Mercedes tapping noise???

[Frank]

The oil was changed with synthetics about 2500 miles ago.Today,I jacked it up and got under and it seems to be the catalytic converters which you can see a picture of at http://www.********************************************************************************************/images/catalytic/MB2256.jpg . It is the thing that has 2 ovals side by side.It stops rattling for a little when you tap it and then it starts again.

So,hopefully that's all it is.

[98Boxster98]

Check out MNBoxster's advice on the catalytic converter:

This is driving me nuts..

[Hoosier]

I've had E-class, M-class, and C-class Benz in the last 3 years. There is a ticking that is normal to all. The key to the ticking is whether it changes with the engine rpm. These cars have a catalyst reclaimer valve. When the engine gets hot there is extra catalytic heat that need to be expelled. This valve opens and closes when needed to release that heat. When it is open it ticks and it does sound like a valve ticking. When the engine cools is stops ticking but again the real thing to watch out for is when you hear it ticking rev up the engine. If the ticking stays the same speed it is the normal valve working correctly. If the ticking increases speed with the rpms get this car in to the shop...or at least tell your father.

[MNBoxster]

Hi,

Good advice so far. To add to the list, it could be a Hydraulic Lifter gone south. A sustained high speed run such as you describe can be a prime breeding ground for this issue, especially on a 100k+ mi. engine.

But, as mentioned, this would produce an RPM variable noise. Hope this helps...

Happy Motoring!... Jim'99

[Frank]

My father and I got the car up and he seems to agree that it is coming from the catalytic convertor and we can't find any loose bolts and the heat shield is secure.

Do you think there is a chance that over time it will work its way out or if we take that part off,could it be shaken out?

Thanks Everyone

[MNBoxster]

Originally Posted by Frank

My father and I got the car up and he seems to agree that it is coming from the catalytic convertor and we can't find any loose bolts and the heat shield is secure.

Do you think there is a chance that over time it will work its way out or if we take that part off,could it be shaken out?

Thanks Everyone

Hi,

If you believe it's a collapsed Matrix in the Cat, then it will probably work itself out the tailpipe with time, but it will diminish the Cat's operation. This could trigger a CEL (on an OBDII Car) or a failed emissions test...

Happy Motoring!... Jim'99

[986Jim]

Does the sound increase and decrease with RPM? You say it's between 1000-1300rpm. If you rev from 1000 to 1300 in neutral does the ticking get faster as the rpm gets higher?

You either spun a main or rod bearing through detonation because of using the wrong gas, or low or dirty oil situation. I highly doubt it's a manual head so you won't have lifters to adjust it will most likely be a hydraulic head. If so you could be low on oil and the lifters are not getting "pumped up" enough.

Is it worse better or no change with cold or hot motor? If its just on a cold motor then Mercedes could be using large ring gaps stock for a Kompressor motor which have noise when the motor is cold. Upon piston expansion the noise goes away as the gap tightens up. I have seen factory turbo/super cars use gaps as large as .0008 before which is stupid large then run with an equally large piston/wall gap.

Other things could be heat shields etc but they wont really change with RPM so much. What you need to discover is if it's mechnically linked to RPM if so then your into the motor for sure. Low octane on a Kompressor will cause detonation and make the motor spin a bearing. For your case I hope thats not it.

Just for educational purposes for those not knowing what a spun bearing is.. Around the crank where it lays in the block there are bearings. They are not ball bearings but just aluminum pieces (two) that go around the crank with a hole in the center like half moons on each side about 3/4" wide. Oil is pumped into the surface of the bearing and the crank floats on it.

When a bearing spins, it attaches it self to the crank beause of detonation where the crank is crashing into it all the time with a lot of force or low oil causing lots of heat and it fuses to the crank and the bearing spins around with the crank. The bearing spinning is now not allowing that oil to get throug the hole in the center which is where the crazy ticking noise comes from. Eventually it lockes up all together from friction and the rod flyes through the block. I have seen it in a few different Mercedes motors before.