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  #1  
Old 08-07-2005, 03:34 AM
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EHA current - help needed

Ok, ongoing mixture issues with my 1987 300E, no cat

Haviing continual problems trying to get the mixture right, the car is running rich as judged by the general exhaust note and the fact it's using a lot more petrol than before. I used to get 20 mpg, which by my calculations is about 7km/litre or about 400km per tankful. Right now I'm getting nearer to 200km per tank.

Another problem I'm having is the mixture seems to adjust itself. Twice now it has gone incredibly rich after running under load. By load I mean with 3 or 4 people in the car, travelling uphill or with golf clubs in the back for more than 20km. The car seems to run fine but when I stop, its sounding really rich at idle and has no power up to 2000 rpm. I solve this by whipping out my 3mm allen key and adjusting the lambda tower screw a 1/4 turn counter clockwise.

I also have an idle problem, sometimes it idles fine at around 800-900rpm, sometimes it drops down to a lumpy 500rpm. Checking the voltage at the ICV, i find that the voltage drops from 6v to 0v when the symptoms occur, I've checked the OPV and traced this to the ECU. I can cause the voltage to drop by rapping the ECU casing, knocking it again brings it back. I've just resoldered some of the connections inside the ECU and so far this particular problem seems to have gone away. I presume this means a new ECU, unless anyone has any other suggestions.

Now, as part of my efforts to resolve the mixture issue, I rigged up a harness to monitor EHA current, and my readings are these:

KOEO +20ma
Deceleration goes to -60ma
Starting (warm engine) +60ma dropping to +3.5ma
Idle (warm engine) +3.5ma
Steady throttle 0ma
Accelerating - anywhere from -5 to -10ma depending on load, dropping back to 0ma

Based on what I've read in other posts, most of the readings are OK, with the exception of idle and accelerating. I'd expect the idle reading to be around 0ma, and I'd expect the acceleration figures to be + instead of -

So, my questions:

How can I adjust the mixture to get EHA current back down to 0 at idle (or -2ma as recommended by stevebfl)? Adjusting the lambda tower seems to have no effect on the current reading.

I've read that there should be around 3mm clearance between the air flap and the fuel plunger. On my engine there seems to be around 1mm or less. Is this likely to be part of the problem?

I've recently adjusted the EHA screw as I've replaced a failing fuel pump and I deduce that fuel pressure is up. I have no way of checking fuel pressure without taking the car to the dealership as noone else on the island has the gauges. Is there any electronic feedback that will help with adjustment?

What's going on with the acceleration readings? My understanding is that a negative current at the EHA means less fuel, is this right?

Why would the mixture adjust itself? Do I need to replace the lambda tower because its not holding its setting or is it likely to be something else?

Sorry for the lengthy post but I hope to get some guidance here. Getting fed up spending my weekends tweaking the fuel injection system.

cheers

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Old 08-08-2005, 08:33 AM
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I had a problem just like that on my 190E. Basically I would set the mixture and then recheck it the next day and it would be diferent. Some days it was rich and other's lean. Replaced the Lambda tower and now it stays where set. Took about 15 minutes time to replace the tower. I would fix that first. Then screw with the EHA.
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Old 08-08-2005, 11:20 AM
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Very similiar problem

Interesting that I was going to post an almost identical problem as far as symptoms go with my 88 TE (198k).
I adjusted the EHA 1/4 turn in an attempt to remedy a rough start problem and ended up with your symptoms. Mainly the idle will intermitantly drop to a rough 500 rpm when the engine is warm or after it has sat in the SC sun all day. It will stall at stops intermitantly if not shifted into neutral and hot starts require the throttle. Readjusted the EHA back to it's previus setting with little change. Fuel milage has gone down also, I assume the mixture needs adjustment. I plan to check the duty cycle this week. Any other ideas out there?

Thakns in advance.

EJC
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Old 08-08-2005, 02:33 PM
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In my experience with my 560SEL, adjusting the EHA screw affects mostly the load at higher rev(2400), with very little effect on idle. Adjusting the screw at the tower affects idle mostly, and very little effect at 2400 rpm.
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Old 08-08-2005, 08:08 PM
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EJC, based on my experience I'd be looking at your Idle Control Valve. On mine, I was losing power to the unit due to a problem with the ECU (i think). Check the voltage across the terminals and see what's going on. Should be about 6v in normal operation, if the voltage drops, the unit will revert to limp home mode, which will give you the lumpy 500 rpm idle and stalling issues you describe. Most likely in your case will be a defective OPV, which delivers power to the Idle Control Valve. In my case it appears to be the the KE control unit that's going bad.

I understand that actual control of the ICV depends on current, not voltage. I tried to hook it up to a meter to measure the current in the same way I rigged the EHA but for some reason it wouldn't have it. Refused to idle, even though there was current and power to the ICV. Maybe the meter is restricting the current flow and affecting operation of the ICV, however it seems to work fine with the same harness monitoring the EHA. Considering we are dealing with 600 - 900ma with the ICV current and a range between + and - 60ma with the EHA i find this puzzling to say the least. Any Bosch wizards out there have an explanation for this?

Tip about the Lambda tower makes perfect sense, mctwin2kman. Will try to find one, hopefully I can get it as a separate part. Anyone have the part number to hand?

Thanks all, on on.
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Old 08-08-2005, 09:12 PM
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Lambada Tower is called "hex key repair kit" on FastLane under Air Intake for my car. Part Number B3012-53135 $19.11
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Old 08-09-2005, 09:56 AM
Lenny
 
Join Date: Jun 2005
Location: Richmond, Va
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Fuel Consumption Problem

It could be the fuel mangement computer (ECU). When mine failed, some strange things happened. Usually, the engine quit altogether but could be immediately re-started. The MB CD ROM provides some disgnostic help. However your best bet is to have an experienced techicial check it out. A new ECU for MB will cost $1,000.
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Old 08-10-2005, 06:27 AM
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My mechanic found me a loaner ECU which I'm running at the moment. World of difference so it looks like i need a new one. Oh woe!

Haven't yet tried to measure the current at the ICV, I'm thinking perhaps the problems I had earlier are linked to the failing ECU, so may try that at the weekend.

EHA current at idle is now 3ma but am seeing no response to adjustment at the lambda tower. should I be?

Current at 2000 rpms is 0ma, pretty much, which I take to be a good sign.

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