My Mercedes 1998 ML350 is leaking coolant from beneath the radiator; at the bottom two corners of it. I am not a mechanic nor do I know much about cars but I did do some stuff on engines. A hose running from somewhere in the car (I'm not sure where) into the radiator, seemed to be lose at the nut next to the clamp. I am pretty sure it is not the lower radiator hose. If someone could let me know what hose that is, that'd be awesome. Also I think this is the same case for the right side of the radiator. Any solutions on how to fix the leak for the right side of the radiator?

Photo of hose running running into radiator. A small nut had to be tightened to stop the leak.

Note: The hose in the image is not a fuel hose, as marked on the hose it is not to be used for fuel. Also a corroded radiator can be ruled out since the radiator has been replaced recently.

2 Answers 2


Sure. That hose is for transmission fluid. Lets start with an image of your radiator.

Cross flow radiator

Your radiator is an aluminum core, cross flow radiator with plastic end tanks. Inside one of the end tanks is a oil cooler for the transmission. And here is a close up / tear away image of that device.

(Edit Here) I updated the image. What I call a sleeve in the photo is actually a threaded nut. That's what keeps the transmission oil cooler tight in the radiator end tank.

Trans Oil Cooler

Here, I updated your original image. Does this make it clear to you?

Mercedes Radiator

My guess is there is a leak at the transmission oiler cooler to plastic end tank seal. That leak is dripping down the radiator. Its possible the seal is degraded and cracked, its also possible the threaded retention sleeve is a bit loose. If loose this is an easy fix. If the seal is degraded, and depending on how the seal is designed this could be a bear to fix. You might do a search on your specific make and model and see of there is a systemic leak at this location. Worst case is a radiator replacement. I know its possible to remove the end tanks from this kind of radiator, but that certainly is NOT an easy job, and rarely recommended.

  1. If the hose is loose to the male cooler fitting, you'd see red transmission oil at the leak.
  2. If the oil cooler is loose to the radiator end tank, you'd see coolant at the leak.

As I look at your photo, I believe I'm seeing green coolant. Is that what you see?

Is this work you are doing yourself? Or are you trying to understand what your mechanic will need to go through?

If you just want to verify the leak location, you can rent / borrow a coolant pressure tester; you pressure the system to 13 psi and look for leaks.

[Note: Autozone has this awesome "free" tool loaner system with great pressure testers there. Basically you leave a deposit (value of the tool) and if you return the tool within 90 days, its a full refund. Great program for the do-it-yourself folks.]

Pressure tester tool

  • Definitely coolant leak; its sweet and is green. What's weird is that both sides have green leaks (both coolant), I don't see any transmission fluid leaks.
    – JandyPilot
    Dec 11, 2021 at 8:55
  • Also yes this is my work. The oil temp does increase, i am not sure if this is because of the lack of coolant quantity or if its because the oil cooler is loose.
    – JandyPilot
    Dec 11, 2021 at 9:03
  • Have you tried tightening the retention sleeve to stop the leak? And if that doesn’t work have you priced out a new radiator?
    – zipzit
    Dec 11, 2021 at 15:06
  • Where exactly is this retention sleeve? Just to reiterate the leak is visible on the right hand side of the radiator.
    – JandyPilot
    Dec 21, 2021 at 7:56
  • 1
    Check my photo update. If that "retention sleeve / retention nut" is currently loose, tighten it. If that sleeve is already tight, then its likely a damaged seal inside of the radiator end tank. And if that sleeve is already tight, tightening it more WILL NOT WORK. If that sleeve is already tight, and the joint still leaks, that means it's time to replace the radiator. And as for the top / bottom conversation? You tell me. There are two fittings to your radiator end tank. Which fitting is leaking, the top one, or the bottom one?
    – zipzit
    Dec 23, 2021 at 17:56

Or the cheap plastic end cap of the radiator is cracked and you are lucky that it is only leaking a little. That is what happened on my Nissan Titan; and it was a pain the replace.

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