the signals coming into the ICV. ALL TESTS AT 'IDLE" - NO ACCELERATOR USED, NOTHING TURNED ON OR OFF.

    Pin 2 13.8V at idle (battery)

    RPM Vdc Pin 1 Vdc Pin 3 Note

    600 6.4 8.3 cold engine
    500 5.7 8.6 warm engine (missed 1 short surge)
    500 5.1 8.8 hot engine
    1400 8.8 5.4 surge lasted ~20 seconds
    500 4.8 8.6 RPMs returned to normal
    700 6.0 8.4 A/C on
    500 4.9 8.9 A/C off again
    800 5.5 7.8 short surge, ~ 10 seconds
    500 5.0 8.9 RPM returned to normal
    700 6.2 7.8 another ~10 second surge

    What I said before about the surge related to the engine temp is wrong - the surges happened after the engine was at normal operating temp.

    Maybe some sensor is sending a bad signal to the computer as it warms up????

  3. #3
    BigBimmer
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    Re: idle surge


  4. #4
    BigBimmer
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    Sonds like a bad MAF sensor

    What is the age and mileage of the car?
    How old is the MAF sensor?
    What are the engine diagnostics codes?

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    Re: Sonds like a bad MAF sensor

    Old car - '95, with original crank sensors (thought maybe that was going weird as it warmed up), original crank sensor, original MAF. Car has 180K miles on a Nikasil engine.

    Checked the resistances of the MAF and they were OK - though I have no way to do this as it warms up and the problem is intermittant.

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    You can't test a MAF by measuring resistance...

    A MAF is a dynamic device. It heats a foil and then measures temperature change of the foil to determine the moving air mass. A resistance measurement of an idle, disconnected MAF is meaningless.

    You really need to read the engine diagnostics codes to know what is going on. Without those, everything is pretty much a guess. Though, a MAF typically only lasts 80K - 100K miles, and your symptoms sound like a bad MAF.
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    Re: You can't test a MAF by measuring resistance...

    I tried reading out the error codes - this is an old OBD1 car, so it's donw thru flashing idiot lights (short & long pulses).

    All that came out, over and over, was a failed Lambda A. I've not yet reeplaced that but will. The same code just kept repeating, endlessly - either it's stored a zillion of these same coeds, or the memory is stuck, or it simply repeats until it is told to stop.

    Maybe i should erase the memory and see what accumulates.

    Wadda ya think?

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    You may need to get codes read by a BMW computer

    I assume that the full-blown BMW diagnostics computer will read more codes than the Stomp Test. But, I don't know personally, because I have not worked on a 1995 E38 like this before.

    Try disconnecting the MAF and driving the car. If the symptom goes away or changes substantially, that might point to a bad MAF. Driving with the MAF unplugged will throw a check engine error, but you can always reset it after the test.
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    The idle surge could be ICV if dirty...

    BTW... what year is the car, model, engine and production date because the problem could be a faulty CPS, if it is aftermarket instead of OE or OEM... most aftermarket are faulty because are technology copies with incomplete circuits...



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    Re: The idle surge could be ICV if dirty...

    Nah -= it's clean.

    THAT'S why I checked the voltages driving the ICV - to see if the voltages were changing without a reason.

    And - see my list of voltages. For some reason the computer suddenly drove the ICV wide open - for a short time - then drifted back to normal. The ICV reacted as it should - opening up and then closing gradually as the voltages in Pins 1 & 3 returned to normal.

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    Re: The idle surge could be ICV if dirty...

    ... hate to be arrogant but in spite correct voltages to possible be right but ohms law is unforgivable because even a dirty ICV does open encloses throwing correct voltages, and there is bit more electronics that communicate failure to each other and corrupted codes too. However, the point is simple... and may have forgot to look under-the-hood... includes every coil, both CPS [cam and crank-shaft] sensors... fuel pressure regulator, fuel injectors... and down stream are O2 sensors. BTW, electronic either digital or analog communicate with each other... and failure of one affects the performance... so, good luck trouble-shooting the root of the problem...



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    Re: The idle surge could be ICV if dirty...

    Thanks, Claudio.

    Have not really checked any of these ...

    Please see my new post to Roy

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    Could it also be just a loose spark plug?


  14. #14
    DanlP
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    Re: idle surge

    Interesting followup

    Since this idle surge problem seemed tightly tied to specific warm up temps, I thought I'd disconnect the temp sensors. Started with all 3, but as you might expect the critical one is the DME coolant temp sensor.

    With it disconnected the idle is smooth with zero surges. Other than at first (morning) startup, engine runs very well.

    Have ordered a new sensor - hope this will end the mystery.

    These damn cars are very idiotic at times ..............

  15. #15
    DanlP
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    INTERESTING UPDATE - CAUSES NOT EXPECTED

    Final word still out, but at the moment this set of symptoms seems to have been the result of 2 unrelated problems:

    1. Wiring to ICV: seems one of the 3 wires (inside the sheath) was cracked and causing an intermittent open. That cause the ICV to surge at times as the wire sometimes passed current and at other times did not.

    2. Bad Coolant Temp. Sensor: seems to have been shorting to ground, but only at certain temps (only at specific points in the warm-up cycle). As the engine warmed the sensor would fail - causing, I think, a DME problem - but then be OK as the temp. continued to rise.

    Who wouldda thunk!

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