Rough starup Idle
 

Hi Guys Been a while But glad to be back.

My 1986 560 SL, 160K Had her for 17 years always ran great Developed a bit of a rough start. Vacuum just off full left, will stumble a bit on acceleration.

If left to warmup for 5 min vacuum goes full left and has just a bit of hesitation going. Once going, she run as she should. Did a smoke test did not see anything.

But when i stop and park and come back out to restart, it may do the same thing again. Or worse if I don't let run a bit. After a while, 30 sec and then vacuum needle back to left. Plan to redo the smoke test, Where is the best insertion location for smoke. Pressure better than just blowing?

Also Want to do a vacuum test. Where is the best place to connect Vacuum Gauge?

Your thought appreciated.

Jack in CA

Rough Idle
 

Adding a bit of information, I found a disconnected plastic tube, It was one of the two that runs between the spark plug wires (#2 & 3) on the passenger side. Connects to something under the coolant reservoir.

The engine idle is about 900rpm. Jack:confused:

While I am no expert on the 560SL, from what you describe as a hesitation upon acceleration, it sounds like because of your vacuum leak, the vacuum advance on your distributor is not providing a timing advance when accelerating, therefore, resulting in a hesitation.

Hi So this hesitation would happen at any time you would accelerate? It could be at a stop light or driving at 40 mpg? Sort of like if you were running on gravel and your firs step your shoe would slide?

OK So I have a vacuum Gauge. Where exactly and how do I connect it to the engine to determine what I have. I assume that I will need to find a timing light to check this out? Reminds me of High School Auto shop. jack

had an accumulator cause a rough idle and slight hesitation on a 450sl once. everything was new and/or working correctly, including the rest pressure test. pulled the return line and a dribble of fuel came out. replaced it and no more rough idle. never has an accumulator do that before or since. just a thought, chuck.

Well I do Have an new accumulator on my parts shelf. I will take it with me when I go to have a MB tech look at it. Know anything about smoke testing? Jack

if your going to have a shop look at it, he should have a smoke machine. otherwise, find another shop.

btw, that accum. was the absolute last thing looked at on that car. no vacuum leaks, perfect ignition scope and fuel pressure tests. it was a what the hell, what's left moment. good luck, chuck.

OK I appreciate that. I just put it on the car. I checked the Idle control valve by disconnecting it and reconnecting it Have not done the voltage check, It does what it is supposed to do, I guess I could take it apart and clean it as has been the suggested on some posts. Thanks

HI All

Tested the Idle control Valve, 4.65 volts, Gave it another smoke test. No smoke that I could see.

If I feather the gas pedal several times quickly it will shudder and if I do it about 6 times it will die.
Still about 900 rpm at idle. vacuum all the way to left at idle. Still looking for where to connect my real vacuum gauge. Thanks, J

Jack M, when was the Oxygen sensor replaced last? I have experienced a delay in acceleration in our 1991 300CE that a new Oxygen sensor resolved. As you likely have read in any Searches there are many possible causes for an acceleration delay. Better to diagnose the true cause rather than throwing parts at the problem.

HI Fred. So the O2 sensor light on the instrument cluster is not on. Does that mean anything? J

Jack M, there was no warning light on the instrument panel with our 1991 300CE either. In an attempt to resolve the delay in acceleration I simply replaced the Oxygen sensor, and the problem was solved. If your Oxygen sensor has not been replaced in 100,000 miles it makes sense to install a replacement sensor … preventative maintenance.

HI Fred. There is an O2 light on the instrmuent cluster. only comes on when you start the car, So it is is bad then would the light not stay on?

Jack M, the Oxygen sensor warning light will illuminate under certain circumstances. I am passing along my experience with our 1991 300CE that had no Oxygen sensor warning light; however, replacing the Oxygen sensor resolved the acceleration hesitation. A local indie suggested that a new Oxygen sensor "may resolve the problem", but he didn't want to charge me for a replacement and not be successful resolving the problem. Since I am a DIY'er I replaced the Oxygen sensor myself. Not difficult, but you need the crow's foot tool with the slot for the sensor wire. Obviously, the best situation for you would be to temporarily install a known good Oxygen sensor and see if that resolves your problem.