Putting RMI-25 in a dead-end system? (BMW)

[Craig in Canada]

I have a 1998 BMW 528i. There is no radiator cap, only an expansion tank cap. The expansion tank is not a flow-through design like some other German cars but a dead-end high point in the system. The e-tank connects to the block, not to the rad. Even pre-RMI, this seems to be where the crud wants to hang out.

I'm going to be adding some RMI-25 to run for 1000km or so before changing coolant and maybe some cooling parts (expansion tank, water pump).

Would it make sense for me to drain the radiator (easy), but not the block (messy and time consuming), add the RMI-25 to the expansion tank, then refill with the drained coolant? This would make sure that the RMI-25 gets right into the system and doesn't take thousands of kilometers to combine.

When filling this system I find that the e-tank never empties, but has an action like a toilet due to the routing of the tubing (like a trap) to the expansion tank. Even when the system is empty, the e-tank never empties. As you fill it to the brim it will then empty into the block.

This is a very annoying set up for adding things like RMI-25 and getting it mixed into the flow of coolant throughout the system. Frankly, I'm not even sure if this arrangement would allow for typical RMI-25 action of floating just into the e-tank.

Craig.

 

[ConfederateTyrant]

Drain the radiator and add the RMI-25 and allowed it to flow into the radiator via the expansion lines. Then fill it up with the drained anti-freeze, that way you'll get a good mixing action.

My Grand Marquis doesn't have a radiator cap, just an expansion tank cap, and I added it in there. It all eventually mixed, it just took a little while.

 

[02Pilot]

Are you sure that the expansion tank isn't a flow-though design? On my 2001 525i there is a small hose under the fan shroud that connects the top of the radiator to the expansion tank (see #6 in this diagram - this picture is for a 1998 528i). It flows coolant from right next to the top radiator hose into the top of the expansion tank constantly. Not sure if that changes your plans or not.

 

[Craig in Canada]

02Pilot:

When I was doing this job I noticed the same thing, but I'm not sure just how much flow there is through this passage.

I did drain just the rad and refill, but unlike every other time this time it gave me problems - wouldn't take the coolant, wouldn't bleed. I did the same process that's led to a perfect, air-free job in the past but lady luck wasn't shining on me this time. A week and a half later I think the air is out.

At any rate, the RMI is in there and doing its thing. Thanks for the reply!

Craig.