I chased a similar problem for approximately two years and it was solved last weekend. The round plate on the throttle body was bent and out of adjustment. (Most likely caused when the engine backfired a couple years ago.) Simply, the engine was getting too much air at low idle, causing it to run rich and idle poorly.

We replaced the round plate on the throttle body with one from old throttle body with the same part number that was lying around and the engine is now purring like a kitten. Just have a little anti-freeze mixing in with the oil after we put the engine back together. Front cover is coming off to look for a busted/leaking o-ring--but that is another story.

The plate has to be centered with a very fine clearance around the edge and, if I recall correctly the height of the plate should line up in the center of the 1mm ridge at the bottom of the opening on the throttle body. The adjustment for this in the SL (107) is a detente under where the fuel line comes into the right front of the fuel distributor as you face it. You can't get to the adjustment without removing the throttle body.

There was one other adjustment that was made after we pulled the throttle body and fuel distributor off. It was at the front of the engine where the rubber piece the throttle body sits on fastens to on the manifold. There was no spec for this that my mechanic could find, so he made a (very well) educated guess in his adjustment.

You may not have a bent plate, but it could just be enough out of tolerance to be allowing too much air into the intake at low idle.

Hope this helps.

I have adjusted the plate as per specs. The picture on the first page is from when I took it apart to adjust.

I am running in closed-loop then as the O2 sensor fluctuates. I will check it again tomorrow since I replaced the injectors and see how rich I am running according to the lambda.

I am going to read through the PDF tonight and learn as much as I can about the Bosch CIS-E.

After driving the 260E for a month while the ignition tumbler was being replaced in the wagon...its easier to see what is going on.

It is very clear that the erratic vacuum begins when the car enters closed loop after being cold. I think my next course of action would be to replace the entire CIS fuel system with the spare I have and see what happens. With that, it will be the potentiometer, fuel distributor and eha. As was said above, maybe even though the potentiometer checks out, it could have a dead spot or something.

My second question -- If the timing is in fact within spec, can I rule off the crank pos sensor and hall sensor? Or could they still be potential problems?

Can anybody also explain to me how the hall sensor affects the CIS? Does it even affect the CIS? Whatever is causing this the rough idle is making massive overfueling and killing my power at the bottom end.

ps2cho,

I have a 88 300TE, and went the same route. Concentrate on the potentiometer/air flow sensor. My sensor would do the same thing when it got warm, sometimes it would read off the correct reading, and sometimes not. (pins 1 & 2 .65v then intermittently to 1.01v - 1.04v)

This would be my second sensor that i've installed in the car in 2 years. First year the rpm gauge was sporadic(dead giveaway), second year(day before thanks giving 09') started to do what yours did in the video with the economy and rpm's,and later on my rpm's/idle got sporadic car started to die on occasion at stop lights or to turn.

Replaced mine, and set it to .65 and it runs great.

Awesome thanks! I will grab 2-3 at the junkyard and give it a try...I have tested mine, but its about the only item I have not replaced so its worth a shot!

I'm pretty sure the hall effect/crank position sensor only affects the ignition system...

I would check your sensors circuit board for scratches and test again to see if the voltage does the same as mine did before spending more money.

The picture is not the greatest nor is the camera but you'll get the idea. The shiny spots on the tracks are grooves made by the brush's that are attached to the sensor plate lever. Every bad sensor i've seen has this same scratch pattern, can't figure it out? And I couldn't tell yuh if a sensor that is working properly would leave a similar pattern or not, never wanted to look since the car was running fine,lol.

I'm guessing that is worn out on my car too as it will do the idle up/down thing for a few seconds when you start the car warm or sometimes when you put it in park....it does always stabilize though. From what I have read, the potentiometer is virtually impossible to find as a new replacement. What happens when we can't find them anymore? :eek:

I have a spare that I found in the junk yard with very little wear on it, (looks like it was replaced before)........but finding a source of new ones would be nice.

I read they do not sell the potentiometers separately -- only as part of the entire airflow unit which is several hundred $$$.

I will pull mine off in the next couple weeks after I get the 260E back on the road and check for the scratches!

You set the voltage to .65v based on its physical bolted on place...there is space for adjustment of the screws if I remember correctly?

What is the method of testing the voltage to .65v while adjusting the unit?

I watched your video. My '88 M103 does the same thing (tach and vac guage fluctuations) and mine has only 59K on it. I thought it was 'normal'. I average 20mpg.

Why .65 volts? Is because my dad has a 86 300e that still runs like it came right out of the factory in 86' and that thing (still trying to buy it) has not been touched in any other way except for oil changes, sway bar bushings, etc,etc, simple crap.

So I use that car for specs and it was pinging .65 volts when the engine was at a warm idle. His car will chirp the tires if need be..............(jealousy kickin in)

Now down to the adjustment, for someone that has never done this it may take you a couple of times to get the adjustment right. I'm posting a link here to give you an idea since i'm more of a hands on guy to show you how to do it, but since im not there this will have to tide you over. If you have any questions feel free to ask.

http://sg-motorsports.com/AFS%20Pot%20write-up.htm

And the sensor you can get it seperate, but it takes alot of searching cuz there is still a handful out there. But once their gone its the whole housing you got to buy.

Oh and after I just put mine in my car, it grew its balls back! :eek:

so I didn't see where you replaced the spark plugs...maybe I missed it...I saw something about NGK spark plugs....very bad!!! my 1990 300TE was idling like crap, stalling, hard warm start....i did the basic checks...idle control valve, O2 sensor, low tension electrical connections, inspected the plugs for gap and regap-ed (they were Bosch of acceptable spec)...i replaced the fuel distributor and the coolant temp sensor (because the engine would always start cold just fine and get worse with warm-up)....nothing...problem kept getting worse....finally i said screw it, this feels like ignition and went and bought a set of the new Bosch Copper-Pluses for $25 and threw them in...car ran like a champ right on first start and has since for 2 years...i swear this is true...it was spark plugs

my point is that i looked right through the solution because i thought it couldn't possibly be something that obvious...gotta be something else

Yeh I've done all the basics like that...I have the correct H9DC0 non-resistors in. That is why I am out of ideas at this point :)

Does anybody know how the potentiometer influences the KE-Jetronic system? I'm just not sure if the rough idle and the fluctuating rpm's are from the same cause. Either way one knocked out, if not both would be great.

I will definitely use .65v as my benchmark. Thanks.

Mercedes recommends .51volts to .81volts(on pins 1 n 2 ), I used the .65 volts because the 86' runs like champ and that is what it was at.

Now for the "why the potentiometer influences the KE, is cuz one it influences the extra enrichment during cold start, and after that it signals how far the sensor plate moves as air flow increases as it pass's by when warm.

"As I said before ps2cho, Concentrate on the AFM!"

Idle issues
 

I may be starting to sound like a broken record on my posts but it is just because it is the only thing that worked on my car. And I do realize the amount of work that you have put into your car. I have followed your posts intently as I am now looking at a head job of my own. And I have to say you have done a wonderful job with the pics and stuff. Keep up the good work. :)
Anyways, back on subject.....;) I was having idle trouble with my 300E and it was the Idle Control Valve..... I didn't see you mention it anywhere in this thread so I am just asking...

Thanks. I always know when I look for other posts on subjects, some high quality pictures really are worth a 1000 words...I'd rather show not tell.

No, but it wouldn't make sense as the ICV opens and closes perfectly fine. I believe the EZL controls the way the ICV opens/closes based on readings from the potentiometer. Also unplugging the ICV did not affect me at all.

If the potentiometer does not pull through, I can always swap the ICV for the hell of it...

I thought the EZL was the ignition module - that shouldn't have any impact on the ICV.

As far as I know, the ICU controls the ICV's movements.

Have you ever taken a look inside your combustion chambers? They (and the valves) could simply be coked up due to age & inferior gasoline. You could try the "sip-in GM top end cleaner" method, or run it at 4,000 RPM for around 20 minutes (it's like putting your oven on "clean" - it pretty much burns off most deposits).

The GM method has worked for me (on other cars), but requires sitting overnight, puts out a buttload of smoke the next day for about 10 minutes, and requires an immediate oil change. The other method just stresses your engine.

Remember, there's a mechanical aspect of your engine, too - it's not all about fuel delivery & spark (though those shouldn't be overlooked).

ICU sorry, you are correct.

Here's my combustion chamber....I got the valves done too :)

http://www.ps2cho.net/downloads/phot...t/rebuild2.jpg
http://www.ps2cho.net/downloads/phot...t/rebuild3.jpg

The only thing I do have is a "ticking" at the back of the engine...I'm not exactly sure what it is though. Its not very loud, but there is a tick.

I am having a similar problem with an 89 300E with 140K miles. This problem would "go away" if I could trick the system to run 100 rpms (if that) higher at idle. The car IMO is not worth the time or effort to go to the trouble the OP has with his car and frankly makes the somewhat minor annoyance of the low idle a daunting problem.

Exactly. As soon as I touch the throttle it disappears even if I give it the smallest amount of rpm's possible. I guess then the potentiometer would make logical sense as its now being controlled by my foot, right?

Thats my thought.

I swear your car was right behind mine in the assembly line. I got a ticking noise coming from the back of my engine too, and it got slightly louder when I had the head rebuilt. I took the head off with cam, and bearings again. Took it to the shop and we both went through it with a fine toothed comb, to find everything in spec. (still can't figure where this noise is coming from:mad:) Could it be the bottom end? wrist pins, rod bearings, somethin along those lines?

It seems You have done a very thorough diagnosis ,leaves only the fuel distributer for further checks . Turn to a lean condition ,test and then a rich condition to determine where it is better. if its rich you may like to check the cold start injector for a dribble leak.
mak
300SE

Any news, Problem Solved???:confused:

Not yet...soon...I broke my forearm snowboarding, so it is slowing me down... :(

Well!! There you go then. You should have had a Mercedes Benz snowboard equipped with anti skid control and airbags and you then would have not done your arm in.;)

As long as your vacuum gauge needle is sitting around the area just off the red at idle the engine will never idle properly. The gauge being a symptom of the problem.

The needle should be right over to the left at idle (in park).

I haven't read all the posts, but I believe there is a microswitch that is closed when the car is at idle. The switch (at least on the 190e) is located at the end of the throttle cable, I think it has something to do with deceleration. Have you checked that this switch functions as it should?

Bummer. But you can tell us the real story man, you were trying to impress some snow bunnie:D

Well its been 5 weeks and I am FINALLY out of a cast....

With some help this weekend I can get the tranny pulled on the 300TE to fix the leak. Once that is done I am right dead on track to look at the potentiometer. I just can't live with this leak any longer as its destroying the road outside :(

I have exactly the same problems that everyone is describing here. I have done just about everything as far as replacing / substituting suspect parts. However I have one exception - an intermittent ECU. When the ECU is working, the rough idle problem is there. In order to determining if the problem is in the ECU or any of the associated functions it tends to I have unplugged the ECU and I am running in open loop, no idle control --- etc. The problems at idle are still there. That tells me that the problem is purely mechanical in nature and not associated with the ECU. Yea I hear ya - the engine is not at lambda, no acceleration enrichment blah, blah, blah etc. Yes I agree. but what I am concentrating on is the problems at idle or in the low RPM range. The engine should perform reasonable well since it is in "limp home" mode. Yes the power is somewhat reduced but it's not bad. I have not been able to track down the problem but I am continuing to work on it.

I would love to get an update to what others have come up with. I will post my results as soon as I have a solution.

This is a great thread. I'm wondering the the potentiometer was ever replaced and problem solved or not...anyone know?

I end up getting caught up with other projects...or other problems...

First the transmission leak...now the head gasket leaking again :rolleyes:

One day...

Well....it sure does look good sittimg there in the picture:) I'm not sure if any of us ever really solve the idle issue on these cars....

My M103 has just 60K miles and I wish it would idle smoother. New injectors, cap, rotor, wires, correct plugs and 02 sensor. Almost there....

I actually replaced the Air Temp Sensor as someone suggested and it improved my idle and stalling today in these hot temps. Then I went and bought Bosch +2 Irridium Plugs ($32.00) and it got even better. It still doesn't want to start though. Must be my fuel accumulator losing the pressure build up to the fuel distributor.

Anything new on the idle?

My idle is not perfect but good enough, not much more I can do about it, 9 on a scale of 10 of smoothness. Car runs super so I've come to accept it.

Interesting that this thread has been resurrected.

I'll take my shot at solving the problem...since I've had a rough idle in my 500SL for over three years now. In that time the engine has been torn completely down and rebuilt. The problem still exists at low idle. Increase the RPM and it's smooth as silk. Tech manual says the idle on mine should be 650rpm +100 or -50. So, from 600-750rpm. Mine sits at 500rpm. Adjusting the mixture on the fuel distributor doesn't make the problem go away. Nor does increasing the idle by manually adjusting the linkage. The electronics bring the idle right back to 500rpm.

My mechanic has told me that the problem is in the throttle valve. Looking at my engine (117.962) manual, I'll guess a weak spring (part # 117 993 03 10). Or it could just be an adjustment to the butterfly plate. Getting to the throttle valve is a bear.

The throttle valve sits at the front of the engine and down low. The throat of the black boot (air flow housing) that bolts to the the bottom of the air flow sensor, clamps to the throttle valve housing. In order to get there, you have to remove the fuel distributor and air flow sensor. Notihing particularly hard about doing that, but it is time consuming.

What the adjustment will be is beyond me. I don't know if there is a screw type adjustment that you can make with an electronic meter or not. However, this is too big a job to tie into just on the hope you can figure it out once there.

Sometime in the next month or so I plan to tear into it and sic my mechanic on the fix.

Don

My '91 300E idles now as perfect as I would expect it to when it was new. However, it got progressively better as I replaced parts and not just from one or two parts. Biggest improvements came from ICV Hoses which had cracks in them and were hard as plastic (biggest idle improvement), replacing O2 Sensor (stopped the hunting idle), fuel injectors (better acceleration), and EHA Valve (which, I'm not sure was bad in the first place but could have been previously messed with and is very expensive for the size of the part). I also replaced all of the vacuum lines which was very easy. OVP did nothing. Needless to say I had tried new plug wires, coil, and plugs first with little noticeable difference at the time. I have 211K on it now and bought with 170K. Next thing is the cap to help the cold start, two cranker. When I start it now, it immediately idles at about 800-1000 rpm then drops down to about 600-700 rpm when warm.

I also replaced all the hoses and fittings during the rebuild, but not the O2 sensor--though it was replaced a while before I started the rebuild of the engine.. Might be worth the few bucks to give it a shot.

Tnx,

Don

O2 wasn't that hard to do. I bought a similar one ('05 Mustang Bosch) and spliced it in.

I know where it is and how to do it. Will take a shot at it this week. Thanks.