[PackPride85]

Originally Posted by F87source

Excellent question, technically if you want to do it right you should buy the vaccum bleeder tool, FCP euro sells a decent unit you can use. Then you need 2 buckets and an air compressor. BMW DIY Guy has a really good youtube video on how to do this bleed.


Now what I did was just bleed using the built in bleeding procedure, and here is why I believe it is acceptable.

1) The way the coolant expansion tank sits, it is the highest point in the loop - if you fill it all the way to the middle point (the seam) and above. This means as you fill the loop all air bubbles should be force up and out through the expansion tank. Now this might not always be the case on tight bendy loops but the reasons below ensure this happens on our n55 engine.

2) The block is tilted, and it fills from the bottom side up. The outlet for the coolant is then the high point and since it is tilted the block will fill all the way up before coolant spills out, this is an ingenious design by bmw to ensure the water jacket around the cylinders are always full. The head will also fill up too because the drain hose is a bit lower than the main oulet port for the block, and this goes to the thermostat which goes to the water pump aka the lowest part of the loop. This will eventually fill up and back up to the head and force air bubbles out via the main coolant exit from the block via the oil pump housing. This leads to the radiator.

3) The coolant will then fill the radiator top down, this ensures that the radiator is full and there will not be air pockets at the top. The radiator hose leading to the radiator has an expansion line that acts as a "air pocket remover" since this is the high point between the radiator and engine block air bubbles like to accumulate there, and the over flow hose siphons air bubbles out and dumps it into the coolant expansion tank. again another ingenious design by bmw. This is why this hose goes to the expansion tank and is above the water level and air doesn't use this path as an entry way back into the coolant loop, because as coolant flows it is pushed into this hose one way and into the coolant expansion tank, this also take bubbles from the highest point in the loop (aside from the expansion tank) and dumps it into the block.

overall you essentially fill the tank, it goes directly to the water pump and you fill the loop both ways (in the direction of normal coolant flow and the opposite direction) from the bottom up. This fills the block completely then as the water level rises any air bubbles are forced to the next highest point in the block aka the expansion hose where it siphons all the air out and into the expansion tank - super smart design by bmw. Literally just by filling the loop you bleed nearly all the air out of the system, and when you run the built in bleed 20x times (like I did) the coolant will only drop for the first run or two then stay steady. But make sure if you do it this way you run the built in bleed method alot.

This might be super confusing if you didn't look at coolant flow diagrams on the n55 technical document and pull apart the cooling system yourself, but if you do it will make sense. Also sorry if my explanation is confusing I don't know how to word it better.



Then after doing this method I logged coolant temps, and cylinder timing to seeing if any knock occured due to a cylinder being hotter than another (due to an air pocket in the water jacket), and everything turned out fine temps are rock solid at 94C in sport+ which is what they were from before.

Also I talked to a few dealership master techs and indy shops, they all do the built in bleed method a few times when flushing coolant on the N5X powered cars. The only time they use the vaccum bleeder is when an s55 show up, because of the mechanical water pumps and the lack of built in bleeding.

Thanks for the write up. I wish they had a better way to drain than pulling the water pump hose off like that. Looks messy lol.