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Stan, Automotive Engineer
Category: UK Car
Satisfied Customers: 3526
Experience:  Bachelor's in Automotive Engineering, MB certified diagnosis technician
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StanI have an s320cdi , 118k miles. Ican hear a rumbling

Customer Question

Hi Stan I have an s320cdi , 118k miles. Ican hear a rumbling from below as if the exhaust is knocking and the car will make a whooshing noise. Its been in to Mercedes but they could nt find any faults. I thought it would be a blocked dpf because this was found on the codes about a year a ago when it kept going into limp mode- I have found the car will only run - just- on Shell nitro diesel

The car will run rough if I put normal diesel in and often will go into limp mode

Submitted: 2 years ago.
Category: UK Car
Expert:  Stan replied 2 years ago.

It is possible that DPF or resonators/silencers have disintegrated and the loose parts inside the exhaust make the noises. It would also cause lack of power due to blockage of exhaust gases.
One thing you can do, is to hold your hand on the tailpipe- there should be a noticeable pulsating airflow. If there is barely any- exhaust is blocked.

Otherwise the complete exhaust should be removed and disassembled to see which elements have blockage or disintegration.

Some install the test pipe and do a DPF delete software update to gain more power.

Let me know how i can help you more on this,

Customer: replied 2 years ago.


I will check the pressure with my hand, if there is no pulsating what do you suggest?

Expert:  Stan replied 2 years ago.
Exhaust will have to be disassembled to see where the loose parts are and what is blocked.
It might be that DPF is intact, but other exhaust components are faulty.
Customer: replied 2 years ago.
Hi Stan, this may sound strange, but I don't think it is a coincidence. I noticed the steering fluid was low. So I topped it up and the car ran a lot better. Done this three times now and it runs better afterwards. It seems there is a leak, not huge but def. needs topping up every couple of weeks.
Is there any chance that strain on the pump and then the belt is causing the rough running?
Expert:  Stan replied 2 years ago.

I would do a test drive with v-belt removed. If the battery is fully charged, it will be possible to run the engine for at least 30 minutes. It should be enough to see if this affects the engine running.
Of course, fault codes will have to be cleared.

Rough running can be caused by the alternator pulley. It is a freewheel design and rotates freely in one direction to reduce engine vibrations.
Steering pump seizure is rather unusual comparing to alternator pulley's failure.

Insufficient oil in steering pump will cause abnormal noise. But it usually does not affect engine performance

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