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Off idle stumble and hesitation

5K views 7 replies 4 participants last post by  H.D. 
#1 ·
I love this site. So much useful information for the noob. Anyway, I just acquired a low mileage 1992 500SL, my first. As typical, it was garage kept and the body is in great shape but due to lack of use (1000 miles driven in the past 5 years) it has some drive ability issues. Idle is somewhat smooth but there is a terrible hesitation and bogging down from just off idle to about 2000 rpm, warm or cold. Above 2k, it pulls strong all the way to 5000 rpm without any noticeable misfire. Additionally, the exhaust smells a little and its pumping out over 1000 ppm of carbon monoxide as registered on my MSA CO meter. I've already replaced the spark plugs, distributor caps, rotors, and dust shields. The wires were replaced in 2008 and all tested between 1700 and 1900 ohms. The coils are fine. Now it's on to the fuel system. New fuel filter plus a bottle of Techron in the first tank and the issue is getting worse. I figured I'd come to the knowledge central here before I get into replacing the injectors and rebuilding the fuel distributor. Any advice would be appreciated. Thanks.
 
#2 ·
I hope I does not mislead you, but I guess you have the KE-jetronic injection system. It sounds like you're mixture is not quite right. Could be a faulty O2 sensor not giving a correct signal to the ECU. If you have a CO meter you can adjust the mixture directly on the FD. Many threads here on adjusting mixture. Adjust in VERY small steps at a time. Congratulations on your new car, and well done filling in your profile on your first post!
 
#3 ·
Next steps

Yep. KJet / CIS. I was planning on adjusting the mix (leaning) by turning the screw CCW 1/8 step at a time and following along with the DVM to hit the 50% duty cycle target. I understand if I have a lazy O2 sensor that the duty cycle basically stays static at 60% value??? I wasn't thinking of using the CO meter- great idea.
I wonder if replacing the injectors would be beneficial overall since they're a 25 years old anyway. It looks like a $450 bill from AutohausAZ with new seals.
 
#6 · (Edited)
... I was planning on adjusting the mix (leaning) by turning the screw CCW 1/8 step at a time and following along with the DVM to hit the 50% duty cycle target. ...
As Ruindr says, there are many threads here on “adjusting mixture” … I’ve visited several of them. But they all displayed a major misconception about the KE-Jetronic, which explanes why it is so widespread.

On a KE-Jetronic the air/fuel mixture - aside from warm-up phase / WOT / limp home mode - can not be adjusted, unless the duty cycle is, of course inadvisably, set to below ~10% or above ~90% !

I explaned that in detail in post 1 (chapter ‘Basics’) of the following thread ... it also containes detailed procedures for checking and changing the Lambda control (duty cycle) adjustment:

http://www.benzworld.org/forums/r129-sl-class/2722145-ke-jetronic-lambda-control-duty-cycle.html

Your descriptions sound like a fuel system contamination problem. Unfortunately I’m a little short of time right now, but as a first step I suggest to check the movement smoothness of the AFS (air flow sensor) plate / control plunger like this:
  • With the air cleaner removed, turn the ignition key to "ignition on" two or three times in order to let the fuel pumps prime.
  • From its base position the AFS plate must move 1 – 2 mm (0.04 – 0.08 inch) without any resistance before you feel it reach & start to lift the control plunger.
  • Then push the AFS plate slowly and completely down and feel the resistance. It should be homogeneous along the complete path.
  • Let go of the AFS plate. It should leave the control plunger behind and quickly and freely move back to it’s base position.
  • Let the fuel pump(s) prime two or three times again.
  • Push the AFS plate completely down again and this time let it slowly move back to it’s base position and feel if the control plunger stays in contact with (follows) the AFS plate along its whole path, which it should.

Particularly pay attention to the first part of the AFS plate's path when you push it slowly down, and on the last part of its path when you let it slowly rise back into its base position.

H.D.
 
#4 ·
I would sort other likely candidates before messing with the mixture adjustment as this is rarely the culprit. If the duty cycle is staying static it could be a faulty Oxygen sensor

I had a big improvement on throttle response after this exercise on my 92, if you can't get one with the tube maybe fabricate one.

(POST FROM ANOTHER THREAD)

Well after Ken said the CIS TB port and polish was a lot more labour intensive than the later M119 because of the air flow plate. Then remembering what it was like removing all the fuel lines without getting a kink in them when I replaced all the injectors, my enthusiasm cooled a little

I still had this nagging feeling that after 20yrs plus if the TB had not been serviced it was probably looking pretty dismal by now. So today plan B, I found this little baby today, (probably available everywhere but new to me) The attachment pipe has 3 small holes that creates a fan pattern.

I first cleaned all round the air flow plate (plenty of gunk there) I then eased the plate down, inserted the hose and gave a real good spray in different positions to try and cover the whole TB area and also up towards the air flow plate swivel area. I left it for a few minutes to soften and did it all again, started the car (spit & fart) and gradually bought the revs up to a 2k then back to idle for 5 mins and turned it off. While I was at it, I removed the ICV and gave that a working over, loads of black stuff came out of that I didn't hear it rattle when I took it out but it did before I put it back.

I can only imagine what a port and polish would be like, as the improvement in throttle response from this clean alone is very impressive. For me the TB clean will be a service Item when I do future oil changes.
 

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#5 ·
Next steps

I used a 1/2 can of carb cleaner on the airflow plate and inside the housing (well at least as far as the 6"plastic tube would get to) without any improvement. I'm going to check the O2 sensor response and the duty cycle once it stops raining around here (No garage to work in so I'm a slave to the weather). I still have to figure out where to measure the fuel pressure from- I haven't gotten that far into the CIS manual yet. after that I'll try replacing the fuel injectors and I'm really hoping I don't have to rebuild the fuel distributor. The missus is rolling her eyes wondering what I've gotten myself into this time....
 
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