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Discussion Starter #1 (Edited)
If your idle starts at 900-1000rpm and as it warms up it races and "creeps" up and goes up to 13-1500rpm, then this is what you should investigate if all else is in order. My situation-1990 560sec. Vacuum leaks-none, OVP-functional, ICU relay-working, ICV-working, Duty cycle-%50 (prior to the idle "creep"). Must be the TPS?! The sensor attached to the throttle body, telling the computer your throttle is more open than it really is. Before you dig into the TPS(throttle position sensor, the 30 dollar part attached to the throttle body housing) and all the labor that entails, I recommend you test your Air Flow Potentiometer Throttle Sensor. You do this by putting a voltage meter on the top 2 pins and look for a reading at idle of .7 volts. If you don't get that reading, then read on. The part is made by Bosch. (Don't buy anything other Bosch IMO). Don't listen to the eBay people selling "the black edition" developed in Germany(but made in China) made in the same factory as Bosch, just not Bosch name, etc. yada, yada yada. You can see them all on Alibaba from China for 1.50-3 bucks. It's sold by Mercedes (and salvage yards) only attached to the airflow meter. The best deal for the switch alone I found on Amazon, Bosch F026T03021 for 253 bucks and free shipping. I would potentially(no pun intended) buy from a salvage yard if I could truly verify and trust the lineage(how many miles on the donor car). It's the black part on the passenger side, rectangular in shape, with a 3 prong wire attached. You remove by 4 screws in the corners, hidden by "half moon" shaped plastic pieces. Outline the shape with a marker before you remove it, as it is adjusted by it's position. When removed you will see "wear patterns" on 2 areas on the carbon electrode. The needle like arms inside the flow meter rub on the carbon areas to translate the mechanical arms action, into eletrical impulses to indicate the position of the air intake plate. Transmitting the info to the ICU when it's in the Idle, partially open & WOT positions. If you look at the attached pictures you will see mine has worn through the carbon to the copper base, at what would be the idle and partially open positions. The needle must contact the black carbon areas to register the correct reading. So as my car warmed up the worn Air Flow Potentiometer Throttle Sensor was sending incorrect air flow rates. This is one of those detail oriented parts that you need to have working correctly sending accurate information. I would buy a used Mercedes/Bosch from a wrecked car with say 100k miles on it for 100 or 150 bucks with an air flow meter and boot attached to it, before I would spend 12-60 bucks on the part only new on eBay.

Hopefully this has helped save someone's time and aggrivation from not knowing wtf is causing the dreaded "idle creep" and throwing money and parts at the problem.
 

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James560SEC, currently I‘m actually not active in this forum, but I‘d like to prevent further misconceptions from being added to the many misconceptions of the KE-Jetronic that are already circulating in this forum. :rolleyes:

I had to do with MB CIS-E engines before they went into mass production and I assure you that on 8-cyl MB CIS-E engines the AFM potentiometer has no influence on idle speed or idle quality … unless (!) idle speed control via IACV (idle air control valve) is not active … for instance, due to a faulty TPS (throttle position switch) or a faulty ICU (N8) or faulty input to it !

Now … before you object :) … consider that “idle speed control via IACV not being active“ does not mean that the engine can not idle. It does idle without idle speed control being active, but idle speed is not controlled in that case … and usually higher than it should be.

If, for instance, the TPS‘s “throttle closed“ signal is missing (although the throttle is closed), not only idle speed control via IACV is not active, but also the CIS-ECU (N3) continues to use input from the AFM potentiometer for a/f mixture changes via EHA control … (which it does not if it does receive a “throttle closed“ signal from the TPS). … And it can do that in an erratic way if there is a problem with the carbon tracks of the AFM potentiometer. And such an erratic mixture change can cause an “idle creep“ like the one you described.

Trust me James560SEC … the fact that a worn AFM potentiometer has an influence on idle speed shows that there is a problem with the idle speed control function chain. The first thing I suggest to do in your case is to check the TPS‘s “throttle closed“ signal input to both the CIS-ECU (N3) and the ICU (N8) !

If you believe this and want to get to the bottom of it, please ask the other forum members for further detailed advice/assistance. They should be able to take over up from this point.

Good luck!

H.D.
 

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Discussion Starter #3
Hello HD, why do other forum members need to take over when you are on a roll here? Which pins on the TPS connector indicate throttle closed and what should the reading be on the voltmeter?
 

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HI Thank you James and James. the voltage stes was -002vcd However my idle starts at about 1050rpm and never gets above 1150rpm So I dont think the potentiometer is the issue. car olnly has 157K miles on it. I think that I will continue down to road to the Idle Module replacement Jack,
 

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Discussion Starter #6
Thanks Mike For the instructions. I went to the Bosch website for their take on things on how/if this part affects idle. Replacing this worn Bosch part worked for me...I'm just sharing my experience and how it solved the surging idle.
Product information-The sensor plate potentiometer is installed in airflow sensors in the KE-Jetronic. It measures the deflection of the sensor plate and forwards this information to the control module.
In conjunction with other sensors such as the throttle valve sensor and lambda oxygen sensor, the control module can identify various engine operating states and correspondingly adapt the amount of injecting fuel.
Together with an idle actuator, the idling can also be stabilized to make engine idling smoother.
Customer benefits
  • delivered as ready-to-use parts set
  • stable idle
  • jerk-free acceleration
 

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Hahaha ... you don‘t trust me James560SEC, do you !? … ;)

My hope that you will believe this is rather limited ;), but maybe the following will at least be beneficial for other readers who own MB‘s with 8-cyl CIS-E engines. These are things that are hardly known and that are, like many other things about the KE-Jetronic, not even mentioned in the service manual.

... Together with an idle actuator, the idling can also be stabilized to make engine idling smoother. ...
You highlighted the less significant part of ^this sentence, James. ... You should have highlighted its first part: Together with an idle actuator, or better yet Together with an active idle actuator.

On the M103 engine, for instance, both components (IACV and AFM potentiometer) are part of the idle speed control function chain and have to work properly for a stable idle. See what I said in this thread: https://www.benzworld.org/threads/m103-engine-idle-issue-up-and-down.3037829/

On M116 & M117 engines, however, only the IACV has to work properly for a stable idle. The CIS-ECU (N3) of these engines ignores the input from the AFM potentiometer when idle speed control via IACV is active. In order to ignore that input, N3 has to receive a proper “throttle closed“ signal from the TPS. Without that signal AFM potentiometer input will not be ignored by N3 while idle speed control is active. In that case idle will not be very stable, especially if the AFM potentiometer sends erratic input to N3 due to worn carbon tracks.

On these 8-cyl engines the AFM potentiometer is not even connected to the idle speed control unit N8. If on these engines a bad AFM potentiometer causes idle to be unstable, then you know for sure that either idle speed control via IACV is not properly / not at all working due to a problem within the idle speed control function chain, or that the CIS-ECU (N3) is not receiving a proper “throttle closed“ signal from the TPS, which on your car … whether you believe it or not ;) … is apparently the case.

So James560SEC, it is well meant when I recommend you to take care of that issue … best starting by checking what I suggested to check in post #2. That, BTW, is not really described in the PDF that luckymike kindly attached in post #4. … Check if both the CIS-ECU (N3) and the ICU (N8) are receiving a proper “throttle closed“ signal from the TPS when the foot is off the accelerator pedal ! ... In case of M116 & M117 engines the ICU (N8) needs a proper “throttle closed“ signal in order to activate idle speed control … and the CIS-ECU (N3) needs a proper “throttle closed“ signal in order to ignore AFM potentiometer input during active idle speed control. So this input should be checked at the respective terminals of both of these control units (N3 & N8) !

As a side note:
It is generally much better to test sensor signals at the control unit(s) that process(es) them. That way the test covers the sensor plus wire(s) plus connector(s). After all, what‘s the use of an intact sensor, if its signals do not arrive at the control unit(s) that process(es) them !?;)

James560SEC, if you take care of this issue, you will be rewarded with an idle that is not only stable, but also within its correct speed range. … ;)

As I said in post #2, I‘m currently actually not participating in threads in this forum. But if you should be interested in checking these things after all and no one else in this forum can give you respective detailed procedures, let me know.

:)

H.D.
 

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Discussion Starter #8
I do believe you. I'm just glad I could elicit such an excellent , detailed response to further my education (and for the benefit of others) as we are trying to understand and fix these Ke-Jetronic issues. I'm far from an expert and I'm not defensive of my postings, especially if they draw out responses that are truly educational that give practical and potential fixes. Not just responses expounding on why something is wrong with a post, but a thoughtful plan to take corrective action. I am interested in checking these things. I have scoured the boards for potentiometer and it's contribution to the surging idle, as it's been a thorn in my side for quite a while. Things wear out, especially on an engine with 332k. Point being I am always interested in sponging up some knowledge of what to test to exclude or include parts that might be defective. Thanks HD!
 

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You‘re welcome, James560SEC.

... we are trying to understand and fix these Ke-Jetronic issues. ...
It‘s a bit like trying to understand a foreign language. There are words and there is grammar that connects the words to logical sentences. DIYers and even professional mechanics often know a few words but hardly any grammar, which in case of CIS-E is quite sophisticated and proper training for logical thinking, requiring some mental discipline … which explains the problems which usually emerge when CIS-E is spoken in forums. ;)

... I am always interested in sponging up some knowledge of what to test to exclude or include parts that might be defective. ...
A really useful key to “exclude or include parts that might be defective“ is the understanding of the KE-Jetronic‘s Lambda control function and the “duty cyce“ that Bosch kindly added to it as an easy to use, yet commonly ignored diagnostic tool. Most DIYers & mechanics don‘t even recognize the duty cycle (especially the non-static one) as a diagnostic tool and think that it is something that just needs to be readjusted via adjustment screw when it‘s out of line … making not only Bosch people shake their heads. :rolleyes:

... I am interested in checking these things. ...
I‘m currently quite busy with other things and would appreciate if other forum members are as kind as to assist you in detail up from this point.

The only quick tip I have enough time for now is one that is not mentioned in the service manual either. Check if the carpet or something else in the footwell is preventing the accelerator pedal to return completely to its rest position. That can prevent both control units (N8 & N3) from receiving a “throttle closed“ signal too. … If I‘d write a service manual for CIS-E cars, that would be the first thing I‘d suggest to check in case of high or unstable idle. ... Don‘t underestimate the time & money saving potential of a tidy footwell. :)

If your accelerator pedal is not hampered, check whether the throttle valve is really in touch with the stop at its closed position anyway ! ... If that is the case, continue with the test I suggested in post #7: check for proper “throttle closed“ signal at the respective terminals of both control unit connectors (N3 & N8) when the accelerator pedal is not pressed !

:)


H.D.
 
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