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Walt Kowalski
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1988 Mercedes 560 SL rough idle, stumble

I'm having some trouble with this car.. It has an inconsistent rough idle, and stumble upon acceleration that I cannot pinpoint. I have searched high and low, near and far for vacuum leaks and I am 100% confident there are none. At a given time, the idle may be smooth, but at perhaps the next light, it will be very rough, yet idling higher, 1200-1300 rpm. It has a new cap and rotor($200!!!), plug wires all test fine via voltmeter, new fuel filter just installed, spark plugs are all fine, gapped to spec. I hate problems that come and go, I absolutely hate them. I also do not fully understand how this mechanical fuel injection setup works. This problem developed overnight. It is strange, as sometimes I will hop in the car, start it and all will be well, but the next time I get in it, it will be very rough. I don't get it. Any specific problems with these, things in particular to look for? The IAC appears to function correctly, and that wouldn't affect acceleration anyway. Acceleration is affected at any throttle position. Light-footed driving, WOT, makes no difference, when it's rough, it stumbles no matter how little or how much gas you give it. When it's not rough, it makes tons of power and has no trouble getting up to 140, so it can't be that big a deal.

I'm lost

edit: the stumble is a random, inconsistent misfire that is very prominent, though the engine never dies nor seems like it is about to die, it just runs rough.
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Old 02-27-2009, 05:14 PM Walt Kowalski is offline  
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SmashingPumpkins
 
fuel pump?
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Old 02-27-2009, 05:16 PM SmashingPumpkins is offline  
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asa
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i think if the fuel pump were going, it'd be more consistent
possibly a piece of trash or something blocking the pickup and/or filter?

nevertheless, i'd check fuel pressure
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Old 02-27-2009, 05:45 PM asa is offline  
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StarDeath13
 
id be looking at a clogged injector or one not working properly.i would only suspect it tho if each cylinder has its own injector.
you pretty much covered the electrical side.
im not sure if the computer is adjusting the idle higher to compensate for the issues or there really is a hidden vacuum somewhere.
Old 02-27-2009, 05:50 PM StarDeath13 is offline  
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miasman1
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Besides the fact that K/KE-Jetronic is a absolute PITA to deal with, those Mercedes M116/M117 V8s are known for developing major vacuum leaks with age - you're going to need to replace each and every vacuum hose/elbow on the upper intake/fuel distributor+air flow meter, the injector nozzle seals+holders, the rubber donuts between the upper and lower intake halves, and the boot from the air flow meter and fuel distributor assembly to the lower intake.

Also, you may want to check the duty cycle and adjust the EHA on the back of the fuel distributor. Go to the MercedesShop forums if you want to know more about this.

And K/KE-Jetronic systems found on Mercedes and VW are continuous systems - they'll keep spraying fuel into the cylinders as long as there's pressure from the fuel pump and the motor's running. Both K and KE use a mechanical fuel distributor linked to a air flow meter which is also mechanical. The only electric bits in a K-Jet system is the fuel pump/warm-up valve/IAC valve, and they use a crude ECU for O2 sensor control. KE-Jet introduced some self-diagnosis and management and better control of the fuel mixture via an EHA.
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Last edited by miasman1; 02-28-2009 at 12:31 AM..
Old 02-28-2009, 12:20 AM miasman1 is offline  
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Walt Kowalski
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Quote:
Originally Posted by miasman1 View Post
Besides the fact that K/KE-Jetronic is a absolute PITA to deal with, those Mercedes M116/M117 V8s are known for developing major vacuum leaks with age - you're going to need to replace each and every vacuum hose/elbow on the upper intake/fuel distributor+air flow meter, the injector nozzle seals+holders, the rubber donuts between the upper and lower intake halves, and the boot from the air flow meter and fuel distributor assembly to the lower intake.

Also, you may want to check the duty cycle and adjust the EHA on the back of the fuel distributor. Go to the MercedesShop forums if you want to know more about this.

And K/KE-Jetronic systems found on Mercedes and VW are continuous systems - they'll keep spraying fuel into the cylinders as long as there's pressure from the fuel pump and the motor's running. Both K and KE use a mechanical fuel distributor linked to a air flow meter which is also mechanical. The only electric bits in a K-Jet system is the fuel pump/warm-up valve/IAC valve, and they use a crude ECU for O2 sensor control. KE-Jet introduced some self-diagnosis and management and better control of the fuel mixture via an EHA.

there are no vacuum leaks. i have checked every single possible source of vacuum leak three times, there are no leaks.
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Old 02-28-2009, 07:32 AM Walt Kowalski is offline  
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BlueNautilus
 
1988 560SL my personal experience

I purchased recently an 88 560Sl, a time bubble, 27,500 original miles.
Great car, near brand new condition but a similar problem as yours has made me very frustrated.
I am lucky to have one of the best MBZ specialists near me and we have approached the issue in the most conservative and methodical fashion, it is still not completely resolved.
When the air temp is very hot (common here in SoCal) the engine will stumble then eventually stall, it happens almost always near idle or very low RPM, it will restart almost right away in most cases.
All the obvious has been checked, vacuum leaks, fuel pumps, etc... We did the trouble shooting by trial and error testing, replacing the reasonably priced parts and swapping good used parts (the expensive ones) for my original parts and checking for the same symptoms.
We swapped the ECU, ignition CU, coolant temperature feeler.
We replaced with original MBZ parts: the overload relay, fuel pump relay, O2 sensor, fuel filter.
We checked the Air flow mass meter, the readings show a messy signal, we replaced the potentiometer with a new one from a brand new old stock box, it was almost impossible to set correctly and get the car to run right so we put the original back on, we had made reference marks on the housing in case, it was still tricky to adjust correctly but we got the car to run very nicely, we adjusted the mixture to slightly richer from the adjustment screw inside the tower (tube) that runs all the way up through the air filter. The signal reading is a little better but still not clean but I still think we hit a better sweet spot after we reinstalled the potentiometer. This afternoon, thinking we had isolated the problem we swapped back to my original ignition CU, within a few blocks after I drove away it stalled 3 times and restarted right away :-( Tomorrow we'll install that used ignition CU back on and try to validate that symptom over the next few days.
I hope I don't need to replace the ICU or the whole air flow mass meter as they are very expensive and this is still an almost new car.
I hope my experience will help you, if you think you can add anything to help me out I am all ears.
I will also email this to you privately in case you do not visit the forum regularly.
Old 09-09-2009, 04:27 PM BlueNautilus is offline  
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JJ-KK
 
May be some problems with your car

you can go to the repair shop to check it.

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Old 09-09-2009, 08:18 PM JJ-KK is offline  
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BlueNautilus
 
Hello JJ-KK
And.... What is your point?
Old 09-09-2009, 08:54 PM BlueNautilus is offline  
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BlueNautilus
 
Latest update
The fuel accumulator is good and I forgot to mention before that we had checked the idle control module and valve and they are fine, so it seems by process of elimination that the culprit is the ignition CU. They are very expensive new and I was wondering if any of you might have a suggestion if and where I can get one rebuilt or find a good used one.
Many thanks
Old 09-10-2009, 07:11 PM BlueNautilus is offline  
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d333gs
 
Simular problem

Hi all , I am new to the site.
I have an 86 560SL which skips/misses du to some sort of fuel starvation.
Miss disappears after 1500 RPM.

Spark is good, compression is good,fuel pressure is good, injectors have been changed, fuel distrubitor has been changed, injector seals have been changed ..........It has been a year of frustration........

Any ideas??

Thanks
d3
Old 05-11-2012, 11:00 PM d333gs is offline  
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[H]ard|On
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Quote:
Originally Posted by BlueNautilus View Post
Latest update
The fuel accumulator is good and I forgot to mention before that we had checked the idle control module and valve and they are fine, so it seems by process of elimination that the culprit is the ignition CU. They are very expensive new and I was wondering if any of you might have a suggestion if and where I can get one rebuilt or find a good used one.
Many thanks

Those are very unique systems, you'd have better luck on a Mercedes board tbh.

I for one know nothing about mechanical FI.
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Old 05-12-2012, 12:40 PM [H]ard|On is offline  
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asa
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Quote:
Originally Posted by [H]ard|On View Post
Those are very unique systems, you'd have better luck on a Mercedes board tbh.

I for one know nothing about mechanical FI.

Dude, it was a necrobump. Try and help the new guy.

Posting goatse from my phone, damn it's cool to be in the future
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Old 05-12-2012, 01:09 PM asa is offline  
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d333gs
 
"Those are very unique systems, you'd have better luck on a Mercedes board tbh."

No luck

Now I'm desperate........
Old 05-13-2012, 01:37 AM d333gs is offline  
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theWITNESS
 
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I got all excited seeing a Rang3find3r thread, thinking he may have come back, but no, it's a necrobump

d333gs; try posting over on benzworld.org
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Old 05-13-2012, 08:20 AM theWITNESS is offline  
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