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Old 08-11-2013, 05:02 AM   #1 (permalink)
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88' 560SL rough idle question

Purchased an 88' 560SL in June 129,400 miles and notice it did not idle well when engine was warm. Not sure how long the car had been sitting prior to purchase (old gas?). I had it tuned-up including plugs, all fluids and filters replaced. Put injector cleaner in tank. Had vacuum hoses replaced. I have driven it about 200 miles and the rough idle now comes and goes. Sometimes it idles like a kitten while other times (5 min later) like an old tractor. It has even died a time or two when trying to leave a red light and then is a little hard to start. Once underway it runs great. My diagnosis is a partially clogged injector(s) which clear and then gets reclogged again and should improve over time and with additional injector cleaner. My mechanic says it may be a vacuum leak at the intake manifold, and he also mentioned fuel pump relay, but why would these causes be intermittent? Are there anyother thoughts? Is this a common issue in these cars?
Also yesterday out of no where the tachometer stoped working for about 5 min and then started back fine. This is the first time this has happened???
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Old 08-11-2013, 12:57 PM   #2 (permalink)
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This sounds to me like the ignition control module is on its way out. It sends the RPM signal to the tach and idle speed control relay.

Vacuum leaks are not come-and-go, and the fuel pump relay will kill the engine, not make it idle rough.
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Old 08-11-2013, 01:11 PM   #3 (permalink)
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when I replaced my fuel distributor with a used eBay unit, my rough idle mostly stopped. Apparently, the many orings in it deteriorate causing blockages and internal leaks.

good luck,

John
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Old 08-11-2013, 09:31 PM   #4 (permalink)
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Check also the Idle Air Valve

Idle Air Valve is adjusting the idle speed constantly - if this air valve is dirty or clocked the result is high or rough idle and stalling depending if it is too open or too closed. Cylinder shape valve can be cleaned with WD40 or similar.

Idle Air Valve is top and front of the engine, between FD and ignition distributors.
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Old 08-12-2013, 05:01 AM   #5 (permalink)
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I have exactly the same issue with my 380SLC 5.6. Thanks for posting this thread. Hopefully, we will find a solution. If I find one I will post the results here as well.
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Old 08-23-2013, 09:51 AM   #6 (permalink)
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Thanks everyone for the replys. I am getting it out of the shop Monday with a new ignition coil and new fuel relays. I will report back if this takes care of the issue.

MM
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Old 08-23-2013, 10:41 AM   #7 (permalink)
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I am curious to see if either of those fixes your problem.
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Old 08-30-2013, 05:49 PM   #8 (permalink)
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Update please
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Old 09-09-2013, 03:10 PM   #9 (permalink)
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UPDATE:
2 weeks post repair and the 88 560sl is running much better. The rough idle has all but cleared. It has not died or tried to stall once at idle since the repair. It starts quickly and runs well at all speeds. I have now taken it for 4 separte 20 mile interstate runs at 80+ mph on hot days near 90F and it runs out great. I have to believe it was the ignition coil given the tachometer issue I notice. The fuel relays were probably over kill. I am very happy with the results. Next up Odometer repair and then air conditioner next spring. I would also like to find some extended lug bolts for the stock wheels.
I like this auto more and more each day:-)))
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Old 09-09-2013, 10:44 PM   #10 (permalink)
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Originally Posted by 2-th-max View Post
UPDATE:
2 weeks post repair and the 88 560sl is running much better. The rough idle has all but cleared. It has not died or tried to stall once at idle since the repair. It starts quickly and runs well at all speeds. I have now taken it for 4 separte 20 mile interstate runs at 80+ mph on hot days near 90F and it runs out great. I have to believe it was the ignition coil given the tachometer issue I notice. The fuel relays were probably over kill. I am very happy with the results. Next up Odometer repair and then air conditioner next spring. I would also like to find some extended lug bolts for the stock wheels.
I like this auto more and more each day:-)))
In the meantime, before you have your odometer repaired, if you have an iPhone or a copy of one, there is an app that you can download called Odometer+ that will track your milage on trips. My odometer went out too, but I used the app to track a 6500 mile trip I took inn my '85 380SL this past June-July for monitoring my gas milage. BTW I averaged 22 MPG...
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