Very Sick Coolant System
 

Hey all,
I bought my r56 S back in October with 105xxx miles on it.
A couple months ago, I started leaking coolant and running hot, so I stopped driving it in the meantime. Diagnosed it as thermostat housing. As I was putting the new thermostat housing on, I of course broke the old crossover water pipe.

Today I've been working on replacing the water pipe (what a PITA!!!) and I just pulled out the old pipe to find a ridiculous amount of blue-green-gray sludge (see pic). What on earth could cause this? Previous owner just never changing coolant or mixing different brands? Or should I be worried about possible head gasket issues causing oil and coolant to mix? I think the head gasket should be fine, no telltale signs when I parked the car at least. It did overheat that day, which is why I stopped driving it, but it didn't seem like that would've blown the HG.
https://cimg7.ibsrv.net/gimg/www.nor...b3cc4c2947.jpg

maybe some sort of radiator stop leak additive has been used by previous owner....

Well that would make sense; that pipe is the water inlet to pump, and with that amount of restriction in the system, you were starving the pump (cavitation); this explains the overheating ... lack of water to the engine block and cylinder head. I would pull the pump, and inspect it at a minimum. Or, you could go ahead and change the pump.

That's not oil you see in the pipe. Mixing water and oil won't make the grease like mixture you have there ... no thickening agent present. I agree with Barola, somebody's added something, and that something may be blocking critical cooling passages in the block, head, radiator, and heater core.

Now what do you need to do to get all that junk out of your cooling system? Here's what I would do based on the mileage of your car and the experience I had with my MINI.

Step 1: Replace pipe and perform a complete cooling system flush; this isn't covered in the Bentley service manual, so send me a private message and we can discuss the details.
Step 2: Cooling system leak test after the flush. You need to know if the cooling system is holding pressure. If it does hold pressure, you can skip the remaining steps. OBTW, there is no compatible coolant on the market that meets MINI/BMW coolant specs ... well, I haven't found one yet by comparing MSDS sheets, so make sure you use the right coolant.
Step 3: Perform a compression check on the engine and see if you have any low cylinders. If you have a low cylinder, see step 4.
Step 4: Perform a leak down test ... Need results of this test before you proceed.

Worst case scenario is that the cooling system was losing water through the head gasket and the previous owner couldn't find the leak, so he/she added the magic stop leak stuff that never works. Best case scenario is that there was a minor cooling system leak, and the aforementioned idiot put stop leak in the system. Here's to hoping you have an easy fix.

OBTW, overheating is more likely to cause you to drop a valve seat than it is to blow the head gasket. Your results may vary. But, if you soon hear what sounds like a bag of marbles rattling around ... think valve seat. Been there, done that.

Could the sludge be caused by mixing different types of coolant? I’ve read that doing so can create a gel-like consistency.

I was under the the impression that Zerex G48 meets BMW specs. Is that not the case?

I too am curious about commercially available coolants apart from what MINI/BMW sells. Following.

No.

As for the Zertex, I don't have any data that supports that claim.

The Zerex G48 product information sheet states that it is approved for BMW GS 94000. I don't know if that's the current BMW standard.