This is a 1986 300E, 117KMi. I've had this car for about 4 years now, and this is my first serious attempt to adjust the fuel mixture since the missing o2 sensor was reinstalled.

I was able to successfully adjust the idle mixture for close to 0 mA (+/- ~2mA) at the EHA, and very close to 50% duty cycle. But when I rev up the engine to check if it maintains the mixture at 2000 rpm, the EHA current pegs to -10mA in a matter of about 8-10 seconds (the controller is obviously kicking in full-tilt trying to lean the mixture). At that point the o2 sensor reads .94V (very rich), which also confirms the overly rich condition. Also, duty cycle goes down to 20%. Once the engine is allowed to go back to idle, after a few minutes mixture will go back to normal, and the system will get back to closed-loop mode (o2 sensor reads between 0.3 and 0.7V), Duty-cycle goes to 55-60%, EHA current stays within +/- 2mA. BTW, if I disconnect the EHA with engine at 2000 rpm's there is no appreciable change in the mixture. Nor if disconnected at idle because its already set close to 0mA, so basically its having minimum intervention at that point.

Adjustment was done with engine at operating temp (~82 C), at idle, no A/C. Water temp sensor measured 355-ohm at said temp, so it is within the target zone, and both sides to ground (its a 2-pin unit) reading the same resistance.

Performed the usual static tests before commencing the mixture adjust (engine off, ignition on):

- Controller: Initial duty-cycle indicates 70% (standard 49 state controller, status ok).
- Iddle contact: Slightly deflected the airflow plate, duty-cycle dropped to 10%, as it should.
- Full load contact: Open throttle completely, duty cycle went to 20%, as expected.
- EHA coil measures 19-ohm (within spec).
- EHA current with engine off: 20mA.

Also, in preparation for today's work, last night I stayed up until 4am reading treads in this and other forums regarding the mixture adjustment procedure, and also the magical "1/4 turn CW at the EHA".

The o2 sensor was just installed a few weeks back, and seems to be functioning correctly as I've monitored the output voltage, responds quickly to changes in mixute, and voltage readings coincide with what the duty cycle is also reporting, and what the EHA is trying to accomplish (aprox. voltage range between 0.3 and 0.7V when system is operating closed-loop, but only at idle). When engine is revved to about 2000 rpm o2 sensor reading pegs to 0.94~95V (full rich mixture).

I don't have a fuel pressure measuring gauge, so no idea what the upper and lower chambers are running at. When I took off the EHA there was obvious tampering on the brass cover, so previous owner was obviously at it. Tried rotating the 2mm hex base mixture screw, first both ways a 1/4 turn. CCW rotation would cause engine not starting, CW rotation by the same amount past the original position made no difference to the initial condition, besides that I had to slightly readjust the 3mm mixture screw on the air intake tower to lean out the mixture in order to compensate and get back to 0mA on the EHA and about 50-60% duty-cycle. Further CW turns, some made in 1/8 and some of 1/4 turn increases did nothing to change the situation. On every occasion I just readjusted the mixture screw and got back the proper idle mixture ratio and closed loop mode. But as soon as the engine was revved up to 2000rpm the EHA would again peg to -10mA in a matter of about 10 seconds, and then return back down close to 0mA after a few minutes.

Something is letting a lot of fuel into the system past idle. Since I disconnected the EHA and it made no difference, I would think that the controller and its sensors could be excluded. I checked bellow the airflow plate after engine shut-off for any signs of fuel leaks, and it was bone dry, so this is probably not a leaking o-ring problem.

My main suspect now would be either that the chamber pressure differential of the fuel distributor (FD) is way off, and might be causing the rich condition only when engine rpm's are brought up. Not sure if this scenario is possible, I'm only familiar with the innards of the FD in a general way. My other suspect would be the FD itself is malfunctioning, or maybe its just the EHA that is bad? Wish I had a spare to perform that test. Probably will order one next week, hopefully someone on ebay will have one at a decent price. Will also try to borrow a fuel press gauge from a mechanic and check the differential pressure setting.

Any comments and suggestions, or additional tests anyone would like me to perform, will be very welcomed. Thanks for reading all this.

 

The EHA could be designed to "fail in place", that is, when power is disconnected it does not move either direction. If that is the case, it would not change the mixture when disconnected. I'm not sure how to check it.

 

did you check the temp sensor that controls the EHA?

 

Thanks for your input.

If you mean the engine coolant temp sensor, yes. That was my first suspect. It measured 355-ohm at 82c, so it is within the target zone, and both sides to ground (its a 2-pin unit) reading the same resistance.

I was just now looking at the KE injection diagram and noticed the "Regeneration valve" that is connected between the fuel tank charcoal filter, and throttle body. I wonder if there might be any way it could somehow be sucking some fuel fumes through there that would upset off-idle mixture?

 

not sure if this is useful to you or not....
http://www.humanspeakers.com/audi/tuning-cise.htm

 

Maybe, guess the only way to find out is getting another EHA to test. In any case it seems to me that at least the signals its getting are correct, and to some extent it has the desired effect on the system when it makes mixture corrections at idle.

Its basically a variable fuel valve, so one would have to feed some test fluid into it and measure the output on the other side while varying the control voltage. Not so easy to set up.

Thanks.

 

Looks good. I'll dig into it now, see if there's anything that is not in the books and shop cd's I've got here. Thanks.

 

Found this... it certainly sounds plausible, and worth checking out:

"An air leak will throw everything off at idle, you will adjust for it, but it will be off by different amounts at other rpms and loads."

 

Idle control valve hoses are not cracked right? Also there is a little O-ring under the fuel distributor that may start to leak after some years. I would check the injectors for any leaks on the seals. I figured out all my fuel injection problems by changing the temp sensor ( black 2 posts by the 6th cyclinder)and changing my injector seals. I also replaced the hoses from the idle control valve since they cracked.

 

Yes, and I understand that the usual symptom is some raw fuel under the airflow plate after the engine is shut off. Mine seemed dry when I checked. In any case that would also affect idle mixture which appears to be fine. Unless the additional suction of higher revs should be able to suck out some extra juice past that seal.

Engine has a very slight non-periodic stumble at idle, checking those seals was next on my list to try and smooth out the idling quality.

That's the sensor I tested today. Resistance readings appear to be in spec, but I only measured it at 82c. Factory manual says one should test it at several points to make sure its tracking correctly. But since my tests where at that temp with correct readings, I doubt the sensor is at fault.

Let me go and take a look at those hoses. They would be prime candidates for an air leak if cracked. I'll be back...

 

Well I just took a look at them, and from the permissible angles I could not detect of feel any cracks, but then there are areas that are obstructed either by the Fuel Press Regulator, or the manifold itself. Are they supposed to have a rubbery feeling like the radiator hoses? These feel rather stiff.

 

They are pretty hard to get to but they usually crack and split open at the ends. They shouldn't be too hard but feel like your breather hose but a little softer. I keep thinking your EHA valve is bad. It should be regulating the fuel going to the fuel distributor and since it's been messed with before and it's probably not working since it doesn't make a difference when disconnected. When the fuel distributor goes bad your engine will stall and run like crap unless you make the mixture really rich and adjust the throttle position sensor to keep it running. The plunger thing in the middle of the fuel distributor shouldn't be lose like when you press it down it should have some resistance or come back out slowly.

Sorry I don't get into specifics of duty cycles, ohm resitance, or air volume. I'm not good at explaning stuff but i know how to fix it. I take my car to a friend and adjust the fuel mixture before doing the smog to put the HC and NOx within specs. I run my car now a little lean and drive slower to save on gas. I still pass smog on a 22 year old car so i think im in good shape.

I think i have a fuel distributor and eha in stock but i have to look at my parts garage. ( I have a metal shack full of spare parts )

or you can get the fuel distributor and eha here..
http://cgi.ebay.com/ebaymotors/MERCE...spagenameZWDVW

 

Well these feel a bit harder then the breather hose, I guess from exposure to the engine heat over the years. I also thought they would tend to split on the ends, as those would be the obvious pressure points on them. I will check those areas again today in daylight. The one that goes under the fuel distributor area looks like a bi***h to get to. Did you have to disassemble much stuff in order to get the new hoses installed?

The EHA is not making any difference when disconnected (at least at idle) because once he system is in closed loop very little current flows through it as required corrections are small. So that is why it doesn't have much effect. Also by disconnecting the EHA the electronic part of the fuel system is being excluded. Still there is a possibility that the EHA is bad, maybe partially clogged, etc.

What I'm worried about is the base mixture setting screw on the back of the EHA, which as I said had obvious signs of tampering, but then again it is also possible it was actually set right by someone knowledgeable. This 2mm hex screw basically sets the pressure differential between the upper and lower fuel distributor chambers. Factory says it should be set to 0.4 Bar. Many top mechanics recommend to set it to 0.45 bar on high mileage cars as it compensates for any small unmetered air leaks due to wear and tear, and will make the car run much better (that is usually known as the "1/4 turn miracle screw"). Will try to borrow another known-good EHA, or order one, just to make sure this one is not bad, and the other EHA might also have the base mixture setting screw closer to the correct position for sure, so I would actually be performing two tests at once.

I suspect that maybe in my case the differential pressure is simply so out of spec that it might be causing the off-idle rich mixture problem. But again, I'm not sure this is possible to happen in this way. I did try reseting it to both leaner and richer setting without any effect other then the car would not start when I leaned it out too far. Problem is I don't have a fuel pressure gauge to check the FD chamber pressures, but will try and borrow one next week and some some tweaking.

So you mean to compensate a car would have to run very rich at idle, or overall too rich? If so it sounds somewhat similar to my problem. The airflow plate has some resistance when pushed down, and returns up immediately when released.

Yeah, I also have a small cache of parts in storage (don't we all have some?).

Is this your own ebay auction, or of a known recommended seller?

Thanks again for the input. Don't worry about not using much of the technical terms, most of them are also new to me. As long as there are ideas and useful info to share, the important thing is being able to pass them on.

 

Maybe you should set the trim screw so that it is as lean as it can be at idle, with the EHA modulating to full rich position just barely keeping the mixture rich enough. Then it will have more room to modulate towards lean at part throttle.

Or, set it so it maintains proper mixture at part throttle and see how it does at idle. I would think a lean idle would not be a bad thing as long as it doesn't stall or idle roughly.

 

I had to remove the fuel pressure regulator to be able to pull it out. The rubber shouldn't be too hard because even if it's not damaged at the ends the hard plastic could cause an air leak by not maintaning a tight seal.


Yes, when my fuel distributor was bad i had to adjust the idle control screw to make it run but it was running way too rich across all rpms. The EHA would work like it should but it sputter all over the place because of the overly rich mixture when starting and accelerating.

Read this...this may help...
http://www.mercedesshop.com/shopforu...+sensor&page=3

No, sorry but the feedback looks pretty good. Only 8 negs in one year after 600 transactions...


I will take my voltmeter out and test the settings of my EHA and O2 sensor ...

 

As I mentioned in my initial post, the EHA current starts to progressively go towards full tilt lean (-10mA) in the 8-10 seconds after the throttle is opened and the revs go up from idle, this to try and compensate for the additional fuel that is being injected into the system and making the mixture overly rich. So I don't believe that any amount of leaning out at idle would help much towards part-throttle. Besides this would not exactly the right way of trying to fix the problem by covering up the symptoms. I much rather want to get it fixed and working right for good.

Thanks anyway, appreciate your input.

 

Right, hard rubber wont seal very well. BTW, I meant how to get at the other end of the hose, the one under the fuel distribution assembly. Did you have to remove it?

 

Well i wouldn't remove it unless i have a replacement because you will damage it when removing it. Like i said you will need to remove the fuel pressure regulator, un bolt the wire harness and try to make as much space for your hand to reach and be able to pull it off.

 

No, certainly no intention on removing those hoses until I get the replacements, maybe just a closer inspection if there is an easy way to get to the difficult places without having to disassemble too much stuff.

I just wanted to know if you had to remove anything to get to the OTHER end, not the one under the FPR.

 

You mean the one by the cold start valve?

 

Thats right.

 

So did you have a hard time getting the hose installed on that end? I recall that the intake manifold and the fuel distribution system seem to be pretty much in the way.

 

What? Masking symptoms is not a solution? Tell that to the FDA who allows thousands of drugs to be prescribed that do just that, if anything at all.
[Sorry]

It sounds to me like the EHA is just not doing anything. The coil may read correctly but mechanically, who knows? I like your idea of replacing it.

 

Ron, it is possible of course that the EHA is not responding and the controller just keeps trying to compensate further and further until it reaches the maximum current excursion of -10mA and pegs to the limit. Exchanging it with another known good would be the only way to find out. In any case also really need to get a fuel pressure gauge to be able to read the differential between the two distributor chambers to make sure that little EHA base mixture adjusting screw is not the cause for the whole problem.

But... in any case if you are reaching your conclusion because there is no change in mixture when I disconnect it at idle, remember that it is at almost zero current, or minimum compensation, when the system is operating closed loop (idle), that is why there isn't any change when it is disconnected.

 

Yes, it was kinda hard but you need to have really small hands lol. Also a flat screw driver would help. You basically remove the 2nd injector line and the idle control valve. You'll have about a two inch gap to reach underneat the intake manifold grab it then you'll use a screw driver on top to help you pull it off.