When I fill my expansion tank to the top, I see a steady stream of bubbles coming out of the little hose that goes across the radiator. However, I don't seem to be using any coolant (only driven the car 800 or so miles), don't have coolant in the oil, or oil in the coolant. When cold, pressure doesn't build up in the cooling system unusually fast. The radiator cap is good and after a hard run the hoses are rock hard. After a recent VERY hard run (multiple 0-100 runs), a pint or so of coolant leaked out... which is when I found the bubbles.

I think it is likely a cracked head.... IIRC, one of the (many) reasons a #14 head would fail is high exhaust back pressure from a clogged trap ox. Well, there wasn't a trap on this car when I got it, but the cat. was so clogged up the only exhaust flow was through the EGR valve (that was interesting to diagnose!).

Here are two videos of the bubbles....

http://www.youtube.com/watch?v=kYcg8uUoG5I
http://www.youtube.com/watch?v=pARyVwRha6Q

Thoughts anyone?

Edit: Sorry... this is for my '87 300TD

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k

is the cap on during the video shoots?

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Nope.

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k

then they really don't mean much.

Put a good cap on and try again. The coolant will boil without a pressurized system.

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The coolant was nowhere near boiling temp though...

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k

oh, sorry. That was a cold engine?

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It does it a little bit when cold (although the hoses don't get hard quickly?), but those videos were at operating temperature. I wouldn't think there would be localized boiling in the coolant system.

edit: the hoses don't get hard quickly when the cap is on

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k

Personally, it would be much more convincing if you could create that condition with a good cap on.

If you can repeat w/ cap, cheapest thing to do is pull the head and hope for a bad head gasket. That would seem unlikely, but quite possible.

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do the hoses get hard?

I agree, it doesn't sound too good. But you might consider going further before making a conclusion.

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Umm, if the cooling fan is working right I'd think you couldn't get the coolant hot enough to make it boil. This is my experience with motor driven fans on VWs, I'd think the belt driven system on a Mercedes should be similar.

Looks like your head is cracked When I had coolant boil over once it was like an eruption... the coolant level rose quickly once the boiling point was reached and the car puked quite a bit of it out of the coolant tank. Your car does not look like it is doing that.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The radiator cap is good (new & tested) the viscous fan clutch is new (newer OM606 clutch and blade), the thermostat is new, and every single coolant hose is new. I did this shortly after buying the car in order to avoid a cracked head.

Thinking about it now... I put the radiator cap on after all this testing, and I don't remember the bubbles stopping. Also, the hoses got hard, but they didn't get hard instantly. It took a while.

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k

Sounds like you got some good nailing going on there. But there is definitely more bubbles than usual. But the nailing and bubbles could be related to a cracked head or blown gaskets. .

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1983 123.133 California
- GreaseCar Veg System

Then you have my condolences. Good luck, hope its just an HG...

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Some bubbles may be just air being pumped through the system. My 603 has a cylinder head leak and it will pressurize the coolant system after idling for a few minutes.

Start engine, let idle a few minutes, shut off, open expansion tank cap, listen to air escape.

After you let it get hot, the upper hose has no give. There is a lot of pressure in there.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

What if, while hot, you relieve that pressure. If I understand things correctly, the pressure shouldn't start building again correct?

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John Robbins
'05 E320 CDI - 240k
'87 300TD - 318k