Hey guys,


I noticed a little bit of coolant on my garage floor last night. My 300D is the only thing that's been parked in there in the past few days. I checked around the engine bay but didn't notice anything obvious. I know the previous owner had a new Behr radiator and a few other coolant components replaced, so I'm going to look at the records and see what exactly has been replaced.

Anyway, back when I had a Mustang that developed a coolant leak, I rented a coolant system pressure tester from Autozone. You'd screw this onto the coolant system via the cap, and pump pressure until your found your coolant squirting out.

Is this same type of tool useable on these cars? I feel like the only radiator cap type cap I've seen in my engine bay is the one on the coolant tank, but there is a little open ended hose that comes off of that, which I assumed was to drain over flow.

That would just leak the pressure, right? Should I just block it off? I've been reading and found plenty of people mentioning pressure testing these cars, but I cant find anything verifying how exactly to do it.

Any help is appreciated. Thanks!

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1984 Mercedes-Benz 300D

Did it get cold over night ?

They seem to leak at the 3" piece of hose where it joins the thermostat housing. Wait a while, and the coolant disappears. Only happens when it gets cold over night. Not sure why or what it means.

You can check the cooling system by putting a bit of pressure on it. You indeed use the coolant tank.

The bit of open hose from the neck of the coolant tank is there to let coolant flow on the ground if the cap opens because of high pressure. Normally that wouldn't happen, but it can happen if you overfill or if the temperature is high enough.

Hmm, that might be. I mean, i'm in Fl, so the word cold is subjective, but it was on the chillier side last night, albeit I wouldn't say it was super cold.

I came home from work around 8pm, parked and turned the car off, and came back out around 11:30 to move it so I could do some other work in the garage. The coolant could have dropped before, but it seemed to be fresh. I wiped up my little bit of oil and the coolant(they had not come from the same source based on there placement on the ground) and left the floor nice and clean. I took the car for a fairly spirited drive and came back and parked it, until 12pm today, when I left again. No signs of coolant.

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1984 Mercedes-Benz 300D

I would look at that 3" piece of stubby hose at the thermostate housing. And yes, it is a drop in tempature but it doesnt have to be super cold. Not sure what causes this other than the hose is old and weak and a drop in tempatures must cause some sort of stress on this part of the system since it is the only place for a leak that I have noticed other than a defective hose. Does your water pump have a weep hole ?

Hmm, Well in that case thanks for the info. I'll be sure to check that out. And I don't know if mine has the weap hole, but I was reading about the weap hole and how it'll start to trickle out when the pump is on the way out so that'll be on my checklist.

Any ideas on the pressure test for the coolant system?

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1984 Mercedes-Benz 300D

Just like a radiator cap, the tester cap seals below the level of the overflow nipple. There is no need to block anything; just connect the test cap and pump up the pressure.

If you are just testing for external leaks, pump the pressure up to 15-20 psi, then look and listen.

Oh, duh, that makes perfect sense. haha I've become used to the fact that all the MB's I've worked on usually have their own special version of tools, so I wanted to make sure this wasn't the same. I suppose I'll rent me a pressure tester on the way home and see what I get. I'll report back with results. Thanks guys!

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1984 Mercedes-Benz 300D

Fortunately, the W123 designers didn't feel the need to reinvent the standard radiator cap. Too bad a long list of other design teams could not have followed suit.

Haha, you and me both man! When I looked at the radiator cap and saw that it just looked like a standard radiator cap, I went into denial. It cant be, no way, that would be too simple.

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1984 Mercedes-Benz 300D

Well it ended up being a loose fitting at the tube that comes out just below the thermostat housing. The tube sits right over the alternator bracket. it's a narrower tube, about 1-1 1/2 inches, and runs off to an assembly in the empty space behind/below the right headlight, then that runs up to the firewall. Didn't look much into where it went exactly.

Anyway, the thermostat housing piece has a metal tube that the rubber hose slips over and is clamped onto. It appears the clamp had worn out and wasn't sealing very well. Of course, at first I suspected a crack in the metal tube or a split in the hose.

I drained the coolant from the plug on the radiator, but the coolant remained in that tube. I just had to cover everything below it and pull the hose off and let the coolant drain from there. I inspected everything, but found no cracks or splits. I put it back together, got a nice beefy new hose clamp on there, and all is well.

At this time, since I'm working on this, I'd like to drain whatever mix of coolants is in there and put in the proper G-05 stuff. I know the drain is below the radiator, but after my experience today, that doesn't drain the whole system.

What is the most effective way to drain the system completely so I can fill it with a fresh batch of the proper coolant?

EDIT: Nevermind, found the info on the block drain. Tomorrow I'll be draining and putting in new coolant.

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1984 Mercedes-Benz 300D

The block drain above the starter can be a real bugger to remove. I made up an adapter so I can flush the cooling system with a garden hose. 1/2 inch pipe nipple fits into the heater hose with a valve connected to the garden hose. I can flush fresh water through the system both ways. Then I blow out what I can with compressed air and refill with distilled water. Just an alternative if I cant get the plug out.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

This is one of those posts where it would be helpful for the poster to have filled out their location. If they are paranoid about exact location, at LEAST put in your state.

IF you are in a colder area, you might have experienced a cold weather leak. Sometimes a water pump will develop a leak that will show up only when setting overnight in coldweather. When this happens it's usually the water pump, and usually can't be found via a pressure test unless it is cold while doing the test.

Hope this helps somehow.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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