three years rough idle sl320 r129 v6 m112

Page may contain affiliate links. Please see terms for details.
Poor idle

Just a thought have you checked inlet manifold gasket as this would cause varied combustion cylinder temps and poor idle but once over 1k revs no effect on engine performance would have thought "star" would have picked this up hope this helps
 
Several things that can go wrong with the propshaft. If the centre bearing has ever been renewed its possible the flexible mounting might be slightly misaligned where it attaches to the body. The prop shaft has to be split to renew the bearing but its important its reassembled in the same way as it was found as AFAIK its balanced as an entire assembly. The propshaft itself often has balance weights welded to the shaft- sometimes these can come off. The doughnuts can have bolts with either balance nuts 1.5,3,5 gms for M10 bolts or 8,10,12 gms washers for M12 bolts that can be added to suppress vibration. Not saying this is your problem but vague but discernable symptoms are often down to incorrect assembly or substitution of genuine parts and while perfectly safe may display annoying symptoms like this.
One example of this I had many years ago was a Macpherson strut car pulling constantly slightly to the left. Tracking was checked--- it was OK The mystery was finally resolved by loosing the strut top mounting bolts with the car's weight sitting on its wheels and allowed to settle. Bolts retorqued ----- tracking perfect.
 
Last edited:
I know you are a long way but if you got the car to me I would be happy to get to the bottom of your problem for you.
There must be a sign in your values!!!!


hi
Thank you but I am in Cork in Ireland, What does "There must be a sign in your values" mean? Do you think it is something in particular?
Any advice please thanks
Darragh
 
Just a thought have you checked inlet manifold gasket as this would cause varied combustion cylinder temps and poor idle but once over 1k revs no effect on engine performance would have thought "star" would have picked this up hope this helps

hi
Are you talking about the valve cover gaskets? I did them and everything is sealed up properly.
If the inlet manifold gasket is the butterfly valve, I removed and cleaned it and replaced its gasket,
Thanks
Darragh
 
Several things that can go wrong with the propshaft. If the centre bearing has ever been renewed its possible the flexible mounting might be slightly misaligned where it attaches to the body. The prop shaft has to be split to renew the bearing but its important its reassembled in the same way as it was found as AFAIK its balanced as an entire assembly. The propshaft itself often has balance weights welded to the shaft- sometimes these can come off. The doughnuts can have bolts with either balance nuts 1.5,3,5 gms for M10 bolts or 8,10,12 gms washers for M12 bolts that can be added to suppress vibration. Not saying this is your problem but vague but discernable symptoms are often down to incorrect assembly or substitution of genuine parts and while perfectly safe may display annoying symptoms like this.
One example of this I had many years ago was a Macpherson strut car pulling constantly slightly to the left. Tracking was checked--- it was OK The mystery was finally resolved by loosing the strut top mounting bolts with the car's weight sitting on its wheels and allowed to settle. Bolts retorqued ----- tracking perfect.


Hi
Thank you for the information
This centre bearing has not been replaced in the life of the car. I'll have to talk to my mechanic about the prop shaft.Another mechanic was suggesting bad torque convertor but he lost interest and let me down.
It has been confirmed by many that the mounts are not the issue but I wonder if I undid the engine mounts and re tightened them might something "settle"?

Thanks
Darragh
 
Last edited:
I think some back to basics have to be carried out here..

- Proper traditional compression test will reveal any valve issues.
- fuel pressure needs to be checked
- fuel injectors tested
 
I think some back to basics have to be carried out here..

- Proper traditional compression test will reveal any valve issues.
- fuel pressure needs to be checked
- fuel injectors tested

Hi
This vibration hasn't improved nor worsened ever. I had air? cylinder hot and cold proper compression test done.
Fuel filter was replaced.
Nothing related to fuel came up on the various star machines I have employed.
Re- fuel pressure - is that a diy test or will I have to pay the man?
Also I intend looking at fuel injectors - what would be the origin of fuel injector problem...this car has always been taken care of...
I know also that parts just die too....When I bought the car at 65000miles it had a slight and sporadic 'loose' shake. One mechanic suggested mounts would fix the problem. They were installed (and have been checked several times by other mechanics). When they were installed the vibration became 'tighter' and harder. Since they were installed at 78000miles the vibration has not softened or loosened but has stayed the same and the car now has 137000miles.
I want to win this one....(I lost to a rusty 107 years ago and pride and love drive me on)
Tell me more, thanks for your response,
Darragh
 
Last edited:
What rpm does it idle at and are you sure it is running on all 6 at idle?
 
Sp!ke,
I am sure it is running on all six at idle... as per various STAR diagnostics...If I unplug a coil then I know what running on 5 cylinders is..
Rpms are correct and as they should be.
Throttle body clean and its gasket replaced, air filters are good and nothing inhibiting intake.
Keep suggesting please,
Darragh
 
I'm a little confused by this. If you have a rough idle causing the car to shake then surely its either idling too low or its missing.

If its not idling too low and its not missing and the engine mounts have been replaced then what's left that hasn't been covered?

At this point I'm thinking the car needs a fresh set of eyes and diagnosis. Where in the country are you? Perhaps we can recommend someone to take it to.
 
hi
Thank you but I am in Cork in Ireland, What does "There must be a sign in your values" mean? Do you think it is something in particular?
Any advice please thanks
Darragh

What I meant is that reading live data with the engine idling must show something amiss. If its not drivetrain related then star must give some clue somewhere!
I remember there was a welsh guy on here with an M112 in a W202 C class. He had exactly the same issue as you do and tried everything. Apparently a darned good thrashing/ long run cured it.
 
I am beginning to suspect you may simply be experiencing the inherent imbalance of a 90 degree V6 engine. Mercedes had to implement a complex crankshaft with 30 degree offset split crank pins to achieve an even 120 degree firing on this engine. Together with a contra rotating twin offset balance shaft and harmonic balancer this was still not sufficient to make it as smooth as M104 straight six at idle . One thing to try is to get a run in another M112 engined SL for comparison?
piccy of 30 degree offset crank
113_0602_11_z+aluminum_buick_v6+crankshaft.jpg
 
What I meant is that reading live data with the engine idling must show something amiss. If its not drivetrain related then star must give some clue somewhere!
I remember there was a welsh guy on here with an M112 in a W202 C class. He had exactly the same issue as you do and tried everything. Apparently a darned good thrashing/ long run cured it.

Hi
I swear, nothing ever shows up on the star machine...absolutely no codes seriously.
I was advised that before and I tried a damn good thrashing but it did not work.
thanks though
Darragh
 
Last edited:
I am beginning to suspect you may simply be experiencing the inherent imbalance of a 90 degree V6 engine. Mercedes had to implement a complex crankshaft with 30 degree offset split crank pins to achieve an even 120 degree firing on this engine. Together with a contra rotating twin offset balance shaft and harmonic balancer this was still not sufficient to make it as smooth as M104 straight six at idle . One thing to try is to get a run in another M112 engined SL for comparison?
piccy of 30 degree offset crank
113_0602_11_z+aluminum_buick_v6+crankshaft.jpg
Hi,
One mechanic showed me the exact same symptoms in the same engine which was in another model I forget which. In engine bay it was exactly the same but virtually unnoticeable from inside the car, probably due to the bigger chassis....
Thank you
Darragh
 
this is an interesting article which shows quite graphically the differences between the torque characteristics of a straight 6/60degree V6 / 90degree V6 with offset crankpins and the torque characteristic of a [ non compensated] 90degree V6 with common plane adjacent crank pins-- not a pretty sight!
ET-X-6%20Cyl.gif


ET-X-6%20Cyl-odd.gif


from this article

Torsional Characteristics of Piston Engine Output, by EPI Inc.


ps you may also be interested to know that Mercedes are developing a new range of in line 3,4, 6 cylinder engines which will first see the light of day in the 2016 E class and 2017 C class. Despite the possible requirement of more main bearings there are less parasitic loads on these engines if they don't require power sapping balance shafts and twin cam shafts [ perhaps air modulated valve trains a-la Fiat / Alfa Romeo are on the cards?]

shown here also

http://www.autozine.org/technical_school/engine/smooth3.htm#V6
 
Last edited:
Hi
I swear, nothing ever shows up on the star machine...absolutely no codes seriously.
I was advised that before and I tried a damn good thrashing but it did not work.
thanks though
Darragh

They don't often throw codes for a minor misfire. The live data is you need to look.

As has been said before. It is very normal for them not to be completely smooth at idle.
 
found this short video- is your car anything like this? -its actually a M112 in a C203 [YOUTUBE]Gq8cQZZ6bCI[/YOUTUBE]
 
That doesn't look promising... I can see that this vibration would likely transmit through to the cabin... even with all the mounts being healthy.
 

Users who are viewing this thread

Back
Top Bottom