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Running rough when at idle and when in drive at a stop

4K views 21 replies 12 participants last post by  therling 
#1 ·
1988 560SL. 144,500 miles

For the past few thousand miles the car has been running rough when at idle and also when in drive sitting at a red light or stop sign. Also, sometimes the car would stall when first started, but not again after it ran a few minutes.

The car ran ok when driving, except that if you tried to accelerate at wot from a stop it would bog down bad and then take off.

I just replaced the plugs and wires with bosch and the car is still running rough. There is a noticable difference in how the car runs and definately accelerates better and there is no more bogging down when at wot.

I did notice that when I start the car, it seems like the exhaust smell is pretty lean. The spark plugs were all very white, which I believe is also a sign of running lean.

Any suggestions or ideas why the car is running rough? No vacum leaks indicated by the economy guage and no check engine light.
 
#2 ·
Oh boy. not my expertise but here's a few things.

Does it run fine when cold? Might be the WUR, check the encyclopedia for that.

New plugs, are they gapped properly? You have to check them if they are the old style that were recommended for the car.

Fuel pressure?

fuel filters?

Timing chain slop?
 
#5 ·
I don't have any chain slap at start up, so I don't think it's that.

I've run injector cleaner through and no change and I've run it really hard to get it hot and "blow" everything out. No change.

So i guess I'm going to have to send it to the independent merc garage to have them check it and the mixture.

I'll check back in once it's fixed.
 
#12 ·
Follow-up

I finally got the car to the indie shop. I explained to them what was going on, what was done and that I wanted it fixed because I'm considering selling the car.

After a week at the shop, the same shop that has always performed great service on the car in the past, I stopped to pick it up.

They let me know that they made a bunch of adjustments on the car, changed the oil and put new exhaust hangers on.

I asked them if they actually found the problem. They said that I probably need to replace the fuel distributor but they adjusted the co2 and air mixture and to just drive it.

I started the car, it still runs rough, but no where near as bad, but now the check engine light is on and it feels like the car has been "dumbed" down.

I asked about the check engine light and they said that since the idle was hunting, they disconnected the o2 sensor and that took care of it but they had to turn the idle up to 1100 rpm's in nuetral or park.

WTH...I asked for this car to be fixed, not to rig it and then be told to just "drive it". I obviously don't want to deal with this shop again. I need to send it to another shop, but at this point I don't even know what to tell another shop. There is no way that I can afford to send it to the dealer right now. I gurantee they'd be in the 4-5K range. Any advice?
 
#13 ·
That sucks, please go back to my original suggestion and hone in on the problem cylinder. The easiest thing to do is to crack open the fuel lines coming out of the distributor and then tighten them. The engine should change tone when you do so.

The line that does not cause the engine to change tone is the one with the problem cylinder. You can then hone in to whether it's a fuel, compression or ignition problem.
 
#16 ·
Could it also be fuel pump assembly issues? I've had intermittent rough idle/stalling at a stop issues.

First thing I did that cured it was a new fuel filter. Lots of crud in the old one, and with the new fuel filter it had more zip as well as no rough idle or stalling.
 
#18 ·
My '87 560SL would idle rough everys so often. Sometimes went away when the temp hit 80c. sometimes not. What I did was remove the idle air valve, very easy to do, and clean the sliding valve piston thing with brake cleaner. don't KNOW if that was the problem or coincidence, but now idles smooth as silk at any RPM or engine temp. Since the 560 is semi computer controlled, strange things can affect the fine tuning of the idle circuit.

A good ration of injector cleaner can never hurt, but the air system is just as important. If it runs good at higher RPM and has good power, probably not fuel or ignition related. The idle air system complete with the rubber hoses is more critical at low engine speed I think. All your indie did was treat the symptoms, and never fixed the root cause, I believe.
 
#19 ·
Well...my problem is that when the car is first started and the injection pump is on , I am able to rev the engine but as soon as the air pump cuts off then the car would bog down and trying to stall.
If I get it on the road I have to feather the gas pedal in order to pick up speed otherwise it will stall.Once stepped on the gas it would hesitate for about 2 sec and then accelerate.
What could be happening?
I have already changed : fuel pumps,injectors,FP relay,O2 sensor,vacuum lines ,Fuel distributor ( used),put injection cleaner, ran compression test(all cyl excellent).
 
#22 ·
Sounds like you've gone through all the likely problems in the fuel delivery system. Now start to look elsewhere.

As someone above had suggested, have you pulled the valve covers and checked the timing? Also check the ignition circuit; could be something as simple as a bad ground for the coil (which I found on my 560SL when it wouldn't start after washing the car), an Idle Control Unit that's gone bad, (something I also found on my car), or a bad fuel injection control module (a/k/a the CIS-E Control Module), a bad electronic ignition module (a/k/a the EZL), throttle valve switch, throttle linkage, etc.

I understand it's a frustrating experience, it drove me crazy when I was trying to figure out how to cure a high idle problem. It's unfortunately one of the downsides to trying to keep a 27 year old vehicle running.

You can either 1. shop it around to other mechanics for diagnosis, or 2. go through Section 7.3 (Mechanical/Electronically Controlled Gasoline Injection System (CIS-E)) of the engine manual and systematically go through the testing procedures as described in pages 1-167.

I wish there was just one simple solution, but it's a pretty complicated system. See p. 7.3-005/1 to see some of all the variables involved. And there are several different systems that these cars used over the years. The 560SL has the KE Jetronic system. This thread also goes through some of the possibilities:

http://www.benzworld.org/forums/r-c107-sl-slc-class/1695262-87-560sl-ke-jetronic-fuel-system.html
 
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