TUNE YOUR EHA WHILE DRIVING - Mercedes-Benz Forum
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post #1 of 48 (permalink) Old 02-06-2018, 01:42 PM Thread Starter
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TUNE YOUR EHA WHILE DRIVING

So I finally gave up on changing EHA's that bleed like they've been shot with a Thompson submachine. Went to the yard and picked a few used old ones and they leaked like a sieve too.

I finally had it. Took one of the old ones and decided to try to fix the leaks of my 1/2 dozen valves without opening them. But before I needed to see what they looked like inside to get a better understanding of what really leaks. The pictures below show a sequence of the disassembly which required paper-thing layer removal with milling machine.

Before I continue, let me add my two cents on my experience with servo-valves: I worked with them for many years using them in dynamic shock absorbers dynos for NASCAR and INDY racing and also for projects related to NASA and I can seriously say with some authority that the 560SEC EHA valve is "one piece of craftsmanship", for having been designed back in the 70's and for the use in a car.

The valve comes apart in three pieces: the body with coil, the inner valve fulcrum and the cover. There are two oval rectangular O-rings that seal the fulcrum plate against the cover and the body. Also we have the mixture adjusting screw which I managed to have access to for tuning while driving and WITHOUT having to remove the EHA valve every time I adjust (more below).
TUNE YOUR EHA WHILE DRIVING-img_0020.jpg

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TUNE YOUR EHA WHILE DRIVING-img_0023.jpg

TUNE YOUR EHA WHILE DRIVING-img_0025.jpg

TUNE YOUR EHA WHILE DRIVING-img_0026.jpg

The main reason these valves leak is not a design problem, but rather the result of changing the formulation of our gasoline throughout the world. The oval O-rings which resemble an Indy-500 race track map, were designed with Nuna-N (NBR, Nitrile) and even though many chemical compatibility charts say it is O.K. to use with alcohol (E85), they eventually succumb to the dehydration effect of the alcohol and shrink in size.

This shrinkage make both oval O-rings to reduce its cross section and they leak through the brass-to-plastic case overmold. I did find a solution and made my previously leaking valves operational again for a while by soaking them internally with 6DPP antioxidant and methyl salicylate along with a stabilizer and after 24 hours soaking the valves were operational at full pressure again. The only drawback is they will eventually dry up again if used with the same gasoline.

Lastly, in order to tune the EHA without having to take it off the CIS head, I made this adapter below that works perfectly for tuning the 1.5mm Allen setscrew mixture adjustment. Now I will have to find the forum post where it details how to tune the EHA while monitoring the X11 mixture ratio and all the other stuff, so I'll let you know later how it went.

Cherry2728
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Last edited by Cherry2728; 02-06-2018 at 01:45 PM.
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post #2 of 48 (permalink) Old 02-06-2018, 01:50 PM Thread Starter
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More pictures on the EHA TUNEUP NA479-00 adapter:
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post #3 of 48 (permalink) Old 02-06-2018, 02:10 PM
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Absolute genius! Many thanks, always wondered what was inside these. Love the adapter idea, tuning my eha took an age with all the driving needed and removal every time.

'86 420SE
'90 420SEC


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post #4 of 48 (permalink) Old 02-06-2018, 02:35 PM
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Cherry, you never fail to amaze. Fantastic work.

I've got some experience with adjusting the EHA so if you don't get what you need from the archives, let me know.

Wonderful. Just wonderful!
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post #5 of 48 (permalink) Old 02-06-2018, 05:29 PM
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Great post and photography! Additionally, it's always music to my ears when I hear praise about the craftsmanship of these cars or parts thereof. Usually it's from those having experience with lesser marques, but in this case I am especially proud to see a NASA comparison
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post #6 of 48 (permalink) Old 02-06-2018, 08:08 PM
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Wow that is genius work! A few newbie questions.

Is it still possible to buy a new EHA from the dealer and if so what is the approx cost? Do any companies rebuild them?

What are these EHA 'adjustments' that people mention? Do they need to be tweaked from time to time?

Cherry2728 would you be willing to produce and sell your adapter that facilitates the adjustment process?
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post #7 of 48 (permalink) Old 02-07-2018, 09:32 AM
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Yes from the dealer, mine was about £100 I think. They come with an average setting which apparently works for most peoples cars. But sometimes it needs to be tuned a little with the screw. After that its not something you should really need to mess with again.

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post #8 of 48 (permalink) Old 02-07-2018, 03:43 PM
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Quote:
Originally Posted by Cherry2728 View Post
... I can seriously say with some authority that the 560SEC EHA valve is "one piece of craftsmanship" ...
How refreshing to read something like this, Cherry2728 (and Tyrone D.) ... … IMO, the whole KE-Jetronic is a remarkable piece of mechanical engineering … and the decision for that system is one of the most typical representatives of the spirit 1980‘s MB‘s were designed with.

Quote:
Originally Posted by Cherry2728 View Post
... in order to tune the EHA without having to take it off the CIS head, I made this adapter below that works perfectly for tuning the 1.5mm Allen setscrew ...
From the pictures it seems that the adapter redirects the fuel flow 6 times by 90° per line (inlet and outlet). … If so, these 12 redirections may cause a pressure loss, that leads to a setting of the adjustment screw that‘s too far CCW when the EHA is mounted back directly to the FD.

With a design according to the attached sketch fuel flow redirections per line are reduced to 2 negligible soft (~45°) bends … of which the one on the right isn‘t necessary either. …

The actual purpose for this adjustment screw is the new EHA‘s factory calibration. Changing it is an option that should only be considered in case of a problem inside the FD (Fuel Distributor), if replacing or refurbishing the FD does not come into consideration. … Also a refurbished FD might require EHA tuning, for instance, if its diaphragm or its springs differ from original parts.
And in case of a problem with the EHA itself (besides removable contamination) I suggest to replace it.

Before the EHA adjustment screw is touched, I suggest to make sure that everything else is in order, including the ignition system. Also things that are often overlooked, like e.g. the air flow meter plate‘s “zero position“ and it‘s play, should be in order. … The EHA tuning should be done according to EHA current and lower chamber fuel pressure.

H.D.
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post #9 of 48 (permalink) Old 02-08-2018, 01:12 AM
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Yes I still wonder what problem I may have been compensating for on my SEC when I went on my EHA adjustment journey after a couple of used ones and then new.
The car runs extremely well since then, very powerful and creamy smooth. But in the back of my mind I do still wonder what it was, unless the original EHA was just faulty due to ethanol and sitting. My new EHA still produced a hesitation although not so bad. I adjusted it carefully by experimentation until the car was spot on.

But I digress. Love the sketch HD, looks like art.

'86 420SE
'90 420SEC


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post #10 of 48 (permalink) Old 02-08-2018, 11:46 AM Thread Starter
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Thank you all for the nice comments on my post. I do agree in more than the design and advanced technology of the EHA, but the entire CIS system is one piece of work that it probably might not be reproduced today for a production car. A time gone bye, bye I'm afraid.

Good points HD on the turns, but I may have been a few steps ahead of you. The adapter only have four 90 degrees portings turns per outlet (8 total for both port outlets) and the loss of hydrodynamic friction due to changing flow directions was compensated by suitably adjusting the size of the porting and piping on the adapter. Once hydraulic pressure is stabilized between the EHA internal cavities and the CIS head, the flow is identical as if the EHA were installed directly on the CIS head. I've driven my car with the adapter on and off and measured the only parameters that I can from the X11 connector (ratio), and there it seems to be no difference in ratio nor performance.

The idea of straight pipes like your sketch was also my initial first version of the adapter until I validated with a computer program the losses due to cornering of the portings, and since it was not going to be a problem, I implemented the 'monolithic' block approach instead.

The last new Bosch EHA I bought (October 2017) was from RockAuto and it cost me $175.00 Dollars. Yes they are still available from many sources , but the average price is in the $220 to $250 everywhere else.

Cherry2728
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