Adjusting an EHA. It IS possible!!
 

My '87 260E has always been a somewhat reluctant starter and needed to warm up for a minute before it would "take the throttle" with any enthusiasm. I attributed that to age & 200,000 plus miles. Something to live with. Replacing half a dozen rubber bits and a couple of hard vacuum line helped lots, but not completely.

Then I got a copy of Stu Ritter's book, "MB E-class owner's bible". It's got some good reading. It's more of a "how things work" than a "fix it like this" book. I stumbled across a paragraph on M103 drivability & the problems described fit me to a T. There is an adjustment to the electro-hydraulic actuator (EHA) that hangs off the back side of the fuel distributor. This is the gizmo that takes the commands from the ECU and adjusts the fuel distributor pressure. It's the "muscle" for the ECU's "brains".

Anyway, pull the air cleaner box, the connector and, two screws later, the EHA is in my hand. There are two small O-rings that stayed on the side of the fuel distributor. Gas does squirt out when loosening the screws, but only a few drops. It would smart to get it in the eyes. On the side of the EHA that faces the dist, there is a brass screw with a straight slot. This is just a cover. Unscrew it about ten turns (very, very fine threads) and underneath is a brass setscrew with a 2mm internal hex. 1/4 turn clockwise, and reassemble all "in reverse order" as they say.

The difference was immediately apparent on my somewhat cooled off engine. About two hours since driven. Fired up quicker & idled smoother. The acid test will be tomorrow AM when the engine is completely cold. Well, cold for NJ in September.

Stu says this isn't really making the engine run richer, it's only making up for normal wear & tear. Bringing it back to the original spec's, as it were. OK! :cool:

Smooth,

First I've heard of this adjustment, but adjusting the duty cycle as explained at:

www.landiss.com/mixture.htm

will usually solve cold start problems if you set the duty cycle to about 45 percent at idle.

Duke

Never heard of a "2mm" screw. All of the specialty tools made for EHA adj. are 3mm.

Interesting.

Excellent link, Duke. That adjustment would need to be made after the one I described. That one affects the mechanical operation of the fuel injection that's varying the upper chamber fuel distributor pressure. That adjustment is really only an idle adjustment. It does change the duty cycle, but only at idle. MB calls it the CO at idle adjustment. That's why the steel ball over the screw, to keep you & me out of it. :) good idle quality generally means a slightly higher CO at idle. Big deal.

The part that the duty signal goes to is the EHA. The adjustment I described changes the lower chamber pressure in the fuel distributor. This is kind of an "over the whole range" adjustment. It seems to have the most effect on idle & just off idle operation. I'll bet there's an interaction between the two adjustments that the old-timers knew about, but never got written down. Ever get the feeling you're re-inventing the wheel? Or maybe just re-discovering it?

Mike, I just re-checked. It is a 2mm hex drive. It's the only adjustment on the EHA itself. FWIW, it's the second smallest of a Bondhus balldriver set. Smallest in my set is 1.5 mm.

BTW, it worked wonderfully. 90% improvement. Maybe another 45 degree turn this afternoon to make it 100%.

Hi nglitz

I like to know how things work
Is this the brass screw you are refering to ( see picture)? and the other for adjustment?

In one of my pevious posts I asked the question of what was the use of that screw. Unfortunately no reply.

I guess the picture didn't come through. The adjustment screw isn't visible with the EHA on the fuel dist. Once you undo the electrical connector and remove the two screws holding the EHA to the dist (watch for the fuel spritz) you can pull it off. Make sure the two O-rings that seal the EHA to the dist aren't lost. The face of the EHA that's against the fuel dist, has a couple of holes in between the attachment screws' holes. Below that is a brass, slotted screw that's flush with the surface. This is the cover or cap. Unscrew it CCW, it's probably lightly siezed and will take about ten rotations to come off. It has very fine threads, possibly to make a fuel-proof seal. Under that is what looks like a brass setscrew with the 2mm internal hex. Moving this will adjust the lower chamber pressure in the fuel distributor. There is a MB special tool to measure this. CW rotation increase mixture richness and CCW leans. 1/4 turn made a lage difference in my car. Putting the cover screw back in, it feels like it would be very easy to cross thread. I used only my fingetip until I was sure the thread was correctly engaged.

Exactly what's inside the EHA, I have no idea, but it's a heavy little sucker for its size.

Started fine at lunchtime, too,

I think you might have a misconception of how a modern 02 sensor fuel system works. Once the engine and 02 sensor are warmed up sufficiently to allow closed loop operation the system maintains an average stoichiometric A/F ratio except at WOT. A stoichiometric ratio is required to achieve maximum oxidation and reduction efficiency from a three way catalyst. The duty cycle wave form is generated by the ECU as a representation of how much time the system spends slightly lean of stoichiometric, and it should be in the range of 40-60 percent to allow maximum correction authority. If it is near zero or 100 percent the system will not be able to provide sufficient correction to maintain a stoichiometric mixture. When the O2 sensors reads lean the electronics orders the EHA to slightly richen. Then when the O2 sensors reads rich the electronics order sthe EHA to lean the mixture. This feeback control loop operates continuously other than cold start/warmup and WOT at a frequency in the range of 0.5 to 2.0 Hertz.

By using the mixture adjustment screw you are setting the basic mechanical calibration, which determines how the KE system meters fuel before closed loop operation is achieved, and the base setting that is corrected as necessary by the electonics during closed loop operation. Setting it at about 45 percent at idle will be slightly rich, and I have found this aids cold starting in mild weather. The procedure also calls for checking at 2000 RPM no load and both readings should be in the range of 40-60 percent IIRC.

Regardless of where you establish the basic mechanical adjustments, the electronics and 02 sensor will maintain a stoichiometric mixture during closed loop operation unless the mechanical adjustment is biased so far that the electonics runs out of adjustment authority, in which case it will very likely bust an emission test big time.

Duke

I may try this on my M102! I have to wait about 30 seconds in the morning to take off and it stumbles at first, then once I am moving for a few feet and the engine has done some work it clears up the hesitation. I also get a slightly rough idle. Well it is a heck of a lot better now than before, but I can still feel her running and I want it a hair smoother. I have replaced all the rubber hoses and all the vacuum connectors and that helped a lot as you said but still not perfect. That and I think I need to open my Idle Air Control valve a hair more to get her to idle at 1000RPM's instead of about 900 that it does not when cold. When I unplug my switch on the throttle linkage, to simulate the gas pedal not at the stop my RPM's go to 900, I assume the IACV closes at the point the switch is disenganged and that of course seems like the high idle point. Oh well I will try the EHA first if my problems sound like yours was running.

Duke, I do know how the O2 sensor works. I can see the duty cycle fluctuating on my meter. I've had feedback carbs and fully electronic FI systems. My 260 needed significant throttle to fire (cranks like mad) and I had to keep it revved to keep it running until it warmed up. If I took my foot off the throttle, it would run for 5 seconds until the cold start valve quit and then die. After four or five of those, it would stumble along for a minute or so. It was just way too lean at cold idle, it even sounded "dry". Needless to say, all of this is in open loop. Once warmed up and in closed loop, it ran fine. Until then crapola.

1/4 turn of this magic EHA adjustment screw and the world is wonderful again. Tickle the throttle while cranking, it fires; foot off & it idles like it should. According to Stu Ritter, this sets the lower chamber pressure for the fuel distributor. That's what it regulates the upper chamber pressure on at idle with a cold O2 sensor. Once the O2 sensor kicks in, it just sets the duty cycle to a little more "off time" to compensate. Now I've got to find a CO meter I can borrow to set the "flap" for a nice warm idle. Stu's recommendation is to use a CIS fuel pressure gauge to set the EHA screw. Original spec is 0.40 bar, his recommendation on an older car is 0.45 to "compensate for basic wear & tear and any unmeasured air leaks that have developed over the years." As I didn't have the specialized gauge, I just tweaked the screw a little. It worked, Stu was right.

One of my favorite engneering sayings is "If it happened, it must be possible". This happened.

Richer,

Well did mine 1/4 turn and now she fires up almost instantly and has minimal if any hesitation when cold. Need to test her on the highway for WOT under load at speed operation. I was getting a dead spot when trying to accelerate up a mild grade while at 75, just holds its speed and does not really increase much. Even my Saturn SL1 with 100Hp and a 4 speed auto would pull at 75, not dramatically but she would accelerate. I am not expecting the 190 to pull like the W203, but I expect a little more from a vehicle designed for highway driving. Will let you know what I find after lunch today.

Second try with the pictures.
A question may raise when adjusting the lower chamber pressure. Does the middle membran of the fuel distributor still resist and how long?

Well the highway test was nice. She actually accelerates going up a slight grade now where before it held speed when floored and accelerated like a turtle heading into the wind! She actually goes faster now as time progresses......:D

CC260 that's the beastie, though I never pulled mine that far apart. Not quite sure I understand your question, "Does the membrne still resist?" I'll ass-u-me you mean does it resist the increase in pressure? As there's all of a 11% change if I hit Stu's numbers, I'll take a wag that it's ok. I've never had one apart & hope to never need to. People who have, tell me they look simple but never worked quite the same after re-assembly. The lower pressure is constantly being changed as the system regulates mixture anyway. Both the inner arm of the flapper and the ECU's duty cycle sent to the EHA are making changes constantly.

mctwin2kman Excellent!! :cool: I stayed with the single 1/4 turn CW adjustment on the EHA, but needed a 1/8 screw turn CCW on the idle CO adjustment on the flapper arm to offset it once it warmed up. That was pretty subtle. I've GOT to rig up a way to read out duty cycle & maybe even O2 sensor voltage inside the car while driving it.

I just adjusted my EHA. I hope to see the same level of improvement. Thanks for the great write up.

Yeah my mixture changed of course so I had to adjust a hair leaner as when the EHA was adjusted it made it rich accross the board. I know get better response when the accelerator is depressed and do not need to let the engine warm up for a minute or so at cold start now since I am not getting the stumble anymore. I still think I need new injectors as I can feel my engine idle ever so slightly and I have been in other 190's with the 2.6 and 2.3 engine of all ages and they idle smooth as glass. But it is far better than when I got the car and could shake a martini by placing the mixer on the dash! Worked great so far and maybe I will get better gas mileage now that I do not need to floor it to get a response from the engine. I take it that after 18 years and 160,000 miles the fuel distributor may not be as good as it once was for regulating flow properly. So it needs to be tweaked.... And Tweaked it is now and running almost like a new car!