S350 Blutec vibrating/rough idle

[S350L]

Hi everyone

Hope you're all well

I have a 2011 S350 Bluetec, which started running very very rough and vibrating and 1/2 a mile from home last night. I put it in neutral and rolled for about 100yds and the issue was still there. I parked up, put it in park, but the vibration was still there.
It sounds like it's coming from the front end and seems to ease slightly when the accelerator is pressed.
I thought it could be a misfire, but wouldn't that get worse with more revs.

Any ideas please?


Thanks as always

 

[whitenemesis]

Engine and/or gearbox mounts

 

[DSM10000]

Engine and/or gearbox mounts

As above.

They would be the first thing I would check although they tend to deteriorate gradually although one could just collapse.

Is the idle rpm stable and at its normal value (usually around 700 rpm)?

 

[S350L]

Thank you for your replies

The idle is stable and seems norma (other than the horrendous vibration)

 

[DSM10000]

Thank you for your replies

The idle is stable and seems norma (other than the horrendous vibration)

Can you have someone rev the engine whilst you look at how much the engine twists?

 

[S350L]

I'll ask my Mrs to do it in the morning. I take it I'm looking for some movement? How much should I expect?

 

[MikeInWimbledon]

They’re asking you the right questions.

It would help: to know what mileage on the car and have you done anything odd with it recently? (Used it cross country SUV style, or run it low on fuel, or stressed / raced the engine?)

When was the car last serviced and MoT’d ?

 

[S350L]

Mileage is 149k. Was last serviced (B) and MOTd in Feb. No I have done anything g out of the ordinary and yes, it has been low on fuel a few times in recent weeks.

Just had the RAC out, he didn't put his machine on as he felt the engine was OK. He thought the engine mounts looked OK as hardly no movement. He said, it could be gearbox mounts or even a loose heat shield.
The later I'd find strange as the vibration is a very solid one and not a tinny or metallic one

Does gearbox mounts sound plausible?

Ta

 

[DSM10000]

Gearbox mount is very plausible and especially at that mileage.

I had the gearbox mount on mine replaced followed shortly by the engine mounts at 154k

The gearbox mount is very easy to replace, 4 bolts from memory and was circa £40 from MB.

My engine mounts looked OK and there was no excessive movement but they had collapsed enough to transmit vibration through the car.

 

[MikeInWimbledon]

Hard to diagnose this from a distance, especially if an RAC man has just been in and said he can't see the problem

Mounts are likely, but I'll just chew the fat on cr@p in the fuel line.

It's a problem that often occurs in both 3 litre and 2 litre petrol 3 series quite early on (from about 50k miles onward. Poor quality Johnny Foreigner or British 95 octane fuel, and muck at the bottom of the petrol tank can give misfires. more typically under load. Usually a bit of fuel line cleaner, a small bottle added to a tank of fuel, can tidy things up without pulling the fuel line apart.

I'm wondering whether our other experts on the 6 cylinder petrol engine and fuel line can weigh in here.

(I've never, ever, had this problem with all the MB V8's, but then I've never driven one past 70k miles so I don't know how they age and coke up.)

But the misfires I'm talking about are less than smooth acceleration under load, the not thumping and clunking of engines and gearboxes moving around more than they should.

Just thought I'd put it out there. Don't hesitate to shoot it down if I'm barking up the wrong tree.


Mike

 

[W1ghty]

At that mileage , you need new engine and gearbox mounts anyway , mine made a noticeable difference at 100k when I changed them .

 

[S350L]

Changed the the rear gearbox mount myself yesterday and made no difference. The old mount had a little bit of play in it, so im glad I did that anyway.

Regarding the fuel line, mine is a diesel, would the same school of thought apply?