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  #1  
Old 10-12-2009, 08:16 PM
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Question 300SE Coolant Leak

Hey everyone. Odd situation and before I take it to the shop I wanted to see if there were any ideas in regard to the cause of my coolant leak.

Car is a 91 300SE with about 65,000 miles. I warm it up from time to time and last time I did, the low coolant light was on. I looked, and sure enough it was a little low on coolant (which it never is).

I pull it out of the garage and there's a stream of coolant (looked like water) left on the driveway. The missing coolant had been collecting in the plastic under-the-car-engine-bay-protector cover thingamajig (under engine) and had not dripped on the floor which is why I had never noticed a loss of coolant until I actually moved the car. No puddle until driven.

So, I refilled the reservoir and marked it with a pencil. I drove around for an hour or so (car runs very well) and when I get back the reservoir is about an inch to an inch and 1/2 low. So, its losing coolant somewhere.

I don't smell burning coolant, I checked the oil and (thankfully so far) nothing odd is in there. Coolant is also good looking with no oil or funkiness either. I checked all the obvious places for leaks, radiator, hoses - nothing. Its coming out someplace at a decent rate, but I can't see where.

So, where can I look? Any ideas where this leak is coming from? Is this a common kind of thing with the M104 or is my car a freak? I'd like to keep her, but stuff like this gets me nervous.

Thanks.

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Old 10-12-2009, 08:42 PM
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OK, Probably doing this bass-ackwards, but I found an old post where someone had a similar problem and it turned out to be the water pump going bad. Sound likely?

So, is the waterpump visible? How does one know the water-pump is going south? Would I be able to see coolant coming from it or is it buried in the engine bay someplace?

I'd love to just verify and be able to go to the mechanic and say 'the waterpump needs replacement'. Thanks!
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Old 10-12-2009, 09:02 PM
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1993 300E 2.8- M104
 
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if you see coolant on the bottom of your oil pan, then it is most likely your water pump....

check the visible hoses in the front of the engine bay, then check the hose at the rear of the engine bay (that goes to the heater core in the firewall). if you don't see anything on all those then it is either your radiator, or your water pump. (it could be more serious, but lets assume it is not for now). most likely it is your water pump since you say coolant puddles on the plastic shield below the engine...

not a serious job to change the pump, but very crammed and annoying without the right tools....

good luck
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Old 10-12-2009, 09:15 PM
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Thank you. I did the best I could, nothing in the front end looks to be leaking, but I'll try to look @ those hoses in the rear that you mention. Assuming it is the water pump, are there any other wear items that should be replaced while everything is apart?

An indy MB mechanic would be doing the job. Thanks again!
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Old 10-12-2009, 09:20 PM
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1993 300E 2.8- M104
 
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new hoses (obviously!), thermostat, radiator pressure cap ...coolant flush and new mercedes or Zerex-g05 coolant. No other coolant brand is recommended only those two. if you go to a mercedes tech, he will probably use the right stuff anyway....

my only concern is that your car is low on miles for a mercedes water pump to go bad, but it happens...

maybe other more experienced members will chime in on this one ,,,,,

good luck

edit: when looking for leaks, do that when the engine is on (system is pressurized ), that will make leaks more obvious
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Old 10-12-2009, 09:26 PM
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Thanks again. Yep, when I got it (2005) I flushed it and refilled w/ G-05 so I know at least that's correct! I've had water pumps go on other cars and I always remember grinding or gurgling sounds. None of that here.

Yeah, its got low mileage. Like I said - a little nervous.
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Old 10-12-2009, 09:32 PM
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its probably simple, not much can go wrong with these engines, and you don't have milky oil or oily coolant so don't worry, at the rate of coolant lose you mentioned, it is definitely going outside the engine somewhere and not inside ....(i think you have an m103 by the way)

see what the mechanic says, but i bet it is either the water pump seal, or a hidden radiator leak somewhere,,,,
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Old 10-12-2009, 09:45 PM
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M103 + W126 = my 300SE! I'm hoping that its nothing too bad. I know it will cost a bit but this car looks showroom new. I don't see any on the road around here that look so good.

I just like going to the mechanic with some idea of the issue, and if I can verify myself (with forum help) all the better. My mechanic is pretty good but still...

Thanks!
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Old 10-13-2009, 07:55 PM
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OK, well I took a look today in the light - its hard to see but sure enough under pressure I can see a good drip - drip - drip coming from the water pump. Looks like we found it!

Thanks for the assist latief!
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Old 10-14-2009, 07:17 AM
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Here's probably what you will find.

Changing the water pump is a DIY job if you are handy with the tools. Just takes a little time. You can do a search and you will find others have done this.

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Old 10-14-2009, 10:02 AM
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1993 300E 2.8- M104
 
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its a good thing you found it!!!! at least now you know your engine is ok!!

change that thing out fast........you don't want to overheat for sure ...

good luck !!!

Edit: Mine was leaking from the rubber seal between the block and the pump, not from the weep hole ......interesting!
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Old 10-14-2009, 04:33 PM
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Well, I can't see where its leaking from, I can just tell its coming from that general area. Perhaps its just loose? No weird sounds or anything, but I guess its on its way out. If its leaking like yours was, is it really the water pump or the gasket that is bad?
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Old 10-14-2009, 04:36 PM
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Quote:
Originally Posted by nj300se View Post
Well, I can't see where its leaking from, I can just tell its coming from that general area. Perhaps its just loose? No weird sounds or anything, but I guess its on its way out. If its leaking like yours was, is it really the water pump or the gasket that is bad?
Either way, it is either the gasket or the pump. once you are in that cramped space, you will want to change them both because the last thing you need is to do the job again another couple of months....

a new pump will come with new gaskets/seals........change the whole thing out for the peace of mind
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Old 10-15-2009, 06:12 AM
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nj300se, try tightening the hose connections on the water pump. It's alsoe possible that a coolant hose has cracked and is leaking. Normally a water pump seal wouldn't suddenly develop a leak that drips steadily. When a water pump seal starts to fail it will weep a bit, but the engine heat evaporates the coolant and leaves a white residue. Use a flashlight and a mirror to inspect the weep hole (that's what the arrow points to in the picture of Ivanerrol's post) closely for evidence of a coolant leak. The last thing you want to do is replace a good water pump. As mentioned, the job is a real PITA.

Another possibility is the thermostat housing. Some of them were black plastic which crack and leak. If yours is plastic replace it with a metal housing. While you're at it may as well replace the thermostat and gasket too.
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Old 10-15-2009, 12:54 PM
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Thanks again guys! I'll try getting in there with a mirror or something to see if I can pinpoint the leak. Would a water pump that is going bad make noise? Its hard to hear over the motor, but it doesn't sound like anything is wrong - but for the leak.

So if there is no evidence of a leak from the weep hole, what does that mean? Loose or cracked hose?

Do have the plastic housing on the thermostat, but no leaks from that area @ all.

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