Bad Coolant Expansion Tank Cap?

[Joncf]

After driving approximately 40 miles today, I noticed a small puddle of coolant under the car just beside the front passenger side tire. I saw this a couple hours after the car had been shut off. It seems to be coming from the overflow tank behind the fender, because it originated in the wheel well area just behind the wheel. The expansion tank and all other cooling hoses look perfectly dry and the coolant level hasn’t dropped whatsoever. Also, operating temperature is normal.

The reasons I’m suspecting it’s the expansion tank cap are:

1. When the engine is at operating temperature, the upper radiator hose is very firm and hard, clearly under pressure. I understand the system is pressurized but I’m not sure if this is normal.

2. Upon inspecting the cap, I am almost certain it’s a URO part. It has no part number or manufacturer’s logo on it and looks absolutely identical to this: https://www.amazon.com/URO-Parts-124-500-0406/dp/B0066RZHZE
Although this is very distressing, I’m quite surprised to find this on the car. The PO was absolutely METICULOUS about maintenance and repairs, as he almost exclusively had it serviced at dealerships who, of course, used genuine MB parts only. This is the first part I’ve discovered on the car that isn’t OE and it’s totally unacceptable.

Is it possible that this cap might not be releasing/opening at the appropriate psi, which is allowing excessive pressure build up in the cooling system? It appears the only outlet would be the vent tube on the top of the overflow tank for coolant to escape. From what I’ve read, people usually have the opposite problem where the cap doesn’t seal properly but I’m assuming the opposite is completely possible. (I will be replacing it either way)

 

[gerryvz]

Note that there is a coolant overflow tank behind the front wheel that is connected to the expansion tank by a braided hose. This also could be a source for leak. The braised hose also gets brittle.

I would order a new cap (either MB or Reutter aftermarket), but don’t be surprised if it doesn’t solve the problem.

HOW-TO: Replacing Coolant Reservoir Overflow Tank Hose | "HOW-TO" Tutorial Articles

 

[Joncf]

Note that there is a coolant overflow tank behind the front wheel that is connected to the expansion tank by a braided hose. This also could be a source for leak. The braised hose also gets brittle.

I would order a new cap (either MB or Reutter aftermarket), but don’t be surprised if it doesn’t solve the problem.

I actually saw the HOW TO write up you posted a few years ago on replacing that braided hose, which will be very helpful. Since it’s braided and not the newer solid rubber hose, it must be pretty old. Anyway, thanks for ALL the HOW TO instructional threads you’ve created over the years. They’ve been extremely helpful.

 

[gerryvz]

The braided hose is actually still available from MB, but the part number provided in the EPC / ISPPI is a smooth rubber hose that I used.

The @gsxr I believe knows the correct braided hose part number.

Triple Trouble (@a777fan) I believe has a photo how to on replacing the fender mounted overflow tank. It’s a prudent item to do while they are still available.

 

[gsxr]

AFAIK, the braided hoses is NLA from MB, but an equivalent is available via aftermarket if you want original/stock appearance.

You need to do 3 things at a minimum:

1) Replace the cap with OE, or OEM Reutter.​

​

2) Remove the fender liner to access the overflow tank, remove it, and empty it of old coolant​

​

3) With the engine cold, verify the coolant level is at the seam on the tank (~1 inch below the top)​

​

4) Optional: Replace the hose if needed.​

Here's the aftermarket hose with cloth braided cover:

Hose Cloth Braided Fuel 7.5 mm

German braided fuel hose sold by the meter. 7.5 m/m internal measurement.

www.partsklassik.com

 

[sheward]

I'll add that recently I discovered the brass insert at the top of my very nice looking expansion tank, the opening/fill point that the cap goes in, had become loose in the tank resulting in lack of pressurization and some coolant on the ground after parking. It took a while to identify. It also caused an increased operating temp. Although the tank looked recently replaced it was leaking. I replaced it with a new tank and all is well.

drew

 

[Joncf]

AFAIK, the braided hoses is NLA from MB, but an equivalent is available via aftermarket if you want original/stock appearance.

You need to do 3 things at a minimum:

1) Replace the cap with OE, or OEM Reutter.​

​

2) Remove the fender liner to access the overflow tank, remove it, and empty it of old coolant​

​

3) With the engine cold, verify the coolant level is at the seam on the tank (~1 inch below the top)​

​

4) Optional: Replace the hose if needed.​

Here's the aftermarket hose with cloth braided cover:

Hose Cloth Braided Fuel 7.5 mm

German braided fuel hose sold by the meter. 7.5 m/m internal measurement.

www.partsklassik.com

Coolant level is perfect and hasn’t changed.

This Rein cap is actually Reutter, correct?

Mercedes Expansion Tank Cap - Rein 1245000406

Mercedes Expansion Tank Cap - Rein 1245000406

www.fcpeuro.com

 

[Jlaa]

Coolant level is perfect and hasn’t changed.

This Rein cap is actually Reutter, correct?

Mercedes Expansion Tank Cap - Rein 1245000406

Mercedes Expansion Tank Cap - Rein 1245000406

www.fcpeuro.com

ECS Tuning lists Reutter separately from REIN (as opposed to FCP, RME, or AHAZ, which don't show Reutter), but heck, the radiator cap is not that pricey to begin with, maybe just get an OE cap? Here's what is costs from my local dealer - MB of SF, if I pick it up locally:

 

[gerryvz]

I got several Rein boxed caps a couple of years ago, and they were indeed German-made Reutter caps in the box. I put them on all of my vehicles.

 

[Joncf]

I got several Rein boxed caps a couple of years ago, and they were indeed German-made Reutter caps in the box. I put them on all of my vehicles.

Good, thanks.

 

[Joncf]

AFAIK, the braided hoses is NLA from MB, but an equivalent is available via aftermarket if you want original/stock appearance.

You need to do 3 things at a minimum:

1) Replace the cap with OE, or OEM Reutter.​

​

2) Remove the fender liner to access the overflow tank, remove it, and empty it of old coolant​

​

3) With the engine cold, verify the coolant level is at the seam on the tank (~1 inch below the top)​

​

4) Optional: Replace the hose if needed.​

Here's the aftermarket hose with cloth braided cover:

Hose Cloth Braided Fuel 7.5 mm

German braided fuel hose sold by the meter. 7.5 m/m internal measurement.

www.partsklassik.com

With the cooling system filled up to the proper level on a cold engine, theoretically the overflow tank in the fender should be empty, correct?

 

[gsxr]

With the cooling system filled up to the proper level on a cold engine, theoretically the overflow tank in the fender should be empty, correct?

Yup. The overflow tank should remain empty unless the engine gets really, really hot (or, if the cap fails and releases pressure far below spec).

 

[Joncf]

Yup. The overflow tank should remain empty unless the engine gets really, really hot (or, if the cap fails and releases pressure far below spec).

After looking under the car again, I actually have no idea what really is leaking now. I haven’t taken the wheel off to look at the bottle yet but I’m starting to question if that’s really the problem.

I took some pictures for orientation. You can see the puddle is forward of the wheel instead of behind it, where the overflow bottle resides. The edge of the plastic under-tray caught some of the drips where there was some runoff. The trouble is I can’t find ANY wetness anywhere on the car in that area. The lower radiator hose and all visible cooling hoses are dry, as is the wheel well liner. The hose from the expansion tank to overflow tank is hard but not leaking.


It is almost directly underneath the windshield washer fluid reservoir but there is no wetness underneath it in the engine bay.

You can see the coolant and washer fluid are both blue, which doesn’t help at all, but the level is correct and hasn’t dropped.

I guess it’s possible the coolant traveled from the overflow bottle to that spot via the inner fender liner, but I’d think there would have been more of a mess along the way. I’ll probably have the wheel off within the next few days to look closer. Does it look like anything obvious that I’m missing?

Also noticed this wetness on the strut dust cover. Top of the strut mount is dry.

 

[Kyiv]

To my amateur eyes, the "puddle" looks small-enough to have any noticeable effect on fluid level

 

[gerryvz]

In one of your photos, I can see white coolant stains on one of the hoses/hose clamps where the coolant hose goes into the windshield washer tank (a coolant heater hose).

You need to check the hose connections on your expansion tank, as well as the hose and pump connections on your windshield washer tank, very carefully.

Coolant tends to have a slightly oily or slippery feel to it. Windshield washer fluid tends to be more watery in feel.

Put a paper towel down there where the stain is, for the next time it happens.

 

[gerryvz]

It looks to me, when I blow up your photo of the windshield washer tank coolant hose connections, that the left-hand hose is loose, and has partially pulled back / slipped off of the raised area in the metal hose.

You need to push that rubber hose back ONTO the metal coolant line going into the tank cap, and then tighten that hose clamp (actually, tighten BOTH of them). I bet you'll find that your leak disappears.

 

[JC220]

I've had m119s push coolant out of a bad coolant cap before when up to temp.

Fitting a new genuine cap cured the issue.

If someone has an old cap you can get by in a emergency by flipping the seal 180 in the cap.

 

[Joncf]

It looks to me, when I blow up your photo of the windshield washer tank coolant hose connections, that the left-hand hose is loose, and has partially pulled back / slipped off of the raised area in the metal hose.

You need to push that rubber hose back ONTO the metal coolant line going into the tank cap, and then tighten that hose clamp (actually, tighten BOTH of them). I bet you'll find that your leak disappears.

Good observation. That’s actually paint that was put on there by whoever installed those hoses. I checked both hoses/clamps and they are tight. The washer fluid stains around the cap have been there for a very long time.


I closely inspected every coolant hose connection in the engine compartment and found no wetness anywhere. I also looked at the windshield and headlight washer pumps and the base of the washer fluid reservoir, which is all completely dry.

I did notice the rubber mounting grommet for the expansion tank looks damaged, when looking at it through the wheel well, but no wetness there either.

Also, I have the new Reutter Cap installed.

 

[gerryvz]

That's normal dried washer fluid residue for washer fluid caps with no inside rubber seal and shrunken/old rubber grommets on the heater lines.

 

[Joncf]

I've had m119s push coolant out of a bad coolant cap before when up to temp.

Fitting a new genuine cap cured the issue.

If someone has an old cap you can get by in a emergency by flipping the seal 180 in the cap.

It doesn’t appear to be coming from there, but I did just replace the cap with a new Reutter one just to be sure.

 

[gerryvz]

I think you are 100% going to find that it is windshield washer fluid. And most likely, one of the rubber grommets in the front of the reservoir where the windshield washer and headlight pumps press into.

The coolant reservoir is WAY too far aft of where that leak is. Fluids are going to leak as straight downward as gravity allows, although an obstruction may cause them to flow horizontally a bit. However, in this case the reservoir is half-way to almost fully aft of the front wheel, meaning the fluid would have to travel horizontally FORWARD nearly a full foot before dropping down. Not probable.

The windshield washer reservoir is RIGHT EXACTLY above of where that leak is. Particularly the two washer pumps/grommets.

The only other thing it would be is the SLS reservoir, which is right inboard of the washer fluid reservoir. However, that leak is far too far to the outside of the car to be the SLS. Again, the SLS fluid leak would have to travel OUTWARD a good number of inches before dropping, something that is not probable.

95% probability that the leak is your windshield washer reservoir. You need to examine it very closely with an LED torch.

It is most likely the washer pump grommets. Part number 123 997 36 81. You can get them at your local dealership -- they are very cheap. Buy four of them, to have spares. They do tend to leak after a number of years.

 

[gerryvz]

 

[Joncf]

I think you are 100% going to find that it is windshield washer fluid. And most likely, one of the rubber grommets in the front of the reservoir where the windshield washer and headlight pumps press into.

The coolant reservoir is WAY too far aft of where that leak is. Fluids are going to leak as straight downward as gravity allows, although an obstruction may cause them to flow horizontally a bit. However, in this case the reservoir is half-way to almost fully aft of the front wheel, meaning the fluid would have to travel horizontally FORWARD nearly a full foot before dropping down. Not probable.

The windshield washer reservoir is RIGHT EXACTLY above of where that leak is. Particularly the two washer pumps/grommets.

The only other thing it would be is the SLS reservoir, which is right inboard of the washer fluid reservoir. However, that leak is far too far to the outside of the car to be the SLS. Again, the SLS fluid leak would have to travel OUTWARD a good number of inches before dropping, something that is not probable.

95% probability that the leak is your windshield washer reservoir. You need to examine it very closely with an LED torch.

It is most likely the washer pump grommets. Part number 123 997 36 81. You can get them at your local dealership -- they are very cheap. Buy four of them, to have spares. They do tend to leak after a number of years.

I agree, this does seem to make the most sense. The grommets don’t appear to be wet, but the tank could easily be cracked on the underside.

 

[gsxr]

You can also stuff paper towels around all potential leak areas, and see which ones remain dry...

 

[Joncf]

The culprit of this leak turned out to be the lower radiator hose clamp. It wasn’t loose but clearly not tight enough. The coolant dripped onto the plastic under-tray before flowing onto the ground, forming the puddle near the wheel.

I confirmed this by pressure testing the cooling system, using Uncle Kent’s pressure brake bleeder tank. I’d actually already had it from a while back, and only needed to purchase the fitting for the expansion tank.





I literally located the leak within 30 seconds, so it worked great. I replaced the oil pressure sender as well since I was in the area.



Thanks everyone for the help

 

[a777fan]

Triple Trouble (@a777fan) I believe has a photo how to on replacing the fender mounted overflow tank. It’s a prudent item to do while they are still available.

No honch style ‘how-to’ but there are pics in my owner thread if I remember correctly!

(Edit: waaaay late to the party here. Glad you found the culprit!)

 

[Joncf]

No honch style ‘how-to’ but there are pics in my owner thread if I remember correctly!

(Edit: waaaay late to the party here. Glad you found the culprit!)

Thanks.. I’ll get to that one day but it hasn’t been a problem so far!

 

[gerryvz]

Coolant tends to have a slightly oily or slippery feel to it. Windshield washer fluid tends to be more watery in feel.

One note: If you’d have been able to ascertain this up front, as to the oiliness of the leaking fluid, it would have given a big pointer as to the source of the issue & probably saved the whole washer tank wild goose chase. That said, your tank coolant hose connection did look a bit loose, and I’d go through those connections and make sure everything is in order.

 

[Joncf]

One note: If you’d have been able to ascertain this up front, as to the oiliness of the leaking fluid, it would have given a big pointer as to the source of the issue & probably saved the whole washer tank wild goose chase. That said, your tank coolant hose connection did look a bit loose, and I’d go through those connections and make sure everything is in order.

I did do this actually.. it did have a slightly oily consistency as you said it would. After confirming that, I decided to pressure test. I’ll double check the tank connections. Thanks