Eghad! A hole in the expansion tank?!

[andyman32]

I'm losing a little coolant, only when the car is hot. The water pump is new, the t-stat and t-stat housing, hoses, and radiator, all new. And, there is some dried coolant on the top of the expansion tank, that doesn't look like it came from the cap.

I have the expansion tank in the rear-top of the engine compartment, above the accumulator. Maybe it just escaped my notice until now (doubtful), but today I was topping off the coolant and I noticed there's a hole in the expansion tank cover, about 1/8", cleanly drilled, that goes clear down into the coolant. It's in the little dip on the top cover of the tank, dead-center between the pressure cap and the coolant level sensor. Do any of you know what this is? Is it supposed to be there? Am I going CRAZY?!

[andyman32]

Just wondered if anyone has any suggestions here. I plugged the hole today with a screw and the radiator blew when the car was hot. I'm not sure they're related (this is only the 3rd or 4th time I've driven it since I put in the new radiator, and the first time I really "pushed" it). But I do want to know what that hole is, if it's supposed to be there, and whether or not I should plug it.

Can someone else w/ a 540 just have a look at their expansion tank? Like I said, it's centered, in the dip between the filler cap and the coolant level sensor.

Thanks guys!

[tim s]

http://www.tntcomp.com/res_tank.jpg
i would believe that is a drain hole so if you spill the fluid you don't make as big of a mess.
that would be my guess.
do you have the correct cap. there is a cap with a lower psi pressure rating.
tim s.

[tim]

If you have an automatic, I can almost guarantee that it is your transmission fluid cooler lines that are leaking. They need to be replaced, as the rubber degrades and no amount of tightening the hose clamp will stop it. Both lines cost less than a $bill from BMA.

The job itself is a royal bitch, but still only a one wrench job- not hard just tedious and hard on the knuckles if you have large hands like me. I'll post a picture of the spot I'm talking about soon.

As Tim S. said, we all have that hole in the expansion tank, although if plugging it makes you feel good, it won't hurt anything ;)

[tim]

Pictures:

edit:
hmmm, having trouble with pictures, maybe related to site upgrade. Take off the motor cover and look directly beneath the expansion tank. Alternatively, look at your tranny cooler (the rectangular metal deal in front of your tranny)from under the car and I bet the driver's side of it is caked with chalky green stuff.

re-edit: check this post:http://www.bimmer.info/forum/showthread.php?t=7276&highlight=tim

[andyman32]

HI Tim & Tim s. Thanks for the pic. That hole is precisely what I'm talking about. I was very worried that by plugging that hole, the system got too much pressure in it & blew the core. Though... that's a silly fear, one would think that a hose would pop off or something before the core blows out.

I'm at least relieved to know that that hole is there for everyone. The new radiator should be here in about an hour, but I feel better now (that this is probably a bad radiator and not a bad install). I didn't want to have to pay for two radiators in two weeks!

Tim: there is not a leak in the rear. I checked all the hoses around the accumulator and return hose, there's nothing leaking. Everything is clean, no corrosion, no dried coolant, and the hoses are all good. I think this lingering leak is in the neighborhood of the bottom-left bolt of the thermostat housing, that one that's a f*ing b*tch to get to. I thought I had tightened those bolts pretty well after I put on the water pump, using a little 1/4" u-joint, but I saw some coolant just sitting there in that little nook on top of the engine block, right beside the t-stat housing, before this radiator fun. That's really the only thing that could have leaked there.

The pressure cap is original and does not appear to leak.

After I install this one, I'm going to spray off the whole engine compartment (since it's pretty much all covered in dried coolant spray from the first radiator blow-out), heat up the car and see what I see.

[tim]

Well, since everything else has been ruled out, it's got to be the radiator. Good luck.