Anyone experienced a lemon water pump
 

No, not a soda stream, a new water pump that fails in short order.

Been getting an intermittent sound from the front of the engine. From inside the cabin, this sound is faintly noticeable at startup. I'd describe it as a high pitched jingle bell. It usually abates after 4-5mins of idling/slow driving. Much more noticeable from the front of the engine, where it sounds more like a tin can with a pebble in it. Because of the timing, I had suspected an alternator bearing, but managed to pinpoint the sound to around the water pump.

Thing is, the water pump is a brand new, MBZ unit I installed last September.
It's probably seen about 15k miles since?

Anyone else ever experienced a water pump failing in such short order?
Is there something else I might be missing?

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Anything can fail, it doesn't matter who made it. In the manufacturing industry it's called "Infant Mortality Syndrome" meaning there's a bathtub curve associated with an increased failure rate when new, relatively steady low failure rate during normal service life, then an increased failure rate when aged out.

To confirm it's the pump, drop the belts. If the noise stops, you know it's probably something in the auxiliary drive system. If it doesn't stop, you could have an issue with the vacuum pump, idler sprocket, chain tensioner, or timing chain.

Copy that -will attempt this afternoon.
Is there any protocol for running the engine with the alt belt removed?
Duration, etc?

So long as you turn off electrical loads like the blower and stereo, you can run a 240D forever with the alternator belt removed. No electrical power required for them to run. With the fan belt off, keep your running to a few minutes at most. You won't have any water circulation, so you don't want to overheat the engine.

Anyone experienced a lemon water pump
 

I had similar noises.

I removed all of the drive belts but didn’t need to run the engine - the cooling fan/water pump was wobbly.

Every brand of water pump fails eventually and none of them have 100% defect-free production. I get the reason to buy MB/OEM but for some parts I buy at a local parts chain so I can get a warranty replacement ASAP. $0.02


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Great test, thanks gents.

Appears to be the water pump indeed...
Now here's a fun one: has anyone developed a method for draining the block without spraying the starter and losing all that brand new coolant?

Put a pan under the front and take out the pump bolts.


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Double check the fan, A/C belt and engine mounts. If the fan wobbles, the A/C belt is stretched or the mounts have collapsed, the fan blades can contact the shroud which will give you the "jingle bell" sound.

Here's a cool way to drain without losing much: funnel, hose, jug; remove lowest bolt from water pump, position funnel below pump, then remove highest bolt. Pours out from the lowest bolt hole like a water cooler.

Side question: why are these impellers cast iron? It's been on the block six months and it's already rusted.


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Anyone experienced a lemon water pump
 

I thought about that as well and thought that just perhaps the thickness/depth of the c.i. impeller allows it to sit in the water pump mount in such a way that when the bearing fails it limits how off-axis the shaft can get, and therefore the seal stays intact.

Pure conjecture, however.

As for corrosion, it shouldn’t happen with correct ratio/type coolant.


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Yes. On my 300d the po had the water pump changed at the Mercedes dealer 25,000 miles before I got the car. It literally was wobbling and dropping bearings when I first got it home. I couldn’t believe it when I looked at his receipt for $600 to replace it just a couple years prior to it blowing up.

I thought the car was ready to throw a rod when I got it into my garage. I was really happy it was just a pump. That was an MB pump form the dealership. Unbelievable!

Thanks for sharing that. I will show it to my wife the next time she complains about "all those tools" that I own.

Wow, okay.
I'm happy to get another data point.
It seemed like such a simple and robust little part... really surprised me.


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$600 - what a rip. I replaced the water pump in my '81 300SD about three or four years ago and it took at most about one hour. Maybe less. Really easy job.

Water Pump Diagnosis and Failures
 

Most quick water pump failures can be directly traced back to improper fan belt tension .

When running any engine sans fans belts to ascertain noises, be SURE to turn the heater FULL ON as hot spots can occur very quickly and spoil your day .

Glad it wasn't a vacuum pump failing .