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1989 300SE 232k miles
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Discussion Starter #1 (Edited)
Hello All,

This regards my 1989 300SE with 234k miles. Today I tested my engine vacuum system for leaks, and I got to looking at my air meter assembly. I have the bar that goes across it, and the plastic piece on the bar. And I have the piece that moves up and down, and that has a nut on top. However, in the pictures I see, that nut touches the plastic piece on the bar. In my car, when the metal flap is all the way up, there's probably an inch or so from the top of the nut to the plastic piece. Is this something that should concern me?

You can see the air meter assembly in the attached picture, inside the green circle labeled A. The bar and black plastic piece are across the top, and should be visible. The nut is hidden.

Thanks,
Bill

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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It is somewhat difficult to see exactly, but to me, the at-rest or zero position of the air-flow sensor plate seems quite a bit too far down in the bore.

I believe the second picture is more what one might expect to see.

Here's a pdf from the MBCDs on adjustment of the zero position.

Edit: If you have to disassemble the air flow/mixture control, here is a pdf on that noting differing screw lengths in the assembly that are important to keep track of.

Good luck man...

MBL Screen Shot 2019-11-20 at 7.14.59 AM.png
 

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1989 300SE 232k miles
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Discussion Starter #4
Wow! My air flow sensor plate is definitely not angled. Here are a couple pictures I took yesterday, from the side. I can almost get my entire pinky under the spring bracket.

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Looks like I need to fix this. Hopefully that'll help with the fuel economy. I'm pretty sure I'm at single digit MPG since I bought the car.
 

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1989 300SE 232k miles
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Discussion Starter #5
Question: The manual says for a low flap I have to remove the mixture control unit and push the pin from inside. If there's enough of it outside, can I grab it and pull it up? Or is it best to remove the unit and push form inside?
 

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1989 560SEC, 1989 560SEL, 1995 E420
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I don't see why you couldn't grab and pull up, if possible. But remember it's a friction fit so you don't want to make any burrs or cause any looseness. I think it's typically tapped from below in small increments.
 

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1989 300SE 232k miles
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Discussion Starter #8
Is the round plate touching to sides of the opening?
This is a great question. Especially since that would require a totally different repair.

I've looked at it from the front and from the left of the car. I know there's a gap toward the front and rear of the car. And I know there is a gap toward the right of the car. I'll need to look from the right to assess the left side of the flap, particularly as that's where the high point of flap is. I know it stops going up, and I don't think it was touching the side. There's no difficulty pushing it down, which I would expect some increase of resistance when pushing downward if it were touching the side.

I'll post what I find when I look at it tomorrow. Thanks!
 

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68 250S, 91 300se, 98 SL500, 450SEL 6.9, 14 CLS550 Past MB's; 300SD, 300E, 300TE, 190E, ML420
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The nut and the black plastic have nothing to do with setting plate height. The bolt holds the plate onto the part below.
 

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1989 300SE 232k miles
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Discussion Starter #10
I pulled the air meter assembly and adjusted the sensor plate. It was definitely low. My adjustment pin was so pressed in that it was below the surface of the assembly. And it was so stuck I used a 2 lb hammer to move it. But now the height of the sensor plate is right.

My idle is now smooth as silk. That said, my off the line acceleration sucks. I'm going to check the duty cycle. Moreover, my fuel gauge is still at 3/4 of the way to the left when I'm idling in gear. This is why I started all this. My engine vacuum lines have no leaks (Smoke tested again today). So I'm going to look at the HVAC system. My vacuum there isn't working right. And I understand that can affect the fuel economy gauge.
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I included pictures of the depressed pin, the pin after adjustment, and the pin from inside the assembly.
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Thanks everyone. All the help and input is greatly appreciated.

Bill
 

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1989 560SEC, 1989 560SEL, 1995 E420
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Changing the air plate's resting position is going to change the position of the control piston in the fuel distributor and therefore duty cycle and fuel pressures. So I think you're right to check the DC. I'd also set up fuel pressure gauges. I'm guessing if the idle DC is okay, the 2500 measurement will be way off or vice versa.
 

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Discussion Starter #12
Changing the air plate's resting position is going to change the position of the control piston in the fuel distributor and therefore duty cycle and fuel pressures. So I think you're right to check the DC. I'd also set up fuel pressure gauges. I'm guessing if the idle DC is okay, the 2500 measurement will be way off or vice versa.
So, with the DC, I understand I'm hooking up a multimeter with duty cycle ability (which I need to buy), and I'm adjusting that 3mm allen nut in the tower until I get a reading of 50. And I do that at idle. After that, I raise the RPM to 2500 and measure the duty cycle again. And I ultimately want that to read 50 as well.

To which, if the duty cycle is not reading 50 at 2500 RPM, do I adjust the 3mm allen nut while keeping the car at 2500 RPM? Until it reads 50. And then the car's duty cycle is theoretically set at idle and 2500 RPM?
 

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1989 560SEC, 1989 560SEL, 1995 E420
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Oh boy. Not sure how to answer this. Member H.D. has made it abundantly clear that the 3mm screw is not to be adjusted until ALL other systems are working correctly. However, since you had a fundamental issue (incorrect air flap position), I'm not sure you have a choice.

I hope H.D. will chime in. If he doesn't, If you don't have the manual, I would get a copy. They're available on this site and other places. I believe you want at or less than a 10% difference between the idle and 2500 measurements. I believe further that the M103 likes a 35-45% duty cycle.

You make adjustments at idle only. If you adjust to 50% at idle and 2500 is off, then adjust to get 2500 to 50% you'll probably find that idle is now off. There are a couple ways to address this. I want to make clear that I am not an expert. I only have M103 experience on my Dad's 300TE. I was able to get the readings into range by adjusting the EHA. However, I would NOT recommend this until H.D. or some other knowledgeable CIS-e person speaks up.

In the meantime and if no one else speaks up, for now I suggest trying to find a duty cycle which is a compromise between idle and 2500.
 

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1989 300SE 232k miles
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Discussion Starter #14
Indeed. I've been reading and watching everything I can on this adjustment for the last month or so. There seem to be a great many saying, "Don't touch it! You can destroy everything!" And there seem to be a great many that say, "It's an adjustment for time and wear and tear. And: Wow! I made the adjustment on my 200k+ mile car and it runs so much better!" That said, your response was the first time I paid attention to the 2500 RPM part. What I read today on the 2500 RPM, I interpreted as doing all the adjustments at idle. But that didn't make much sense to me. So appreciate your response clarifying it is at idle.

I'm a very precise guy. But I'm also pretty intuitive. And combining those 2 seem to work for me. If no one chimes in, I'll try and find a balance. And I agree, I didn't want to touch that 3mm allen bolt, but I think I need to. ...Though maybe I'll fix my rear window that just broke and drive the car for a while. See if the acceleration improves over a week or so.

Oh, and one other thing, after I adjusted the air plate, my car did the fastest start of any car I've ever owned. I mean EVER! It felt like less than one crank.

Thanks again,
Bill
 

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1989 560SEC, 1989 560SEL, 1995 E420
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LOVE the fast start comment!

Sounds reasonable to detour to the window and give the car some time in it's new 'place'. Best of luck. Looking forward to further posts.
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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Yeah, that fast start comment is just great! I am really glad you are getting results heading in the right direction here.

I too, am no CIS expert, that being said when comparing your fuel distributor/airflow meter images with the one from plinker, I noticed the two apparent hacksaw cuts in the airflow adjustment "body" or "tower": part number 102 070 01 04 .

The airflow plate stop being tampered with combined with this fact, suggests the original setting may have been changed. As this housing contains the actual adjustment screw, I wonder is the anti-tamper ball still in place? If so, that might be good news....

This is a great write-up with excellent illustrations..

Good luck,

M.

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s-l500.jpg
 

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1989 300SE 232k miles
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Discussion Starter #17
Yeah, that fast start comment is just great! I am really glad you are getting results heading in the right direction here.

I too, am no CIS expert, that being said when comparing your fuel distributor/airflow meter images with the one from plinker, I noticed the two apparent hacksaw cuts in the airflow adjustment "body" or "tower": part number 102 070 01 04 .

The airflow plate stop being tampered with combined with this fact, suggests the original setting may have been changed. As this housing contains the actual adjustment screw, I wonder is the anti-tamper ball still in place? If so, that might be good news....

This is a great write-up with excellent illustrations..

Good luck,

M.

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Wow! I noticed those cuts, but being new to this disregarded them as being normal. I did look down that tower and I'm pretty sure the ball is gone. I saw the head of what I believe to be the adjustment screw. Also, I don't see how the cuts really affect anything, so I'll most likely leave the part as is. But thanks for the number.

Though now I am now curious at to why someone would make those cuts....

It's funny, I just read a GREAT thread involving H.D. about the duty cycle for an '89 300SE and decided, because I have no history on the car, I'd replace things like the fuel injectors and O2 sensor before going into the duty cycle. But, I think I need to at least buy a multimeter and see where these are. Here's a link:


I appreciate the compliment on the thread. I never intended for this to be a write-up, but I'll take it ;)
 

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1989 560SEC, 1989 560SEL, 1995 E420
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Those cuts are one of the methods to remove the anti-tamper ball. I'd say your fuel distributor has very likely been adjusted.
 
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