KE-Jetronic Lambda control (duty cycle) adjustment - Mercedes-Benz Forum
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post #1 of 65 (permalink) Old 10-24-2016, 05:44 AM Thread Starter
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KE-Jetronic Lambda control (duty cycle) adjustment

Before I get to my suggestion how to check and how to change the KE-Jetronic’s Lambda control adjustment, I’d like to address some basics in this first post, which might help better understand the matter. I’ll continue with some recommendations and considerations and the detailed procedures in post #2.

Basics:

Here at BenzWorld.org I see the Lambda control adjustment usually being called ‘duty cycle adjustment’, or (not really suitably) ‘mixture adjustment’ or ‘air/fuel mixture adjustment’, which can easily lead to (and possibly often reveals) misconception. … By changing the adjustment the position of the fuel distributor’s control plunger in relation to the air sensor plate’s position is changed, which in case of a K-Jetronic (without lambda control) results in a changed ‘air/fuel mixture’, but - aside from the engine’s warm-up phase, or completely floored accelerator, or limp home mode - not in case of a KE-Jetronic!

What is ‘Lambda control’?
It’s the fine-tuning of the air/fuel mixture to a ratio at which complete fuel combustion takes place, in order to minimize pollutants. That ratio is called ‘λ (Lambda) = 1’, which in case of non-ethanol fuel is given at an air/fuel ratio of about 14.7 mass units of air for 1 mass unit of fuel (14.7:1). It’s a compromise between engine torque and fuel consumption. The highest engine torque would be given at a ratio of about 12.5:1, and the lowest fuel consumption would be given at a ratio of about 16:1.

How does Lambda control work?
I like to use a metaphor for illustration. While driving along the road our eye tells our brain to which side the car is about to drift off-lane, the brain processes that information and tells our hand to turn the wheel a little to the left or to the right, which we more or less alternately do all the time. … Translated to the KE-Jetronic the lane is ‘λ = 1’, the eye is the o2 sensor, the brain is the ECU (electronic control unit), and the hand is the EHA (electro-hydraulic actuator).
The EHA is a valve via which fuel flows through the lower chambers of the fuel distributor’s pressure differential valves in order to control the quantity of fuel injection, hence the air/fuel mixture. The EHA’s baffle plate is electromagnetically moved closer to or further away from its inlet nozzle by positive or negative current from the ECU, by which the lower chamber pressure can be changed. And the lower chamber pressure controls the fuel flow through the upper chambers, each of which has a separate injector pipe port.
In order to detect whether complete fuel combustion is taking place, regardless of the type of fuel, the o2 sensor compares the amount of residual oxygen in the exhaust gas with the amount of oxygen in the ambient air. At the ratio which represents complete fuel combustion (λ = 1) the o2 sensor is very sensitive and generates a voltage of 450 mV. That voltage changes significantly at tiny changes of the oxygen ratio. At ‘λ = 0.98’ the o2 sensor voltage is about 800 mV, and at ‘λ = 1.02’ it’s about 100 mV. And as we can not keep the car in its lane without tiny adjustments via steering wheel, ‘λ’ can not be kept at ‘1’ without tiny mixture adjustments either. The air/fuel mixture is either a touch too lean or a touch too rich and alternately has to be enriched and leaned a touch (micro-tuned) in order to keep ‘λ’ close to ‘1’. Lambda fluctuates with about +/- 0.02 around 1, when the o2 sensor voltage fluctuates with about +/- 350 mV around 450 mV, which with a healthy o2 sensor hapens at a cycle frequency of about 0.5 – 1 Hz. That’s the o2 sensor voltage the ECU ‘wants’ to receive, and it adjusts the air/fuel mixture via EHA control in such a way that it does receive that voltage, regardless of the kind of fuel, which in case of non-ethanol fuel leads to an a/f mixture fluctuating with about +/- 0.3 around 14.7:1. In case of ethanol containing fuel it leads to a different (richer) mixture, depending on the percentage of ethanol in the fuel.
Here’s a simplified example of one Lambda control cycle with non-ethanol fuel, which takes about 2 seconds at idle:
- λ ~ 0.98, (a/f ~ 14.4:1), > o2 sensor voltage to ECU ~ 800 mV
- ECU generates more negative EHA current (duty cycle: 45%)
- leaning the air/fuel mixture
- λ ~ 1.02, (a/f ~ 15:1), > o2 sensor voltage to ECU ~ 100 mV
- ECU generates more positive EHA current (duty cycle: 49%)
- enriching the air/fuel mixture
- next cycle: λ ~ 0.98, (a/f ~ 14.4:1), > …..

What happens when we change the Lambda control adjustment?
Let’s say the duty cycle is fluctuating like in the above example between 45% and 49% at idle. When we change the adjustment by turning the adjustment screw cw the control plunger moves to a higher position, leading to a richer mixture. That immediately leads to o2 sensor voltage not undershooting 450 mV, upon which the ECU immediately reacts with an EHA current fluctuating around a more negative mean value in order to lean the mixture again, which is accompanied by a duty cycle fluctuating, for example, between 33% and 37%.
And when the control plunger is set to a lower position by turning the adjustment screw ccw, leading to a leaner mixture, the o2-sensor immediately reacts with voltage not overshooting 450 mV, upon which the ECU immediately reacts by sending a current fluctuating around a more positive mean value through the EHA’s coil in order to enrich the mixture again, which is accompanied by a duty cycle fluctuating, for example, between 57% and 61%.
No matter to which position the control plunger is set, unless it’s set too high by cw turns beyond the EHA’s ‘Lambda leaning limit’, or too low by ccw turns beyond the EHA’s ‘Lambda enriching limit’, the ECU always adjusts the air/fuel mixture via EHA control, regardless of the kind of fuel, in such a way that it receives o2 sensor voltage that fluctuates with about +/- 350 mV around 450 mV, which represents ‘λ ~ 1 +/- 0.02’ respectively ‘a/f ~ 14.7 +/- 0.3 : 1’ (in case of non-ethanol fuel).
Or, speaking in terms of ‘duty cycle’: No matter around which mean value the duty cycle fluctuates, as long as it’s above 5–10% (‘leaning limit’) and below 90–95% (‘enriching limit’), if it fluctuates, Lambda respectively the air/fuel mixture fluctuates around the correct ratio, intactness of the system provided, of course. However, around (or close to) 50% it does that more precisely than, for example, around 20% or 80% (I’ll get back to that in post #2).
Conclusion: When we change the duty cycle adjustment we change the operating ranges of both the control plunger and the EHA’s baffle plate … but not the mixture!
- Control plunger higher > EHA more open
- Control plunger lower > EHA more closed

What’s this ‘duty cycle’ about?
Parallel to the fluctuating EHA current the ECU sends a square wave voltage with a corresponding ‘on/off ratio’ to port 3 of the diagnostic coupling X11, where it can be measured in 'duty cycle', 'dwell angle' or 'volt'. This fluctuating duty cycle is an easier to check representative of the EHA current, and the duty cycle check / adjustment is actually an EHA current check / adjustment.
A duty cycle of 50% represents an EHA current of ‘0’ mA, a duty cycle below 50% represents negative current (flowing in one direction through the EHA’s coil), and a duty cycle above 50% represents positive current (flowing in the other direction through the EHA’s coil).
Additionally to the fluctuating duty cycle, a non-fluctuating (static) duty cycle while the engine is running serves as error code.

From the above it may also become clear that the air/fuel mixture can not only be ‘micro-adjusted’ (to ‘λ ~ 1 +/- 0.02’) via EHA control, as often assumed, but also ‘macro-adjusted’ - as long as the EHA’s leaning / enriching limits are not exceeded. Especially the leaning capability via EHA control is significant, and more precise.
Let me illustrate this ‘macro-adjustment’ with my driving-along-the-road metaphor:
As explaned above, the ‘micro-adjustment’ via EHA control is like the constantly done tiny adjustments via steering wheel to the left and right in order to keep the lane, no matter whether we’re driving along a straight road or through a curve. The ‘macro-adjustment’ is like the turning of the steering wheel in order to follow the road’s changed direction. The changed direction of the road represents a changed condition of the system … for example, a changed control plunger position in relation to the air sensor plate, or different fuel, or contamination in the fuel distributor, or a false air leak (of limited size of course), etc. …. conditions, all of which without EHA control would result in more or less significantly too rich / lean mixture.
The more or less far to the left or right turned steering wheel represents the EHA’s baffle plate position more or less far away from the EHA’s inlet nozzle, respectively an EHA current more or less far below or above ‘0’ mA, respectively a duty cycle more or less far below or above 50%.
And like we continue with the tiny adjustments of the steering wheel to the left and right in order to keep the lane, no matter whether we hold the steering wheel in the straight-ahead position on a straight road or turned to the left or to the right in a curve, the ECU continues with the tiny adjustments of the EHA’s baffle plate in order to keep ‘λ ~ 1 +/- 0.02’, no matter whether the baffle plate is operating closer to or further away to either side from its center position (closer to or further away from the EHA’s inlet nozzle).

Continued in post #2 …
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post #2 of 65 (permalink) Old 10-24-2016, 05:45 AM Thread Starter
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… continuation of post #1

Purpose of the adjustment:

Since, as I explaned in post #1, the air/fuel mixture is unchanged, no matter whether the system is adjusted to a higher or to a lower duty cycle … what is the purpose of the adjustment?
It’s the EHA’s optimal operating range with regard to:
- Lambda control (keeping ‘λ ~ 1 +/- 0.02’ by fine-tuning the air/fuel mixture)
- the engine’s running behavior.

Regarding Lambda control the EHA’s optimal operating range is given when its baffle plate oscillates around its center position (currentless rest position) – in other words, when the EHA current fluctuates around ‘0’ mA, which is represented by a duty cycle fluctuating around 50%. That way the EHA has its highest dosage accuracy.

Regarding running behavior, however, a slightly further opened EHA with its current fluctuating around a mean value slightly below ‘0’ mA at idle, which is represented by a duty cycle fluctuating around a mean value slightly below 50%, is better. That has i.a. to do with the in post #1 mentioned better leaning than enriching capability of the EHA. A duty cycle at idle fluctuating around a mean value of about 47% or a little lower is usually a good choice for an intact KE-Jetronic in my experience.

I’d like to add that there’s another advantage of a duty cycle close to 50%, respectively of an operating range of the EHA’s baffle plate close to its currentless rest position. That way the air/fuel mixture is almost unchanged if the KE-Jetronic goes into limp home mode due to a failure of the ‘E’ in ‘KE-Jetronic’, and during driving at normal operating temperature most drivers would probably not even notice any change.
That, btw, was a major argument in favor of the KE-Jetronic (as an advanced K-Jetronic) for Mercedes, at a time when BMW was already using the fully electronic L-Jetronic. They did not want to see pictures and reports in the media about S-Classes standing on Autobahn brakedown lanes due to injection system problems ever again either, which they did with the fully electronic D-Jetronic they introduced in the early 70s. Instead they prefered the owner to drive quietly to the dealership and tell the friendly people there that somehow the car behaves strangely for a minute or two after starting it in the morning ... lol.


Please note:

Above target values apply to an intact system (not only CIS) !
Generally problems relevant for the fuel combustion have an influence on the EHA control, hence on the duty cycle. Depending on the problem(s), the most suitable duty cycle could, for instance, be below 30% or above 70%.

Here are a few examples of relevant problems:
Oil or coolant getting into the combustion chamber(s), wrong or bad spark plugs, bad distributor cap/rotor, worn injectors, leaking cold start valve, contaminated metering slits in the fuel distributor, incorrect fuel pressure, false air leaks, clogged air filter, incorrect ignition timing / faulty vaccum advance, false input from o2 sensor, problems with the ECU, EHA’s baffle plate damaged, EHA’s coil damaged (resistance should be 18–21 Ω), incorrectly adjusted throttle linkage, throttle plate not resting against its idle stop, air sensor plate not centered or its ‘zero position’ incorrect, control plunger sluggish or stuck, problem with EGR valve, bad battery, bad voltage regulator, …

On the other hand, a duty cycle check might help to track such problems. A high duty cycle might be caused, for example, by a false air leak, which of course should be fixed instead of turning the duty cycle lower with the adjustment screw. A low duty cycle might be caused, for example, by a leaking cold start valve, which of course should be fixed instead of turning the duty cycle higher.

Correcting the duty cycle to the target value is often done too easily, IMHO, and should only be done if the elimination of the reason for its deviation is an option which doesn’t come into consideration, like maybe for instance in case of a problem inside the fuel distributor.
But, checking the duty cycle is too often neglected as a quicky and easily done diagnostic measure, IMO.

The kind of fuel being used (non-ethanol / ethanol-containing) has no effect on the validity of the above EHA current / duty cycle target values (see ‘Basics’ in post #1). However, it has of course an effect on the position to which the control plunger has to be set (via adjustment screw) in order to get to these values !
After a switch between fuel types, depending on the o2 sensor’s input, the ECU sends different amperage through the EHA in order to change its operating range, accompanied by a correspondingly different duty cycle, so that it continues to receive o2 sensor input fluctuating around 450 mV (which represents ‘λ ~ 1’). Therefore after a switch between fuel types the duty cycle should be checked, and if necessary readjusted to the target values !

I’d also like to point out, that the duty cycle adjustment does not ensure corresponding optimal results if the adjustment of the EHA’s baffle plate has been improperly changed !
Changing the EHA’s adjustment can make sense in case of a changed fuel distributor condition. There could, for example, be contamination, or the pressure differential valve’s diaphragms / springs may have been replaced and differ from the original ones, etc.. But bear in mind that by changing the EHA’s adjustment the mechanically predetermined fuel flow rate through the lower chambers of the pressure differential valves in relation to the fuel flow rate through the meetering slits into the upper chambers is changed, which IMO should not be done without adequate know-how and care. If done improperly the EHA current’s / duty cycle’s informative value is gone! … and it’s proper adjustment, as for instance also required for other tests, impossible !


Measuring device:

I suggest to either use an analog duty cycle meter or an analog voltmeter. Analog meters offer more comfortable monitoring of the fluctuating readings than digital meters.

Duty cycle meter:
Some duty cycle meters show the percentage of the square wave voltage’s ‘on’-time, and others show the percentage of its ‘off’-time. In case of the KE-Jetronic the duty cycle value refers to the square wave voltage’s ‘off’-time. A meter which shows the ‘on’-time would, for instance, read 53% instead of the relevant 47%.
If you’re not sure which version your meter is: With ignition switched on (engine not running) the duty cycle should be about 70% (California: 85%). If the meter shows about 30% (California: 15%), it’s probably the wrong version. And if, while the engine is running, the fluctuating duty cycle drops when the adjustment screw is turned cw, it’s the right version.

Voltmeter:
The voltage is converted to duty cycle according to the following formula:
duty cycle [%] = [1 - (Vp3 / Vp6)] * 100
Vp3 = voltage between X11 port 3 & port 2 (or ground)
Vp6 = (battery) voltage between X11 port 6 & port 2 (or ground) during the respective rev !
Example for a measurement at idle:
Vp3 (at idle): 7.1 - 7.6 V
Vp6 (at idle): 13.9 V
duty cycle at 7.1 V = [1 - (7.1 / 13.9)] * 100 = 48.9%
duty cycle at 7.6 V = [1 - (7.6 / 13.9)] * 100 = 45.3%
duty cycle mean value: (48.9% + 45.3%) / 2 = 47.1% (fluctuating with +/- 1.8%)


Preparations:

  • In case of California version the ECU may have to be switched over to duty cycle output. Check the service manual for instructions if necessary.
  • Warm up the engine to its normal operating temperature. A 10-minute warm-up drive is better than letting the engine idle until it’s warm. Make sure that the engine does not heat up too much during the check / adjustment procedure.
  • Pull off the vacuum line between the throttle valve and the regeneration valve of the fuel evaporation system at the regeneration valve and block it.
  • Keep the A/C switched off.

Check procedures:

  • Connect the meter to the diagnostic coupling X11 port 3 and 2 (or ground).
  • With ignition switched on (engine not running) the duty cycle should be about 70% (California: 85%).
    If you’re using a voltmeter it should read 0.3 * Vp6 (California: 0.15 * Vp6)
    If in case of engine M116 / M117 the duty cycle is 100%: The installed ECU version doesn’t feature fault diagnosis via static duty cycle.
  • Take off the air filter lid and check two other duty cycle values with ignition switched on (engine not running):
    With the throttle closed and the air sensor plate deflected the duty cycle should be about 10%. If it stays at 70% there may be a problem with the ‘closed signal’ of the throttle position sensor.
    With the throttle fully opened and the air sensor plate not deflected the duty cycle should be about 20%. If it only drops to 40% there’s a problem with the air flow potentiometer.
    Put the air filter lid back on for the duty cycle check with the engine running, which should be done with the air filter installed (and clean)!
  • Start the engine, let it idle and wait until the reading starts to fluctuate (it takes a moment until the o2-sensor reaches its operating temperature). If it doesn’t start to fluctuate after a while, the meter may be displaying a static error code (see ‘Static duty cycle’ further down).
  • Increase the engine’s speed and monitor the meter while you keep the speed at about 2500 rpm. The reading should fluctuate! Record the values between which it fluctuates – it should be a range not much bigger than 4%, for instance: valley = 42%, peak = 46% (mean value = 44%). The fluctuation frequency (1 cycle = from ‘valley’ to ‘peak’ and back to ‘valley’) should be about 1 Hz (1 cycle per second).
  • Then check the reading at idle. Again it should fluctuate, and again record the values between which it fluctuates. The fluctuation frequency should be about 0.5 Hz (1 cycle per 2 seconds).
  • The mean value at idle should not differ by more than +/- 10% from the mean value at 2500 rpm.
    In case of engine M116 / M117 of model years ’86 & ‘87 the mean value at idle should be 5-15% higher than the mean value at 2500 rpm.

Adjustment procedures:

  • Remove the plug from the adjustment tower (if it’s still in there), so that the Allen wrench can be inserted. You can put a drop of oil into the adjustment tower if you like.
  • Then start the engine, let it idle and wait until the reading starts to fluctuate again.
  • Please note: adjustments are always done at idle (not at the higher rev)!
  • Then insert a 3 mm Allen wrench into the spring-loaded adjustment pin in the adjustment tower and carefully push it down. Don’t put too much pressure on it, otherwise the air sensor plate’s lever below the adjustment pin might be pushed down, which can easily stall the engine. With the Allen wrench engaged, turn the adjustment pin a little to and fro in order to let it snap into the actual adjustment srew, which is located in the air sensor plate’s lever.
  • Turn the adjustment srew in small steps. Even tiny turns can change the duty cycle by several percent.
    Cw turns lower the duty cycle … ccw turns raise the duty cycle.
  • After each step briefly rev the engine and let it settle for about 10 seconds before taking readings.
  • I recommend to record the total adjustment angle. If you turn the adjustment srew too far the engine will stall. And if you can not remember how far and in which direction you have turned it, you may not get the engine restarted. Then the KE-Jetronic needs to be reset in order to get the engine started again, which is not very difficult, but unnecessary labor.
  • After the adjustment to the desired value at idle, check the duty cycle at 2500 rpm and then again at idle.
    Readjust if the mean value difference between both engine speeds exceeds the above-named allowance.

Static duty cycle:

A static (not fluctuating) duty cycle value with the engine running and the o2-sensor at operating temperature, indicates a problem according to the following list:
  • 0%: problem with the meter
    or diagnostic coupling (X11)
    or too rich setting (beyond the EHA’s ‘leaning limit’)
  • 10%: TPS (throttle position sensor), throttle fully closed signal
    or (if at 2.000 rpm) no/false supply voltage to POT (air flow potentiometer)
  • 20%: TPS, ‘throttle fully open’ signal
  • 30%: CTS (coolant temperature sensor)
  • 40%: no/false output voltage from POT
  • 50%: o2 sensor (aside from not having reached its operating temperature yet)
  • 60%: car speed signal (displayed during driving or engine still running after driving)
  • 70%: CPS (crankshaft position sensor)
    or EZL (electronic ignition module)
  • 80%: IATS (intake air temperature sensor)
  • 95%: micro switch of throttle linkage (4- and 6-cylinder engines)
  • 100%: problem with the meter
    or diagnostic coupling (X11)
    or ECU ‘N3’ (missing connection to voltage supply or to ground)
    or OVP (overvoltage protection relay)
    or o2 sensor signal (short to ground)
    or too lean setting (beyond the EHA’s ‘enriching limit’)

Consider that in case of a static duty cycle reading, there may be just a problem with the connection of a component (loose / broken cable, damaged plug) instead the component itself.

Depending on the running behavior after duty cycle readjustment an additional EHA check which includes overrun cut-off and acceleration enrichment and lower chamber fuel pressure tests under various engine operating modes may be recommendable.

Don’t forget to reconnect the vacuum line of the fuel evaporation system!

H.D.

Last edited by H.D.; 10-31-2016 at 12:42 AM. Reason: Added 2 sentences under 'Please note' (correcting / checking the duty cycle)
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post #3 of 65 (permalink) Old 10-24-2016, 12:47 PM
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Thank you H.D. for your very informative post
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post #4 of 65 (permalink) Old 10-25-2016, 10:49 AM
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I canNOT wait to carefully read this. Thank you, H.D.!

Okay, have read it and am letting it soak in. Great write up, H.D. I really appreciate your taking the time to do this. It's an excellent resource document.
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Last edited by luckymike; 10-26-2016 at 05:05 PM. Reason: update after reading hd post
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post #5 of 65 (permalink) Old 11-08-2016, 11:04 AM
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Finally, an explanation I understand.

Thank you, H.D.
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post #6 of 65 (permalink) Old 11-08-2016, 04:41 PM
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A big thank you to H.D. for this write up. It obviously took a lot of time and effort. It is appreciated.
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post #7 of 65 (permalink) Old 11-15-2016, 12:31 PM
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This is the best explanation of this adjustment I've seen. Thanks for making it so clear!

-J
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post #8 of 65 (permalink) Old 12-04-2016, 03:03 AM Thread Starter
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Quote:
Originally Posted by H.D. View Post
… Regarding running behavior, however, a slightly further opened EHA with its current fluctuating around a mean value slightly below ‘0’ mA at idle, which is represented by a duty cycle fluctuating around a mean value slightly below 50%, is better. That has i.a. to do with the in post #1 mentioned better leaning than enriching capability of the EHA. A duty cycle at idle fluctuating around a mean value of about 47% or a little lower is usually a good choice for an intact KE-Jetronic in my experience. ...
I’d like to add some content to the above "i.a." :

Another reason for a duty cycle preferably below 50% is contamination.

Deposits can, for instance, narrow the 0.2 mm (~ 0.008“) wide vertical metering slits through which the fuel flows from the control plunger side into the upper chambers of the fuel distributor’s pressure differential valves. With the control plunger set to a slightly higher position via Lambda adjustment screw, these metering slits are a little wider open, which, as described in detail in post 1, in order to continue to receive confirmation of the correct air/fuel mixture (λ ~ 1) from the o2 sensor, is compensated via EHA control by more negative EHA current (lower duty cycle). Depending on the degree of contamination an adjustment to a rather low duty cycle might be beneficial for the engine’s running behavior.

However, if the degree of contamination in the fuel distributor requires a duty cycle below 35% to achive the best possible running behavior, and if a fuel distributor replacement or refurbishment does not come into consideration, changing the mechanical adjustment of the EHA’s baffle plate might be an option.
But, like refurbishing the fuel distributor, that requires adequate knowledge and significant care, patience and cleanness ! … I would not recommend to touch that tiny EHA adjustment screw if the EHA is looked at as a “black box” attached to another “black (gray) box”. … I’m thinking about creating a separate detailed thread about checking and changing the “EHA adjustment”, similar to this “duty cyle adjustment” thread.

Talking about contamination …
The KE-Jetronic is not overly enthusiastic about ethanol containing fuel, especially when the car sits a lot. I recommend not to put the car into hibernation for a couple of months with the fuel system filled with fuel containing more than 5% ethanol ! … If it’s unavoidable, I recommend at least to use a good and proper fuel additive, which I recommend in case of fuel containing more than 5% ethanol anyway, even if the car is driven long distance every day.
Ethanol containing fuel can lead to increased deposit forming and to acidification, which can lead to corrosion of the aluminium … and aluminum … parts of the KE-Jetronic.
There are fuel additives on the market which promise to prevent these effects and which, aside from the corrosion, even promise to reverse them … and I don’t say they don’t.

H.D.
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post #9 of 65 (permalink) Old 12-04-2016, 03:10 AM
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EHA adjustment would be very useful. I ended up doing mine by feel/trial and error.
Many of these will be getting replaced, no doubt ethanol has a part to play in the leaking that occurs and leads to replacement.
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