Bubbling noise in coolant resevoir
 

Did a search, didnt see this one...
My '92 300E after driving in warm temps (90F+) has strange bubbling pressure release type sounds coming from the coolant resevoir. Is this a pressure cap problem ?

It could very well be a tired pressure cap or the expansion tank itself is losing pressure. If you see no leakage on/around the expansion tank, try a new pressure cap. On other non-MB cars, I've also experienced this problem if there was air in the system; that sort of thing generally occurs right after a coolant flush/refill.

Happened frequently to me before I realized that the cap wasn't on tight. I guess a weak cap could do the same. One symptom is...are the hoses collapsed after the system has cooled down? If so, the cap needs to be replaced.

I just bought a new cap from dealer today during lunch. Will replace and drive home this evening.
I havent noticed any collapsed hoses but I will look now.

Could this be head gasket related ? Please say "shouldn't be" !:(

If it were the head gasket, I wouldn't expect it to wait to appear until the coolant is hot.

In our case, it was a shot gasket on the cap, followed by not turning the NEW cap far enough on to pressurize, as another poster found too.

Also, don't pull the same oops as me and forget to wait until the engine is completely cold before removing the old cap.

Steve

Well it does really do it after the coolant is hot. Like driven in 90+ degree temps (here locally) for at least 15-20 minutes. After which I can easily here the bubbling noise at the resevoir...
If I change the cap and it still happens what might that indicate as the cause???
PS, no leaks are visible.

The bubbling is just the coolant boiling, because either the formulation is incorrect (not the right mix - nominally 50/50), the pressure is too low, or the temperature is too high. If your gauge is correct, and you have been adding lost coolant with the correct mix, then the new pressure cap should fix it.

After you put on the new cap, wiggle it. If it wiggles at all, it is NOT on completely! I found I was only comfortable pushing down hard enough on the cap if I first supported the tank from underneath with my other hand.

BTW, see if your old cap wiggles - maybe it is just incorrectly installed! If so, check for a bad gasket before putting it on properly.

Steve

You guys were right. On 2 points. The new cap stopped the bubbling sound (or pressure bleed). And when I tested the anti-freeze it was too much water mixed. I'll correct the antifreeze this weekend. Have a good Labor Day all.

THANKS !!