Rough idling with proper rpm both engine cold and warm - PeachParts Mercedes-Benz Forum

motozine · #1 09-14-2009, 05:50 PM

Hello,
Just purchased 86 560SL with 114k miles.
Trying to tackle first annoying issue with rough idling. Rpm range seems fine with about 700.
Doesn't mater when engine is cold or after running for a while. Every time when I stop/idle in traffic light engine runs very rough. When it goes above 1k rpm engine runs very smooth.

One thing to note, my economy gauge is all the way right all the time. appears to be stuck doing nothing.

I suspect something to do with vacuum. Any idea what to check first? thanks.

rocky raccoon · #2 09-14-2009, 06:14 PM

Oh Yes. Vacuum problems for sure. The "economy" needle should be pegged to the left at idle. That gauge reads nothing but vacuum. Look at all your underhood lines first. The issue is usually rotted soft rubber vacuum connectors. The first place I would look is the hardest to see and one that is often overlooked for that reason.

Look between the rear of the intake manifold and the firewall. May have to remove the air cleaner (don't remember if you have to). Use a flashlight. You will find a right angle soft connector that attaches to a vacuum spigot on the intake manifold and a hard vac line that runs along the top of the firewall to a 4-way rubber connector on the driver side. One of the lines coming out of that 4-way goes to your economy gauge.

In my case the right angle connector was rotted to pieces. It resides in a area of max oil vapors and high heat and is the most vulnerable of all the connectors because of that. You need little hands and a lot of patience to service that thing as you will see. I used a piece of thick wall plastic tubing as a temporary fix until I got the proper connector.

Check all of the vacuum connections you can find of course. You can buy them all for chump change but my bet is on the one that I described.

Let us know what you find out so we can add to the data base. Happy Hunting.

motozine · #3 09-14-2009, 07:12 PM

any photos? am i asking too much?

thanks so much for guidance. Will check and update soon as I have time to check it out.

sublettd · #4 09-14-2009, 07:55 PM

...and if it turns out to not be a vacuum problem, it may well be worn valve guides. I chased a similar problem for well over a year. Finally nailed it down when we pulled the engine to replace the head gaskets--which evolved into a complete engine rebuild due to a busted ring.

rocky raccoon · #5 09-14-2009, 09:24 PM

Find your vacuum leak.

motozine · #6 09-14-2009, 10:57 PM

Is this the angle connector you are talking about?
(one with red circle below)

That line goes to connector all the way to driver side corner

If that's the one you advised? then how I check to make sure vacuum is ok? just by wiggling little angle connector/line to 3way(?) connection seems to be ok.
thanks again for comments.

rocky raccoon · #7 09-15-2009, 06:21 AM

that's the one. Pull it off and examine it. If that's not it, follow your hard lines and examine all of the similar connectors. A MityVac is helpful in finding subtle vacuum leaks but yours seems to be massive and you should find it easily. Probably wouldn't hurt to replace all of the connectors as you go if they have never been. Quarter of a century old you know. Available from Phil on this site or dealer. Cheap insurance.

motozine · #8 09-15-2009, 12:15 PM

ok thanks for help. Will go with connectors first, hope i nail the problem soon.
By the way what that yellow thing (valve?) from bottom photo after 3way connector?
Will give an update.

Bill Adkins · #9 09-16-2009, 11:12 AM

I ran a dyno during that cars reign. They did suffer from rough idle for several reasons. The valves were particularly prone to carbon at the valve seats, usage of good fuels like Chevron with Techron seems to have solved that issue. The fuel system is a CIS system with fuel distributor. The injectors had a tendancy to become fouled with age causing undesirable spray patterns. The spark plug wires had a weakness in the resistor ends and did break down causing secondary leakage. I would probably run two fuel tanks with a bottle of Chevron Techron in each tank and then re evaluate the running. If it is still poor you should have it checked with a scope to evaluate the ignition wires and basic function. The injectors are mounted in plastic cups with seals that also cause vacum leaks and poor idle quality. Just some ideas, some of these items are quite expensive so the fuel treatment is where I would start.
Bill

motozine · #10 09-16-2009, 10:26 PM

ok I'll try with techron too.
Other maintenance i probably do during the winter time when i don't use that car.

thanks.

sublettd · #11 09-16-2009, 10:32 PM

Finding the link wasn't as hard as I thought. You'll have to search through the Mercedes books available for the 117.962/3 engine.

sublettd · #12 09-16-2009, 10:32 PM

Guess you'd like the link, huh???

http://www.books4cars.com/search.asp?findmake=Mercedes&offset=550

motozine · #13 10-03-2009, 10:27 PM

Spent some time again to track down vacuum issue but no luck so far.
But found something and need help what those are.

First photo with numbers taken in between MCU and distributor, probably throttle body.
#2 in blue, hole that just open without any connection hose. what is it? where does it need to be connected?

#3 in green is capped off. Is it suppose to be?

#1 hose is just dangling without any connection. that hose routed to driverside front.

Please help on these.
thanks.