560SL Idles Rough When Warm - Mercedes-Benz Forum
 
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#1 (permalink) Old 06-10-2012, 04:58 PM
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560SL Idles Rough When Warm

I have an 88 560SL that I purchased from my neighbor last September. I had been driving it off and on for about 6 months, and then about a month ago it started acting wierd. The engine has about 180k on it and it was running like a bat out of hell before the wierdness. I'm pretty familiar with cars and how the different components operate, but this car is a bit different than anything I've owned in the past. When I start it up when it's cold it runs great, but after about 90-120 seconds, it starts running very rough, the economy guage goes way to the right and it appears like a vacum leak, I've checked the hoses and they are fine, although I'm going to replace all of them in the near future for GP. What I'm trying to figure out is what component "changes it state" when it gets warm. After reading a bunch of posts here, I have replaced the plugs, wires, coolant switch, and finally the coil, and the problem persists. I'm not sure if this is fuel related or not, I've seen some posts about fuel pumps potentially causing this issue.

Any help would be greatly appreciated, I'm kinda stuck and I don't have a money tree to keep swaping out parts.

Thanks in advance
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#2 (permalink) Old 06-10-2012, 06:39 PM
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Could it be related to the fuel injection system? My car's rough idle turned out to be a symptom of just that. Had the 25-year-old parts replaced and it runs beautifully now.
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#3 (permalink) Old 06-10-2012, 10:41 PM
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could be an intake leak. have you considered smoke testing to source out what vacumn leaks there are?
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#4 (permalink) Old 06-11-2012, 08:46 AM
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First off I would do an Italian tune up if you haven't done one. Two bottles of Techron (available at Chevron dealers or even Walmart), a fresh tank of gas and then some highway miles at 80mph plus...and report back. I am wondering if you have a clogged or failed injector.
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#5 (permalink) Old 06-11-2012, 09:27 AM
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Originally Posted by reno4u2 View Post
I am wondering if you have a clogged or failed injector.
This had been the case of my car. But I didn't have that weird problem with the needle moving to the right of the economy gauge.
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#6 (permalink) Old 10-29-2012, 09:48 PM
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OK, so here's what I've done to date. Replaced the intake manifold seal and injector seals as they proved to be leaking, as well as all of the vacum tubing. I also replaced the fuel distributor as the seals and hoses did not totally fix the issue, and this didn't totally fix the problem either. The mechanic I used had to set the fuel mixture very rich just to idle right.

I then had the car diagnosed by the dealer and they said bad O2 sensor, now the car is running like it was, but it still hesitates abit.
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#7 (permalink) Old 11-04-2012, 12:20 AM
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Hello,I had a similar issue on my Mercedes R129 which is a different car but may help point you in some direction My 1991 500SL used to hunt on idle between 550 to 900 rpm after initial warm-up which was about few minutes like yours.I adjusted the fuel mixture bit rich because with age some of these cars get comfortable or should I say prefer to run abit rich or lean than original spec? Little tweaking helps.Also my car sometime surged forward at traffic lights or at stops while my foot was on the brake? I traced the remedy to my idle speed.I had to raise the idle a bit to about 750-800 rpm in park or neutral mode.This helped resolve my surging issue and idle hunt was greatly improved.Hope this helps
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#8 (permalink) Old 11-04-2012, 01:04 AM
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Some air plate and basic adjustment suggestions:

The Cis-E air plate needs basic and zero position adjustments if you have played with the fuel distributor.

Zero position, position of rest:

You need fuel pressure on. Air plate should be flush with the upper edge of the cylindcral part of the air funnel/venturi. If not you have to move the air plate stopper pin deeper or higher. Deeper is easy but higher needs to open the mixture box :-((

Basic position, FD fuel stop/start point:

Fuel pressure on, the fuel must just start or stop flowing at the FD outlet fittings
when the air plate is pressed or released. Adjust the correct operation point by the mixture screw. FD plunger depth position has also impact here.

The idle and mixtue fine tuning must be done with CO meter and read the correct lambda/EHA cycles from the diagnostic socket.

Yka

Last edited by yka; 11-04-2012 at 06:03 AM.
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