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Thread: Surging Idle While in Gear

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    E500E Guru emerydc8's Avatar
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    Surging Idle While in Gear

    Before I go looking for a new (used) ETA, I thought I'd throw this out there to see if there are any other opinions. My idle intermittently surges from 500 RPM to 700 RPM while in any gear (including R) and not moving. It surges at a rate of about one or two cycles per second. When I move the gear selector to N or P the idle smooths out almost immediately.

    I just replaced the NSS, ignition module and crank position sensor, but it hasn't resolved the issue. Occasionally on a start (usually when cold), the idle will stabilize abnormally high, like 900 RPM. Restart the engine and the idle is back where it should be -- around 500 RPM.

    The codes that have consistently appeared after clearing all codes and then checking them four additional times after driving are as follows:

    Module 7: DTC 2 (points to a bad ETA)
    Module 17: DTC 18 (crank pos sensor magnets not recognized)
    Module 19: DTC 6 (ISC inop)

    The ETA (non-ASR) was replaced by a prior owner about 100,000 miles ago with a rebuilt unit. Maybe it's time for a new one.

    Any thoughts would be appreciated.
    Last edited by emerydc8; 6 Days Ago at 09:52 PM.
    Jon D.
    1994 E420
    1995 E420

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    Re: Surging Idle While in Gear

    Jon, first thing would be to rule out vacuum leaks... may require a smoke machine. Don't forget the PCV tubes, vac line to EZL, vac lines to solenoids on passenger fender, and the 8mm PCV tube to the valve near the EZL.

    If you clear all the codes, and test drive, do you get the same codes repeatedly?

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    Re: Surging Idle While in Gear

    Thanks, Dave. I don't have a smoke machine, but maybe I'll have to buy one for Christmas. I checked for vacuum leaks by spraying starter fluid all over the engine compartment to check for any variation in idle, but didn't hear any. I'll check closer tomorrow during the day.

    As for the codes, I checked four times after clearing all the old codes out and those codes appeared consistently. The only other code that appeared twice (but not the last check) was the knock sensor (Module 17 DTC 5), but that may have been because I got on it and I did hear a slight knock during that short acceleration. Do you think a bad knock sensor could cause this?
    Jon D.
    1994 E420
    1995 E420

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    Re: Surging Idle While in Gear

    emerydc8,

    I have an update ETA without ASR.
    If you are willing to pay shipping costs, I can send it and you can try it out to see it it resolves your issue.

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    Re: Surging Idle While in Gear

    Hi Steve,

    If it's the ETA with the straight plug and you want to sell it, I would gladly buy it from you. I had been meaning to get one just to have as a spare. Maybe if I replace my existing ETA, I can have mine rebuilt. Please PM me when you get a chance.

    Jon
    Jon D.
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    1995 E420

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    Re: Surging Idle While in Gear

    Jon, the knock sensor shouldn't cause an idle oscillation. But it may cause other issues with power delivery; if you can hear knock or continue to get that code, I'd look into replacing the sensors (not a fun job).


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    Re: Surging Idle While in Gear

    Thanks, Dave. That's why they haven't already been replaced. I probably should have done it when I did the motor mounts. If I recall, the passenger side knock sensor was pretty easy to access with the engine off the mounts.
    Jon D.
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    1995 E420

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    Re: Surging Idle While in Gear

    I drove it all over town today and experience the same surging at idle quite a few times. Just pulled the codes and the only code that recurred was DTC 18 on module 17 (magnets for crank position sensor not recognized).

    A few years ago, I installed a shim/washer on a new crank position sensor, but it was thicker than .5mm, which is specified in the shop manual below. When I replaced the crank position sensor yesterday, I left off the washer. I remember Klink warning me that it was a bad idea to go thicker than .5mm, but I couldn't readily find a thinner washer and it worked for years like that.

    The next step will be to search for a thin washer (.5mm) and install under the sensor. I did notice it has been taking a bit longer (a few more cranks) to start the engine lately and I think that is a symptom of a bad crank sensing issue.
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    Jon D.
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    1995 E420

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    Re: Surging Idle While in Gear

    Jon, if you cannot make code 17 go away, visually check to confirm BOTH magnets are still in place on the flywheel. It's possible for one to pop out of place, and when that happens, you'll get an intermittent extended crank before start, and code 17 will be there all the time.

    This is almost certainly not related to the surging / oscillating idle though.


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    Re: Surging Idle While in Gear

    Ugh! I remember reading that post a long time ago. I hope that's not the case. The surging idle remains a mystery then. I thought it might be caused by the crank position sensor. I ordered a smoke machine today so I'll be able to check for vacuum leaks when it comes.
    Jon D.
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    Re: Surging Idle While in Gear

    I have heard and read how the w126 420 has had CPS issues and also, missing flywheel magnets.
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    Re: Surging Idle While in Gear

    Can you check the flywheel magnets by removing the flywheel cover/plate and manually turning the engine, or do you have to look through the hole on top where the crank position sensor goes?
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    Last edited by emerydc8; 1 Week Ago at 11:20 PM.
    Jon D.
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    Re: Surging Idle While in Gear

    Quote Originally Posted by emerydc8 View Post
    Can you check the flywheel magnets by removing the flywheel cover/plate and manually turning the engine, or do you have to look through the hole on top where the crank position sensor goes?
    You need to look through the bottom "grille" of the transmission. See attached.
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    Re: Surging Idle While in Gear

    A cigar works for checking vacuum leaks. Just blow smoke into one of the vacuum lines.

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    Re: Surging Idle While in Gear

    Quote Originally Posted by Maui View Post
    A cigar works for checking vacuum leaks. Just blow smoke into one of the vacuum lines.
    Yes, cigars have many purposes (says Bill Cliton).

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    Re: Surging Idle While in Gear

    I saw a billboard with Monica's face stating she was voting Republican in the mid-terms because the Dems left a bad taste in her mouth.
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    Re: Surging Idle While in Gear

    UPDATE:

    Clearing and pulling the codes for the 6th time, I am still getting the following:

    Socket 7 (EA/CC/ISC): DTC 2 (points to a bad ETA)
    Socket 17 (Ign Control): DTC 5 (knock sensor); DTC 18 (crank pos sensor magnets not recognized)
    Socket 19 (Diagnostic): DTC 6 (ISC inop); DTC 25 (knock sensor).

    I thought I'd look at changing knock sensors. Holy crap! At $244.20 each, they must be made of unobtainium. https://www.mboemparts.com/oem-parts...sor-0031534728
    Last edited by emerydc8; 1 Week Ago at 11:47 PM.
    Jon D.
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    Re: Surging Idle While in Gear

    Quote Originally Posted by emerydc8 View Post
    UPDATE:

    Clearing and pulling the codes for the 6th time, I am still getting the following:

    Socket 7 (EA/CC/ISC): DTC 2 (points to a bad ETA)
    Socket 17 (Ign Control): DTC 5 (knock sensor); DTC 18 (crank pos sensor magnets not recognized)
    Socket 19 (Diagnostic): DTC 6 (ISC inop); DTC 25 (knock sensor).

    I thought I'd look at changing knock sensors. Holy crap! At $244.20 each, they must be made of unobtainium. https://www.mboemparts.com/oem-parts...sor-0031534728
    Jon, based on codes alone, your ETA is pretty high on the suspect list... but do the smoke / vacuum test first.

    The knock sensor is a 1-piece assembly, so you don't need to buy two of 'em. Sadly, this is another item that vanished from the aftermarket, and AFAIK only OE is available now.


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    Re: Surging Idle While in Gear

    Jon, I have a nice newer dated ETA from a non ASR 92 400E that was very well maintained and running well. It has the older pancake plug and your car requires the newer plug. I don't know if the newer plug is available or what is involved in switching them but would let it go to you for $300. should you decide you are interested.

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    Re: Surging Idle While in Gear

    Jon, based on codes alone, your ETA is pretty high on the suspect list... but do the smoke / vacuum test first.
    Jon, I have a nice newer dated ETA from a non ASR 92 400E that was very well maintained and running well. It has the older pancake plug and your car requires the newer plug. I don't know if the newer plug is available or what is involved in switching them but would let it go to you for $300. should you decide you are interested.
    Thanks to both of you. I have an ETA coming in the mail soon, thanks to Stevester.

    The knock sensor is a 1-piece assembly, so you don't need to buy two of 'em. Sadly, this is another item that vanished from the aftermarket, and AFAIK only OE is available now.
    So, there's only one sensor per car? Does it cover both sides?

    EDIT: Nevermind. I see now it is one harness that Ys off into two sensors. Just ordered it on Parts Geek but they're not sure they have it, even though it says one in stock. We'll see on Monday. Thanks again.
    Last edited by emerydc8; 1 Week Ago at 04:47 PM.
    Jon D.
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    Re: Surging Idle While in Gear

    Quote Originally Posted by emerydc8 View Post
    So, there's only one sensor per car? Does it cover both sides?
    Exactly correct. Photo attached.

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    Re: Surging Idle While in Gear

    UPDATE:

    Found and fixed! It was a vacuum leak on the elbow that goes into the shift pressure modulator on the side of the transmission. I love it when it's something simple like this -- and I find it on a Friday evening. I don't know that a smoke machine would have shown the leak because it was tucked into the transmission tunnel and hidden behind some of the blanket. I installed a .5mm shim on the crank position sensor, but like Dave said, that definitely wasn't the problem with the surging. We'll see if the crank position sensor codes come back.

    Well, at least I have a knock sensor, a smoke machine and spare ETA on the way. Thanks for the help guys.
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    Last edited by emerydc8; 1 Week Ago at 07:06 PM.
    Jon D.
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