190E squealing
 

My 190E has a nasty squeal whenever the car is running. I originally thought it was the belt tensioner pulley as it had bad bearings but surprisingly it did not fix the noise. I am almost certain the noise is concentrated right above the water pump. The water pump was replaced on it almost a year ago, however the MB brand coolant was not used to refill the system. Could not having used the proper coolant caused the water pump to make noise? Other than the noise, the engine runs at normal operating temperature.

no

If I remember correctly you have an idler pulley up there somewhere. Check it to see if it has gone bad.

I already replaced that

Does the noise go away when you remove the oil filler cap ?

What is the purpose of that?

But ill still try it and see

Some times, the engine builds up vacumm and causes air to be drawn in past the crank seals which can make a loud noise like screeching metal.

Mostly happens to BMW's and Asian engines with PCV's.

Personally, I'd check all of the pulleys for bearing wear and alignment. Although it's not likely, don't forget to check your harmonic balancer.

Mine had a nasty bird chirping noise weeks ago! I removed the valve cover to find that it was the plastic portion of the timing chain tensioner had worn and come off the metal part of the tensioner arm! Causing the chain to ride on the metal instead of the plastic! Causing metal on metal squealing noise.

Been there, done that
 

My squealing was the belt. A quick check is to pour some water on it while its running, it should stop for a few seconds until it dries out. A cheep fix is to buy some belt dressing. A permanent fix is to replace the belt.

Glad you found it.. :) .HOWEVER...do not use belt dressing....this stuff was designed for the old pto leather belts like those on thrashing machines or rip saw drive belts....on an automotive belt like Mercedes uses, it will cause heat due to increased slip. This can melt your plastic pulley wheels. It also bungs up the grooves with road dust and the belt will still slip and that ain't good. :(

DRY belt only. If it's making a noise you may have a pulley siezing or a leak from your p/s pump.

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I ran the engine with the belt off and no squeal, which limits it to belt driven stuff. I can pretty much rule out power steering, a/c and tensioner pulley as they are far enough away from the approximate source of the squealing. But like I said earlier I am almost 100% sure the source of the sound is directed dead center above the water pump. If in fact it is the water pump making the noise, does it mean it is bad and may go out at any time? (pump is only 1 year old)

A water pump is only $7 at the local pullapart so I will get one just in case.
In the mean time I'll try the easy stuff like pouring water on the belt..