rough idle on 91 560 SEL
 

my car started to have rough idle a couple of days ago. I searched this site and have removed the air cleaner housing and inspected all vacuum lines that I could see, but could not find any that is disconnected nor any that would appear to be broken.

Let me be more specific, perhaps it will narrow down my symptoms:
1. rough at idle
2. appears to run normal as it approaches about 40 mph.
3. Trans shifts normally
4. gauges appear to function normally ( idle at 600rmp, oil at about max during idle and in gear)
5. starts normally.

thx in advance for your help.

emmy
91 560 sel

DO A SEARCH FOR ROUGH IDLE 560SEL - This subject has been discussed in depth many many times. You will find the cure with your search.

thx for the input. I have actually searched and read all the posts, and I have the following as my plan to inspect:
1. idle control valve
2. idle air valve
3. Fuel injector nozzles(holders, hoses, guides)
4. recheck all vaccuum lines

I would appreciate if there are more tips/guidance from the experts.

thx again,
emmy

You missed my thread on the fuel distributor screens getting partially plugged from rust from a rusted fuel line. After 1 year or more of a miss at idle - new fuel lines and cleaning the screens to the injector lines cured my miss at idle. Just to be clear - my miss was not major - just annoying to see engine move and feel the random miss. Pick up and acceleration was perfect.

thx for the reminder.

Would you know where the idle control valve and the idle air valve are located at in a 91 560 SEL? not sure if these are one and the same..

thx

Hi emmydotnet,

I think they are the same and it is located in front of the distributor under the air filter housing on a 87 420:

Good luck

Reinhard Kreutzer

It's the longish cylinder with the rubber hoses at the top and side and the connector at the other end. When in idle, pulling of the electric connector should increase the idle speed. Otherwise, it means the valve is clogged.

thx for the input.

OK, so when I disconnected the idle control valve, the idle went up, but still rough idle. does this mean that there is nothing wrong with the idle control valve?

I also noticed that the air intake into the carb ( about 5 in diameter) has a little bit of an opening (about 1/8" maybe a little smaller) after start-up with engine still cold. Is this normal? or does this indicate some vacuum leak somewhere?

Well Emmy,

it means the idle control valve is not stuck. Cleaning still might help a bit.

I would start with an injector cleaner to add to the fuel and see what that does to the idle. It's a rather complex system and this is the cheapest start.

Good luck

Reinhard Kreutzer

i have not considered the plugs, plug wires and dist cap since it runs smoothly at about 40mph and up. good point though. i will do that.

I have also read some posts on spraying carb cleaner over specific vacuum line to troubleshoot hard to see broken lines. Any comment on this method?

with the other recommendation, i found a local vendor for the injector cleaner BG 44K. Sounds like a lot of people are happy with this additive - although pricey at $17 a can. I will try this today.

I added the BG 44K to my full tank, drove it hard for about 40 miles and bingo! the rough idle was gone. I started to notice some improvement after about 20 miles with only random miss at idle. The idle was smooth after about 40 miles. This stuff is amazing...it works! I will observe for the next few days to see if it holds up...

Reinhard Kreutzer, thanks for the tip.

Been there and done that - same results - temporarily. If problem returns after about 500 miles-- more cleaner - smooth idle - problem returns--- if this happens to you - suspect rust inside fuel lines clogging screens in fuel distributor. Hope you are right and I am wrong.

Hi emmydotnet,

I pass that "thank you" on to all the others who have mentioned it before...

You are also to change the oil after this treatment to get rid of whatever made it into the oil...

Regards

Reinhard Kreutzer

i need to update my situation. after the full tank with the BG 44K was consumed, the rough idle came back. in addition, the engine now dies randomly at any speed. It would however recover by itself, sometimes, but other times I would have to restart it. It does restart all the time. now i am back to square one with the rough idle, and the engine cutting out randomly in addition. Fuel pump relay, and injectors, and rusted fuel lines seem to be the well discussed causes in this forum.

The fact that the idle improved when i used BG 44K tells me that it is injector related. The fact that the rough idle came back tells me that something is making my injectors go bad. Could it also be ignition related?

The engine cutting off/ and recovering tells me that somehow the fuel pump relay is defective or the signal triggering the relay is failing in ramdom. I recall one post mentioning about some other device that triggers the relay to let it know that it should turn on to start the fuel pumps. any ideas on this?

Any related leads would be appreciated.

moedip: how difficult was it to change the fuel lines? How many are there? Can you order all that is necessary from fastlane? Thanks!