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Definitive 5.6L M117 Idle Diagnosis and Guide
Hey guys,
If you own a 560SEL, 560SL or 560SEC etc etc, you have most likely tackled the tribulations of high, wavering, rough or plain bad idle quality. I'm making this thread because this question comes up a bit, and feel I have come up with a great checklist for things to check for new owners of the 1100rpm club, or even vets who have tackled it down to 9 or 800. First things first: This generation of M117 have electronic idle control. While the idle is smoother and more efficient, the dependence on perfect vacuum and various valves provides a playground of headaches for the DIYer. Electrical things that affect the idle are: -Coolant Temp Sensor. Produces a high idle to about 900rpm. At least it did in my case. $75 at the dealer. -Idle Control Unit. This little computer is under the false floorboard on the passenger side. I got a second hand one for $20, and turns out it wasn't bad in my case, but now I have a second. -Idle Control Valve. These get sticky from carbon going through. Gets the idle stuck to about 1100-1200rpm. Disconnect it and the idle rises? It's functional, and worth cleaning. Use carburetor cleaner. I personally sprayed it and achieved acceptable results, but some like to soak it over night to get the coke out. -Ignition Control Unit. This is the Big Kahuna computer behind the side trim that slides towards the passenger seat next to the aforementioned false wall. Mine was bad and caused stumbling and rough idle on some days. $700 at the dealer, but mine is second hand. Be aware of part numbers-- only some are interchangeable. -Kick Down Switch-- Mine was alright. I think. Vacuum things that affect idle: -All of the vacuum tubes and air injection ports on the surface of the engine. Roughly $200-$250 at the dealer to replace; highly recommended at these cars ages. These include the injection rails, breather hoses and two L shaped ones that connect to the Idle Control Valve. -Manifold gaskets. Cheap gaskets. Labor intensive R&R. Not for the faint of heart, so I smoked the car after doing the surface rubber. Lo and behold, these were alright. Phew. -Thermovac switches. There's two, they have two elbows on vac lines each. $35 a pop OEM. Didn't do them yet, but about to. I hope this helps someone out-- please chime in if you know of other terminology for these things to help searchers or other things I may have forgotten while doing the write-up! -Carlen
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_______________________________________________ 1987 560SEL (210,000 miles) |
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Very informative. I've done everything you covered but the Ignition Control Unit. You didn't mention the eight dougnut gaskets between the intake halfs. Mine's a 420 but I believe they are present on the 560. Also, the injector O-Rings in the holders, and cleaning the throttle body that lives between the manifold halfs. I've done all this and still have a rough idle. Any idea if the Ignition Control Unit can be diagnosed? Or where I could pick up a used one just to try it. Don't know if you are smog checked or not. If not, do yourself a huge favor and disconnet the EGR system. If you take your intake apart you won't believe the sluge and coke in there. Dave. |
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Think the emissions readings will be fine without the addition of EGR? I lean toward yes.
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I'm not a doctor, but I'll have a look. '85 300SD 245k '87 300SDL 251k '90 300SEL 326k Six others from BMW, GM, and Ford. Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG] |
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All very interesting, BUT...
I did most of that, and still had a rough idle ('89 560 SEL). Took it to a Mercedes only specialist shop that knew well I worked on it, and they showed me a "trick": Very carefully bleed fuel, while at idle, out of all of the eight injector exit ports (one at a time) at the fuel distributor. They showed me how - brown gew cam out, replaced slowly by clean gas. OK, so the idle got a bit smoother. My own "trick" then was to add an in-line fuel filter right before the fuel distributor.
To make the story short - all of this added some MINOR improvements. New injectors, new seals, chasing vacuum leaks, cleaning the idle control valve, all of this, and in the end the idle was still too rough and random. After years of living with this, I finally recently swapped out all three engine mounts - and the rough idle was gone, FOR GOOD. Smooth - just like a new car. Since this is a relatively easy job, with not too much cost involved in parts, I'd do this FIRST before chasing down phantoms in the engine and throwing money at parts. Interestingly, on this site there were others in the forums that had suggested I look at the mounts when I was trying to chase the source of the rough idle roughly ten years ago. I took your approach on my own back then, but they were right in the end. Lesson learned: I'd start with the mounts first. They are rubber, these cars are old, and they are gone if they have not been replaced.
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Henry Bofinger 1989 560 SEL (black/black) 2001 Audi TT Roadster (silver/grey) Last edited by hbofinger; 11-27-2011 at 07:11 PM. |
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In CT, no emissions testing after 25 years of age. Goodbye EGR. Goodbye smog pump. Goodbye Catalytic converters. Hello Horsepower Get this, I've had my EGR disconnected at the vac line since I've owned the car, and I routinely pass smog. Go figure. The gaskets you speak of for the intake I referred to as manifold gaskets; good catch though in case of terminology clash for onlookers hoping to set their M116/7 straight! I don't know how to test the computer; I just knew mine was dead because of oxidation on the pins, where one wasn't even connected anymore
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_______________________________________________ 1987 560SEL (210,000 miles) |
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Most definitely; mine has been disconnected to vacuum for quite some time
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_______________________________________________ 1987 560SEL (210,000 miles) |
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_______________________________________________ 1987 560SEL (210,000 miles) |
#8
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As I figured, thanks. I have euro duals to drop on my car. Sure, not as much power as the four piece manifolds... but I have around $125 invested in mine and I will see a nice jump in power for the money invested. Hell, scrap value on the cats will more than offset the cost.
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I'm not a doctor, but I'll have a look. '85 300SD 245k '87 300SDL 251k '90 300SEL 326k Six others from BMW, GM, and Ford. Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG] |
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You're not kidding, I sold the ones from the 560SEL, along with the one from the 560SEC and got $300!
I had a miss at idle that was caused by 1) bad plug wire 2) stripped threads on the head causing the plug to back itself out Once I addressed both, the idle became smooth like silk.
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With best regards Al |
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