[crrraaig]

Can we talk about the coolant expansion tank? I've been going down this rabbit hole for a few hours now while I look for ways to avoid doing any actual work and I was hoping for some company. Of my 99 problems, evidently two are with the coolant expansion tank.

Problem #1, to replace the bleeder screw or not. It does not take a lot of googling/youtubing to see that the coolant expansion tank bleeder screw is a troubled area on our cars - numerous complaints of the bleeder screw itself cracking under pressure and causing coolant to spray everywhere and make a real mess of the engine compartment. This is such a common issue several vendors offer replacement brass bleeder screws to address this fault.

The cap on the expansion tank is set to release pressure at 140KPa (~20psi), so if the bleeder screw is failing first could this mean that actually the cap has failed to vent and the system is over-pressure? If so, replacing with a brass screw could just cause the tank itself to be the next failure point. However if the plastic screw has simply failed due to becoming fragile over time, perhaps replacing with a new plastic screw before failure is the best option?

I'm thinking back to my X3 and the infamous plastic transfer case gear which fails causing the "3 lights of death". The aftermarket offered a brass gear to replace the faulty nylon gear BMW engineered - but the thing is, by removing that wear point, you moved the failure point to the next weakest component which was much more expensive to repair. BMW actually engineered that gear to be the weak point in the awd transfer case assembly on purpose, and it works very reliably for about 80,000 miles. Of course BMW wants you to replace the entire $800 actuator motor when it is really a 10 minute repair to open the actuator and move the gear to a not worn orientation (it only wears about 1/3 of the gear - the rest of the gear is unused).

So is this a similarly engineered system and we should all just have a few spare plastic bleeder screws handy and swap a new one in every presidential election or so? Or do you think it is better to go with metal?

[crrraaig]

Problem #2, what the actual huck is going on here?!

In order to do another repair (TIC pipe replacement) I needed to remove the expansion tank on my 435i, which was no big as I have already drained the cooling system. Cleaning up the tank a bit and having a look and I noticed brilliant turquoise blue crusties at the bottom of the sensor connector. I've already been working up to a near paranoia state that this car has seen some high water in the past and this really pushed me - I mean, if water made it this high, the entire block would have been submerged!! Further, I noticed that the connector was using two thin wire 'pins' and not blades as I would have expected (and the female plug seemed designed to accept). I used electronics cleaner to get rid of all the crusties and get the socket looking like it should, but once I got home I hit up ebay to see how much a used tank would set me back.

Surprise, surprise - many of the used coolant expansion tanks on ebay also show the same crusties!! And here is a new one that does have blade connectors, not pins... Hmmm, so this is electrolysis? A design failure and not a indicator of flooding?! Looks like this is a pretty common issue based on my ebay survey. I'm still on the fence as to how to deal with this and open to suggestions. Interestingly, the cheap non-oem tanks do not seem to use the same blade connectors - theirs seem to be made of only one material.

Some pics - the first three are used ebay examples and the fourth shows the two part nature of the BMW blade connectors:

[scubajim]

Since you have a concern about possible issues and damage, I would just purchase new replacements, none of which should be cost prohibitive. The age and heat can certainly affect the durability of any of those components. Give yourself peace of mind.

[F32Fleet]

Originally Posted by crrraaig

Problem #2, what the actual huck is going on here?!

In order to do another repair (TIC pipe replacement) I needed to remove the expansion tank on my 435i, which was no big as I have already drained the cooling system. Cleaning up the tank a bit and having a look and I noticed brilliant turquoise blue crusties at the bottom of the sensor connector. I've already been working up to a near paranoia state that this car has seen some high water in the past and this really pushed me - I mean, if water made it this high, the entire block would have been submerged!! Further, I noticed that the connector was using two thin wire 'pins' and not blades as I would have expected (and the female plug seemed designed to accept). I used electronics cleaner to get rid of all the crusties and get the socket looking like it should, but once I got home I hit up ebay to see how much a used tank would set me back.

Surprise, surprise - many of the used coolant expansion tanks on ebay also show the same crusties!! And here is a new one that does have blade connectors, not pins... Hmmm, so this is electrolysis? A design failure and not a indicator of flooding?! Looks like this is a pretty common issue based on my ebay survey. I'm still on the fence as to how to deal with this and open to suggestions. Interestingly, the cheap non-oem tanks do not seem to use the same blade connectors - theirs seem to be made of only one material.

Some pics - the first three are used ebay examples and the fourth shows the two part nature of the BMW blade [...]

Why on the world would you buy a used part from eBay?

[crrraaig]

Originally Posted by F32Fleet

Why on the world would you buy a used part from eBay?

In this case I never suggested or considered that. But as a source of visual information, it was just the thing I was looking for. And in the case of the TIC charge pipe, buying a $25 used pipe so I could have a HENN clamp in my hands and see how it works for the first time was invaluable.

I did consider buying a new non-oem part as it might not suffer the same corrosion issues the BMW tanks do, but in the end I cleaned everything up applied some quality dielectric grease and carefully reconnected the blade connector into the pins. These translucent tanks do not seem to have the reputation for cracking like the black plastic ones on the N55 X5 do, just bad bleeder screws.

Speaking of which, the bleeder screw was nearly fused with the tank and I was lucky to get it out without stripping - I am definitely going to replace with OEM, and get a few extras to have on hand. No need for metal screws, IMO.