W463 with OM642 V6 Diesel 2008

[Woodward]

Hi to all who make this forum so useful....
The engine runs perfect and has now got 100k miles on it with a full and proper MB service history.
Recently, it has started making a tapping noise, specifically and only under engine load and ly when the engine temperature hits normal running temp.
The single tapping noise comes in at approx 1500 rpm and rises with engine speed up to about 2300 rpm and can be heard very clearly with the window down but only when driving.
There is no noise at tickover or when simply revving the engine, only when driving...
When starting from cold I watch the temperature gauge as it moves up to around 90c until this point is reached the engine sounds like new and is very smooth, it's as though once and only when the engine operating temperature is reached something turns on the tapping noise.
Also when the foot is off the accelerator and coming to a stop there is no tapping noise, all is quiet as expected.
Doesn't sound like exhaust leakage or injector chuffing.
Fuel consumption seems stable, no oil is used and no smoke at any point, engine itself is very clean and dry especially around the injectors and turbo.
It's baffling me - does anyone have any ideas for this.
Much appreciate any response....

 

[Woodward]

Tapping noise under load while driving

.

 

[Woodward]

......BUMP.....anyone with any ideas ?

....BUMP....

 

[kid-jensen]

Gearbox/Torque Converter causing it?

 

[umblecumbuz]

If it can only be heard when driving, then why do you conclude that it is probably engine-related and not some other rotating component?

Umble.

 

[Woodward]

Thanks for responding,

I have taken it to 2 MB dealers who have had the car for a day or so and conclude that it is tapping from the "engine" but are baffled for why it only occurs when the engine temperature reaches operating temp at 90c.


When driving and you open the ns window, the noise will emanate from this side and seems to come from the vehicle front, other than that, that's why I'm here to hopefully get some pointers as I don't know much more....but want to get it sorted out.

 

[Woodward]

Gearbox/Torque Converter causing it?

Hi PaulG, would the TC or gearbox start to make the tapping noise once the engine temp reaches operating temp ?
So far both units are running perfect in my opinion, no issues, in fact I had the gearbox oils and filter serviced at MB 4 months ago. It runs really smooth in this regard.
Thanks

 

[kid-jensen]

The Torque converter geta a hard life from the point of view of temperature and pressure, so it's possible that when it expands, something is running out of clearance... it's all guesswork without hearing it, or better taking it apart and looking, but that's a bit drastic.

Realisticly, you're not going to rebuild the Automatic on the off-chance of a problem being inside. You'd just live with it. With the transmission oil changed, you've exhausted the "cheap" options of fixing it, only really expensive ones remain.

Still think it's unlikely to be the engine if it only happens when you're moving....

 

[OlafMaxwell]

Worn mountings could cause loss of clearances of some moving parts with the body or allow exhaust/prop shaft contact. Stopping when foot off accelerator can also point to this, acceleration causes a torque reaction that's significant in a rear wheel drive. 100k miles would have me looking at the mountings.

This can also happen on left or right steering lock, again due to mountings.

Have a listen and see does it change as the transmission changes gear and post again.

But thinking of other areas what of the manifold...? I don't know the V6 diesel engine well but I have noted some issues with actuators/butterflies?

 

[LostKiwi]

I'd go for a slightly loose or leaking exhaust manifold.

 

[turbopete]

I'd go for a slightly loose or leaking exhaust manifold.

^^^ I was thinking exactly the same ^^^

 

[Woodward]

Thanks for your contributions, I hope to update you on any progress over the weekend.

 

[Woodward]

****UPDATE*****

Well it's now all sorted.

As stated in my opening post, this unit is well cared for, properly serviced and very clean - no leaks of any sort so it was a concern when it started to tap under load.
The first visit was to a " well reviewed" MB centre who had it for 4 + hours and found no fault.
I admit to waiting at the MB dealer while they did this since I did not want it to become a stretched out matter where I had no car for a few days - you all know what happens.....
I profess to having some technical knowledge from my days at RR aero, i had a feeling it was injector related and clearly told them this, they plugged into STAR and advised me that the injectors were some way out, but where in good working order - they reset the injectors back to factory settings.
I took it for a test and noticed that it was running smoother- good, smoother gear changes - good, slight reducation in the noise, but still present - not good.

Took it back and was advised that now they thought it was the exhaust manifold blowing or even cracked on the inside ???
4 hours later, they could not find the fault and suggested it was time for the exhaust manifolds to be removed and the injectors to be taken out 1 by 1.
So, a few hundred pounds lighter and still not resolved, I asked if they have injector testing equipment on the premises, they answered no.
They made the next decision for me and I chose not to return to them.
Pretty surprised they could not diagnose the problem, who is teaching them ??

The slight noise improvement etc, reinforced my suspicion for the fuelling system, since the noise was more and more sounding like injection/combustion rattle
Onto a recommended Bosch service centre, they initially plugged it into their STAR kit and again saw that the injectors were within tolerance, but, did question why the MB dealer had reset the injectors back to factory levels on an engine and injector set that was at 100 K miles.

I decided there and then having seen a high level of customer care and specific injector knowledge to have them take a bank of 3 injectors out and fully test them.
They came out very easily with the original white MB grease still on the injector bodies ( it clearly works ) and I would recommend anyone removing injectors to purchase this same product from MB for the refit.

The 3 were tested and performed to specification with the exception of all of them leaking slightly at the nozzle tips
I thought if these 3 are nozzle tip leaking, it's highly likely the other 3 are.

As some of you will know, these Bosch injectors are the Piezo type and as such can not be serviced or repaired, I also learned that the injectors in the MB OM642 engine have a cycle of 5 injections per combustion cycle, correct me if I am wrong, but it's 3 pilot injections followed by 1 main injection, followed by 1 overrun injection ( happy to be corrected on this ).

The Piezo injectors in this engine are injecting 4 or 5 times more than injectors of old, I understand the pilot injections are to reduce combustion noise and rattle prior to the main injection, so rather than 1 big bang, 4/5 smaller ones to smooth out the combustion and flame propagation.
The Om642 should be a very quiet diesel engine if all is working well.

With my injectors leaking slightly and with logical thinking, thats possibly 5 slight leaks of diesel in to the combustion chamber per cylinder per combustion cycle - was this my rattle noise ?? when under load.

My injectors although tested okay, were slightly leaking, I decided to replace all 6, we tested the final 3 and they also had slight nozzle tip leak.

Took the car out, got it up to operating temperature and I can not put into words the increase in low down power, pick up in power, increase in power through all the rev range, the smoothness of the engine, also starting from cold running is much smoother.

Now, I thought prior to this, my engine was running nice and powerful, clearly not the case, these new injectors have totally transformed the engine.

I'm expecting fuel consumption to be improved, and I will advise in time, remember this is a 2500 Kg car that was giving 26.4 mpg, I was happy with that.

The injector rattle under load ? It's completely gone. Thanks For taking my hard earned money MB dealer.

Credit only due to the injector specialist, I won't publish their name here, but I'm happy to provide details by pm if required. TT

 

[Woodward]

***UPDATE***
After doing near 2,000 miles, the average fuel consumption is now at 27.8 mpg - happy with that.
Since my last post, I have also removed and cleaned the 2 MAF sensors, replaced the 2 air filters and fuel filter, this also seems to have allowed the engine to run smoother, the 7G gears now changes smoother and has what I consider to be a more suitable rpm before up shift.
The egr has also been removed and cleaned, albeit it was not really dirty....

Cheers.