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1988 560SL
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30 Posts
Discussion Starter #1
Hi all,

I've been chasing down a rough idle/stalling issue on my 1988 560SL—which, aside from this one problem, is running great—for a while now. I've pored through the forum, found some posts that describe something close to my issue, and done what others have done before me, but I still haven't been able to solve it. Hoping y'all might be able to help me out.

The problem:

If my car is running at a low speed—in stop-and-go traffic, or at a red light, for example—and I lightly tap the gas pedal to inch forward, the car will run rough. It feels like it's misfiring; my oil pressure gauge drops and fluctuates; and my economy gauge swings to the right, sometimes veering all the way into the red. The car shakes palpably. Sometimes the car stalls out completely. I can prevent it from stalling by hitting the gas—whether that's in drive, neutral, or park. Once I've done that, the car smooths out, and everything is fine—but every time I'm moving slowly and tap the gas pedal just a touch, it will run rough and threaten to stall again. If it does stall, I throw it in park, remove the key, crank it, and it starts right back up.

What I've already done:

- Replaced both fuel pumps, the fuel filter, the accumulator, and all of the hoses in the fuel pump cluster (all Bosch OEM parts, excluding the standard 14mm and 6mm fuel hoses).
-Replaced the distributor cap and rotor (Bosch).
-Cleaned the idle control valve (thoroughly, with carb cleaner and Seafoam) and replaced the two rubber hoses connected to it, along with the small vacuum connector bridging them (All Mercedes parts).
-"Deleted" the EGR valve by removing the vacuum line to it and plugging the vacuum line with a golf tee.

I'm at a loss here. Any ideas as to what might be causing this problem, and how I can fix it? Any and all help would be deeply, deeply appreciated.

P.S.: If it would help y'all, I can upload a video of the car struggling with the issue I've described.

Best,

Drew
 

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112 Posts
I wonder if your throttle switch is not working correctly. KE jetronic needs to see idle position And full throttle. At idle position 2 things are happening 1) idle control using the idle control valve and deceleration fuel cut off above 1500 or so rpm’s. If the idle signal from the switch stays closed that can cause problems. The only other thing I can offer is getting a meter that shows duty cycle and hook it up to the diagnostic socket. From those readings we can better determine what the KE ECU is trying to do. Also just make sure the vacuum line is hooked up to the EZL ignition module and has vacuum. It seems to me that this is a fuel injection issue not an ignition one.
 

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1987 560SL (L.Tonk) [92,700 miles]
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378 Posts
My 87 560SL has the same problem (although not as bad as yours). It only happens sporadically, and it seems to happen more frequently when it's a hot day (high 80's). I've not spent time chasing down the issue specifically, but along the way my Fuel Pump Control relay and CIS Idle solenoid have been replaced by my indie shop, so we can rule those out as well.
 

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1988 560SL
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30 Posts
Discussion Starter #5
I wonder if your throttle switch is not working correctly. KE jetronic needs to see idle position And full throttle. At idle position 2 things are happening 1) idle control using the idle control valve and deceleration fuel cut off above 1500 or so rpm’s. If the idle signal from the switch stays closed that can cause problems. The only other thing I can offer is getting a meter that shows duty cycle and hook it up to the diagnostic socket. From those readings we can better determine what the KE ECU is trying to do. Also just make sure the vacuum line is hooked up to the EZL ignition module and has vacuum. It seems to me that this is a fuel injection issue not an ignition one.
Thanks for your input, @Alphasud30. Where exactly are the throttle switch and EZL located? I'll check them both out and, depending on what that turns up, look into hooking a meter up to the diagnostic socket.

Thanks for your advice too, @oncebitter —I'll cross the FPR and idle solenoid off the list of possible causes. I'm a bit perplexed by the chart you linked to; what does "on / off ratio in percent" actually mean?

I found a chart detailing duty-cycle analysis:
Anybody else have thoughts here?
 

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1988 560SL
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Discussion Starter #6
Never mind r.e. the location of the EZL, Alphasud—found it. The vacuum line is connected, though it would be very difficult to reach it to test the vacuum line itself without taking a bunch out of the car, due to its position. All looks OK there, for what it's worth.
 

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1987 560SL (L.Tonk) [92,700 miles]
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378 Posts
I’ve not fiddled with it myself but did find these while researching it:


 

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1986 560SL
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73 Posts
About six months ago I had such a serious surging with the throttle just barely open to maintain speed on a down-slope street, that I turned around and parked it in the garage. One of the old timers here suggested checking the idle switch. I haven't, because the problem hasn't reoccurred. But it makes sense that a little 30+ year old switch, on a hot, vibrating engine, wouldn't last forever.
 

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1988 560SL
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Discussion Starter #10
Thanks for this note, @Turbo510 . From what I've read, an issue with the throttle switch tends to create high idle problems, like the one you described on your car, as opposed to low idle/stalling issues, like the one I'm struggling with. It's quite a pain to get to it, necessitating the removal of the fuel distributor and the throttle plate assembly, so I'm hesitant to go through all that trouble—and potentially screw something up along the way—without knowing that the throttle switch is actually the culprit. Given that all the posts I've seen about the throttle switch are related to "high idle hell," I suspect that my problem lies elsewhere.
 

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Yiur next step is to buy or borrow a meter with a duty cycle function. With that you will be able to tell some basics. I’m curious to know if you are in closed loop meaning your oxygen sensor is working and the ECU is responding. So easy for somebody to mess with the adjustments as well as possible vacuum leaks or fuel distributor issues. Looking at the meter will be the least evasive. when I work on a modern car the first thing you do is check data PID’S and fault codes. I guess I should ask if your car has CA emissions. It will say on the emissions label under the hood.
 

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1984 380SL
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123 Posts
For what it's worth, you originally stated that your economy gauge swings to the red. That makes me suspicious. So, when I was having issues with my idle, I had the same symptom with the gauge swinging. Found a vacuum leak on my ICU. There were other problems, but fixing that leak stopped the economy gauge from swinging. So, I would so a smoke check for vacuum leaks if you have not done so already.
 

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1988 560SL
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Discussion Starter #13
Thanks for your input, @DHartman66 . Good tip! I think a smoke test is a wise next step—I’m going to do that as soon as I can, address any vacuum leaks, and then circle back here.
 
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