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Surging Idle While in Gear

emerydc8

Active member
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.
 
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gsxr

.036 Hoonigan™, E500E Boffin
Staff member
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?

:detective:
 

emerydc8

Active member
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?
 

Stevester 500E

Active member
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.
 

emerydc8

Active member
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
 

gsxr

.036 Hoonigan™, E500E Boffin
Staff member
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).

:duck:
 

emerydc8

Active member
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.
 

emerydc8

Active member
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.:klink:

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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gsxr

.036 Hoonigan™, E500E Boffin
Staff member
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.

:scratchchin:
 

emerydc8

Active member
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.
 

nocfn

E500E Guru
I have heard and read how the w126 420 has had CPS issues and also, missing flywheel magnets.
 

emerydc8

Active member
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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gsxr

.036 Hoonigan™, E500E Boffin
Staff member
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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Maui

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

nocfn

E500E Guru
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.
 

emerydc8

Active member
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/mercedes-benz-sender-unit-knock-sensor-0031534728
 
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gsxr

.036 Hoonigan™, E500E Boffin
Staff member
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/mercedes-benz-sender-unit-knock-sensor-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.

:spend:
 

sheward

Active member
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.

drew
 

emerydc8

Active member
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.
 
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emerydc8

Active member
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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emerydc8

Active member
Just a follow-up question on this surging idle post. I replaced the ETA, and both the LH and T/LLR modules. The surging has stopped, but now about 20% of the time, when I start the engine I am seeing a high idle (1500 RPM) and it stays that way until I restart the engine. Most of the time when I restart the engine the problem completely goes away. Sometimes it takes two restarts. I cleared and pulled the codes and I am still seeing DTC 18 on socket 17 (crank position sensor magnets). I know this is a problem that I will eventually have to deal with, but could this be causing the problem now with the intermittent high idle? It doesn't make sense that the problem completely cures itself after a restart. Could there be some other module that I should be looking at replacing? Thanks.
 
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195910

Intl 500E GTG coordinator
E-Gas controls the idle RPM, and this is a symptom of a bad EGAS module. But from my understanding this would occur constantly.
 

emerydc8

Active member
Thanks. This car doesn't have an E-gas module -- it's non-ASR. From what I understand, the T/LLR module is the equivalent EA/CC/ISC, without the EA.
 

gerryvz

Site Honcho
Staff member
Thanks. This car doesn't have an E-gas module -- it's non-ASR. From what I understand, the T/LLR module is the equivalent EA/CC/ISC, without the EA.
That is 100% correct !!

Only ASR cars have an E-GAS module (all E500Es as well as 400E/E420s equipped with the ASR option). Non-ASR cars use a T/LLR computer module, which does oversee cruise control and ETA (throttle body) functions.
 

e500.org

Member
Dave has already raised it - vacuum leaks. More specifically, have you checked the vacuum hose from the engine oil breather system to the purge valve (located next to the ABS unit / EZL)? It is a thick black hose (not one of the thin translucent ones) - I damaged mine while changing the engine oil breather system hoses and now my car is idling like you describe. I temporarily patched it up (didn't last long however) and it ran perfect. Don't underestimate that breather hose! My local dealer is selling the hose in 25m minimum quantity (pain) so trying to find a similar hose to replace it with...

p.s. it can't be seen from the top of the engine - mine is cracked underneath the plenum and thus is not visible from the top.
 

gsxr

.036 Hoonigan™, E500E Boffin
Staff member
Just a follow-up question on this surging idle post. I replaced the ETA, and both the LH and T/LLR modules. The surging has stopped, but now about 20% of the time, when I start the engine I am seeing a high idle (1500 RPM) and it stays that way until I restart the engine. Most of the time when I restart the engine the problem completely goes away. Sometimes it takes two restarts.
As mentioned above, check for vacuum leaks... do a smoke test if possible. Is the replacement ETA new, used, or rebuilt? If rebuilt, who did the work?



I cleared and pulled the codes and I am still seeing DTC 18 on socket 17 (crank position sensor magnets). I know this is a problem that I will eventually have to deal with, but could this be causing the problem now with the intermittent high idle? It doesn't make sense that the problem completely cures itself after a restart. Could there be some other module that I should be looking at replacing? Thanks.
I had this error for years until I replaced the missing magnet. The only symptom was an intermittent hard start (long cranking time). It should not cause idle problems.

:scratchchin:
 

emerydc8

Active member
Thanks guys. I'll recheck the vacuum lines. I replaced all of them but the line going to the purge valve. I'l have to get under there again and check. I bought a smoke machine, but I'm not too impressed with the amount of smoke it puts out (actually I'm pretty disappointed with it). I guess you get what you pay for. If it is a vacuum leak, wouldn't it be present all the time and not completely disappear after a restart?

Dave:
The ETA was a used unit but it was a late version ETA and the seller said it came from an operating car. I'm glad to hear the magnets probably wouldn't cause the idle issue. I've only experienced extended cranking a few times, but it always eventually starts.
 

gsxr

.036 Hoonigan™, E500E Boffin
Staff member
Jon, if you still have the old ETA, you could send it out to the guy in Alabama (Don Roden, of Roden Foreign Car) who repairs ETA's. Pretty reasonable at ~$300. I think this is the right one for your non-ASR, 94-95 E420:

https://www.ebay.com/itm/MERCEDES-THROTTLE-BODY-ACTUATOR-000-141-93-25-REPAIR-SERVICE-3-YR-WARRANTY/223320835774

https://www.ebay.com/sch/at-hiker1/m.html

Jono says that the late datecode ETA's can still have internal parts wear out, despite having good wiring.

Are there ANY codes on the T/LLR module?

:detective:
 

emerydc8

Active member
Thanks for the link, Dave. I'll ship mine off this week to get it repaired. It is the same part # as the ETA I replaced (000 141 93 25). I cleared the codes yesterday after changing the LH and T/LLR modules and so far the only codes that have returned are the crank position and the knock sensor. I have a new knock sensor in-hand but not too excited about pulling the mounts to get to it.
 

emerydc8

Active member
I just checked the codes again. Now I'm getting a DTC 2 on pin 7 (CC/ISC), which could mean any of a dozen things wrong. Without the digital reader I'm screwed. I did check the vacuum line going to the purge valve and maybe there is a problem with that. When I blow into the end of the hose that goes under the left bank of the engine (the end I'm holding in the picture below), I am getting no resistance as if the air is leaking out. I'm not sure if this is normal.

[EDIT]: This purge line is pulling a vacuum with the engine running so I suspect it's okay.
 

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kiev

Active member
I just checked the codes again. Now I'm getting a DTC 2 on pin 7 (CC/ISC), which could mean any of a dozen things wrong. Without the digital reader I'm screwed. I did check the vacuum line going to the purge valve and maybe there is a problem with that. When I blow into the end of the hose that goes under the left bank of the engine (the end I'm holding in the picture below), I am getting no resistance as if the air is leaking out. I'm not sure if this is normal.

[EDIT]: This purge line is pulling a vacuum with the engine running so I suspect it's okay.

Is there an update, by any chance?
 

emerydc8

Active member
I sent my original ETA out to Don Roden to have rebuilt as per Dave's suggestion. He got it back to me within a few days, but I'm out of town right now so I won't be able to replace it for a few weeks to know if it was the ETA. Otherwise, I'm not sure what else to look for. It's just idling high on startup, intermittently. Restarting usually completely fixes it.
 

jftu105

Active member
I just checked the codes again. Now I'm getting a DTC 2 on pin 7 (CC/ISC), which could mean any of a dozen things wrong. Without the digital reader I'm screwed. I did check the vacuum line going to the purge valve and maybe there is a problem with that. When I blow into the end of the hose that goes under the left bank of the engine (the end I'm holding in the picture below), I am getting no resistance as if the air is leaking out. I'm not sure if this is normal.

[EDIT]: This purge line is pulling a vacuum with the engine running so I suspect it's okay.
See my post on the purge valve under the top end rebuilt. If the purge valve is stucked open, you will have vacuum leak. The purge valve likely is not functioning. You should hear (using a scope to listen) dadada....

jftu105
 

195910

Intl 500E GTG coordinator
Was going to say the same, its impossible to determine if its functioning correctly unless you manually inspect it.

Do you have any issues with the fuel tank or when refueling ?

Also check the brake booster hose, I guess you can unbolt it from the brake booster and plug the end to see if its pulling air through the brake booster.
 

gsxr

.036 Hoonigan™, E500E Boffin
Staff member
I sent my original ETA out to Don Roden to have rebuilt as per Dave's suggestion. He got it back to me within a few days, but I'm out of town right now so I won't be able to replace it for a few weeks to know if it was the ETA. Otherwise, I'm not sure what else to look for. It's just idling high on startup, intermittently. Restarting usually completely fixes it.
Jon, was there an autopsy report from Don on your original ETA? Just curious what he found inside.

:detective:
 

emerydc8

Active member
No problems at all with the fuel system as far as I know. I'll recheck the brake booster, but I think it's okay. If it was a vacuum issue, it's hard to believe it could be completely cured by simply restarting the engine. I'm thinking it's got to be something electronic. The ETA was the first likely culprit but we'll see when I get home and install it.

Dave, I don't know if Don Roden includes an autopsy report or not. I'll definitely let you know if he does.
 

195910

Intl 500E GTG coordinator
Is it just the high idle now? or are you getting abs/asr lights and surging.

If its only the occasional high idle (constant not fluctuating) it may be the E-GAS as with the M120 cars
 

emerydc8

Active member
It's only the occasional high idle now (no surging). I already replaced the T/LLR module. There is no E-gas module on my car.
 

195910

Intl 500E GTG coordinator
It's only the occasional high idle now (no surging). I already replaced the T/LLR module. There is no E-gas module on my car.
Check with Viktor from restoreyourmercedes on this one, its possible the replacement was defective as well?
 

jftu105

Active member
I don't know if this is related. For my E320, 1994-95, there is one situation that if I shut the engine off and restart it right away, the idling speed will stay way high, over 1000 rpm. This will not change until the engine is shut down and restart again. If I shut the engine and wait for a few minutes, the engine will start just fine. I think that it is a software issue of the ECM because I have verified that all other parts are fine.

Finally, for idling surging, one likely problem is the MAF. The MAF could be fine at low temperature before the engine bay heats up. Once the engine bay is hot, the MAF went out. There is something within the circuitry of the MAF. It is not repairable. Need to replace it.

For the purge valve, one can take it off and apply 12V to it. If you don't hear a click, it is not working. You can revivie it by kocking out the carbon deposits and apply some cleaner. Hit it with a deadblow hammer and most likely, it will come back to life. Once you put it in, and when the temperature is above 80C, you will hear dadada.... as the purge valve is open and close at a rate of 75 Hz (or something like that). It is a source of vaccum leak if the purge valve stayed open.

jftu105
 

emerydc8

Active member
I don't know if this is related. For my E320, 1994-95, there is one situation that if I shut the engine off and restart it right away, the idling speed will stay way high, over 1000 rpm. This will not change until the engine is shut down and restart again. If I shut the engine and wait for a few minutes, the engine will start just fine. I think that it is a software issue of the ECM because I have verified that all other parts are fine.
I apologize for my ignorance, but where is the ECM? Is this the one that lives in front of the passenger's knees, or is it contained in one of the modules (e.g., E-gas or T/LLR)?
 

gsxr

.036 Hoonigan™, E500E Boffin
Staff member
I think my "ECM" he means the primary engine control module, which for HFM injection combines the fuel+ignition brains into a single computer. On our cars it's the LH module and EZL as two separate items.

The module below the passenger's feet is the DM (Diagnostic Module), a North-America-only box that turns on the CEL when it detects anything that may affect emissions.
 

gerryvz

Site Honcho
Staff member
For a V-8 powered W124, all of the primary computer modules will be located in the CAN box underhood on the passenger side, just ahead of the firewall. The one exception is the DM (Diagnostic Module), which is located under the dashboard on the driver's side.

For the later (HFM/M104) powered W124 models, the ABS (or ABS/ASR) and HFM injection computer modules are located underhood on the passenger side, in a vertical orientation under the windshield area behind the battery. The DM is located in generally the same area on the six-cylinder cars -- under the driver's side dashboard above the parking brake and brake pedal area.
 

emerydc8

Active member
Thanks, guys. I'll install the rebuilt ETA and see if that resolves the problem. I'm hoping I get an autopsy from Don Roden on the ETA. Otherwise, maybe the replacement T/LLR and LH module are bad too, although I think the chances are probably pretty low. I haven't gotten any codes indicating a bad MAF sensor, but that may be the next step if the rebuilt ETA doesn't fix it:detective:
 

gerryvz

Site Honcho
Staff member
I have a spare ABS-only module from a 400E that did not have ASR. If you want it, Jon, let me know. I will let it go for very cheap (my cost and postage) as it is not of use to any E500E and even many 400E420 owners.
 

emerydc8

Active member
UPDATE:

I got the rebuilt ETA back from Don Roden. I checked with him and he said he doesn't do a postmortem unless specifically requested, so I have no idea if my original ETA I sent him was bad.

After installing the rebuilt ETA, I drove it about 100 miles and started it a few dozen times. Let it idle for at least 30 minutes too. I'm still getting an intermittent high idle. It has consistently occurred after the first start of the day (the second start), but it has only been two days so far. So, I don't know how long the adaption process takes here; and I can't remember how to do a manual adaption reset.

At this point I can't say whether the ETA was the problem. Maybe I should try a third T/LLR module, or try a new mass air flow sensor. Sure is frustrating.
 

a777fan

Active member
Jon...

Two things which may or may not help,

1) did you let the eta ‘adapt’ by turning the key to the ON position and letting it sit for 90 seconds or so? You’ll hear it whirring and clicking in there while its doing its thing

2) did you confirm the throttle linkages are all in alignment per the FSM? Most notably, the engagement of the idle control switch when the throttle is closed? When I had mine in and out, i didn’t adjust the linkages properly and the idle control switch wasn’t engaging. This resulted in a high idle. Its a long shot but perhaps you’re current adjustments are right at the edge of the switch engaging and something about warming the car up is pulling the linkages enough to disengage the switch? Its easy enough to test. Next time it acts up, pull the air box and forcibly pull the linkage on the eta towards idle. If the switch isn’t engaged, you’ll likely be able to pull it closed and hear a small ‘click’ when it engages and trips the idle switch.

EDIT: if you suspect the MAF try disconnecting it and see if it returns to more ‘normal’ ops. Although I’m not sure how ‘normal’ open loop ops for the LH module appear... that might also result in a slightly high idle?


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