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Thread: idle surge

  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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    Re: You may need to get codes read by a BMW computer

    OK, will give the disconnect MAF a try - but it seems to me (from distant memory) that the instant the connector is pulled the engine dies.

    Will check it out.

  10. #10
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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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    Nope, the engine will run fine without a MAF


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    Re: Nope, the engine will run fine without a MAF

    You, of course, are completely correct - pulling the plug on the MAF does not kill the engine.

    Did so, let the engine idle for about 5 minutes - was JUST about to shut it down and go get a new MAF, when all of a sudden the surge happened again and RPMs shot up to 1,500.

    So - I guess this rules out the MAF.

    Any suggestions of what other sensors contribute to the idle calculations? Would a failing temp sensor do this?

    When the engine surges to a high RPM it runs fine at that speed -- I suppose that rules out a failing crank sensor, right?

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    Probably time to connect to a BMW diagnostics comp

    It is probably time to connect the car to a BMW diagnostics computer. Without the detail engine diagnostics codes, it is near impossible to figure out what is going on.
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    Find some Indy with an Autologic unit or a Dealer.


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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: Find some Indy with an Autologic unit or a Dealer.

    (hate not having a job ...)

    Trying to diagnose this w/o paying a stealer ....

    Have another issue going on, maybe related - leading to the question: "do you know what events cause the DME to go into 'limp-home'; are you familiar with what limp-home is like; do the E38 8-cyl have limp-home, or is that only for the V12's?"

    Car has been at times going into a terrible rough idle with almost no power. Have to 'pump' the accelerator to get going like in the old carb cars.

    SURPRISINGLY - when I pull the connector from the DME for a few minutes, the problem goes away. Returns after a while.

    Does this sound like 'limp=home' - what events can cause such a thing? The only error code I have been getting (from stomp test) is a bad O2 sensor.

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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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    “limp home” gives you a specific cluster error…

    There is a special “limp home” mode that can occur when certion sensors or components signal errors. This is called “Engine Failsafe Mode,” and you will always get an “ENGINE FAIL SAFE” message on the instrument cluster when that happens.

    The symptoms you describe sound like a bad catalytic converter. The O2 sensor error is telling you that something is not right with one of the cats. That, combined with the severe misfiring is a signal that one of the cats is likely clogged.

    At a minimum, you will need to get the engine diagnostics codes read by a BMW-specific tool like a Peake code reader. This should tell you the misfire errors and other cat-related errors. If you have a bad cat, then there is no point trying to sort out a MAF. You will need to deal with the cat problem first, and then go from there.

    You are really at the point where will probably have to pay someone for some diagnostics time. The Stomp test does not give enough information to understand all that could be happening. Maybe someone nearby has a Peake code reader-- that will go a long way. However, you will probably have to connect to an Autologic or BMW computer.
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    Re: “limp home” gives you a specific cluster error…

    Thanks for all the help! Really appreciate it.

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