Diagnosis Help - 560 Rough Idle - Mercedes-Benz Forum
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post #1 of 22 (permalink) Old 07-24-2019, 03:38 PM Thread Starter
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Diagnosis Help - 560 Rough Idle

Hi,

I have a 1988 560 SL with 34,000 miles and need some advise diagnosing a rough idle issue. It's not terrible, but it does distract from the driving experience. The car has California emissions and is in pristine condition.

Besides the rough idle. The idle speed fluctuates between about 600 and 700 RPMs in about 1 second intervals. The roughness continues as the throttle is opened. At about 1100 RPMs it either goes away or becomes unnoticeable.

I had a tuneup done last summer. The fuel filter and spark plugs were replaced. The mechanic noticed the rough idle and suggested disconnecting the O2 sensor. He said this is a common practice on these cars. I didn't really want to do this, but since it is easily reversed, I went ahead. As I have done more research, I've come to believe this is a bad practice.

With the O2 sensor disconnected, the idle was silky smooth. I began to notice a fuel smell whenever I would come to a stop. I believe the car was running too rich so I reconnected it and cleared the OBD alert.

With the O2 sensor reconnected, the rough idle returned immediately.

Any suggestions for where to start my diagnosis? Any help would be greatly appreciated.

Thanks,
Jay
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post #2 of 22 (permalink) Old 07-24-2019, 03:54 PM
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Originally Posted by Spunkmeyer View Post
Hi,

I have a 1988 560 SL with 34,000 miles and need some advise diagnosing a rough idle issue. It's not terrible, but it does distract from the driving experience. The car has California emissions and is in pristine condition.

Besides the rough idle. The idle speed fluctuates between about 600 and 700 RPMs in about 1 second intervals. The roughness continues as the throttle is opened. At about 1100 RPMs it either goes away or becomes unnoticeable.

I had a tuneup done last summer. The fuel filter and spark plugs were replaced. The mechanic noticed the rough idle and suggested disconnecting the O2 sensor. He said this is a common practice on these cars. I didn't really want to do this, but since it is easily reversed, I went ahead. As I have done more research, I've come to believe this is a bad practice.

With the O2 sensor disconnected, the idle was silky smooth. I began to notice a fuel smell whenever I would come to a stop. I believe the car was running too rich so I reconnected it and cleared the OBD alert.

With the O2 sensor reconnected, the rough idle returned immediately.

Any suggestions for where to start my diagnosis? Any help would be greatly appreciated.

Thanks,
Jay
I question that mechanic's intelligence. Disconnecting the o2 sensor forces the car into full enrichment hence the smell. This isn't a problem if you don't mind destroying your catalytic converter and reducing the life of your spark plugs.

Given what you've posted, it sounds like there might be a slight air leak towards the top of the engine. False air into the intake causes all sorts of mischief.

A smoke test will reveal such leaks.

The next step if no leaks are occurring would be to investigate the idle management system. These do go bad causing weird or high idles. You can rebuild the idle control relay by reflowing the solder and capacitors.

I would look for leaks first given the age of the vehicle and low miles.
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post #3 of 22 (permalink) Old 07-24-2019, 04:11 PM
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I have a 1986 560SL that idles a little rough but accelerates silky smooth.
Still have it even after replacing my O2 sensor.
My old O2 sensor was sorta a hack job, so I replaced it with a $20 O2 Sensor off Ebay.
I've done a lot of tinkering trying to fix this idle.
Might try disconnecting mine, and might even purchase an OEM O2 sensor to try.

BTW, whats your fuel economy with the O2 sensor disconnected?

Carl

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post #4 of 22 (permalink) Old 07-24-2019, 04:17 PM Thread Starter
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Thanks for the reply. I will try to rig up a smoke test.

I appreciate your help.

Jay
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post #5 of 22 (permalink) Old 07-24-2019, 04:22 PM Thread Starter
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Thanks for the reply.

I haven't paid attention to the fuel economy. I don't put many miles on the car and only fill the tank with enough fuel to last a few weeks. I'm trying to keep old fuel out of the system.

I suspect my O2 sensor is working properly and this is probably a vacuum leak.

Jay
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post #6 of 22 (permalink) Old 07-24-2019, 04:40 PM
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Originally Posted by Spunkmeyer View Post
Thanks for the reply. I will try to rig up a smoke test.

I appreciate your help.

Jay
Good luck, please let us know what you see!

Also, apologies for my terse reply. I've heard and seen all sorts of nutty things before and hearing stuff like that makes me go:

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post #7 of 22 (permalink) Old 07-24-2019, 04:52 PM Thread Starter
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Originally Posted by okyoureabeast View Post
Good luck, please let us know what you see!

Also, apologies for my terse reply. I've heard and seen all sorts of nutty things before and hearing stuff like that makes me go:

No worries. I didn't take your reply as terse. BTW, I've changed mechanics because of this issue. I don't want bad mechanics touching my SL.

Jay
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post #8 of 22 (permalink) Old 08-04-2019, 07:04 PM Thread Starter
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Smoke Test

I finally got around to doing a smoke test on the vacuum system today. I definitely have a leak.

I can't see exactly where the leak is, but smoke is coming from below the idle control valve. It is something around the front of the intake manifold.

Any ideas what it might be before I start taking things apart?

Thanks,
Jay
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post #9 of 22 (permalink) Old 08-04-2019, 11:21 PM
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I would say the AFM booth, the vacuum connections on the throttle body or the manifold donuts.

The vacuum connections are easy to check by just looking down and fidle around a bit.

With a good flash light and a "dentist mirror" you should be able to see if the AFM booth is well seated on the throttle body by the inside.

The donuts are a larger operation as the complete intake manifold needs to be removed.

Good luck B
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How hard can it be?
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post #10 of 22 (permalink) Old 08-04-2019, 11:38 PM
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Originally Posted by Bauk18 View Post
I would say the AFM booth, the vacuum connections on the throttle body or the manifold donuts.

The vacuum connections are easy to check by just looking down and fidle around a bit.

With a good flash light and a "dentist mirror" you should be able to see if the AFM booth is well seated on the throttle body by the inside.

The donuts are a larger operation as the complete intake manifold needs to be removed.

Good luck<img src="https://www.benzworld.org/forums/images/smilies/thumbsup.gif" border="0" alt="" title="thumbsup" class="inlineimg" /> B
X2

I'd also replace the lower plenum donuts , the gaskets that mount the intake to the plenum, and all of the plastic components that sit down there.

Lot of "while you're in theres" worth doing. I did the job twice because of a denseness in my cranium. The issue ended up being bad injectors.
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