E70 4.8i Expansion Tank Failure & Fix

[juthale]

The expansion tank on my 2008 4.8i E70 X5 decided to end it all at 109,000 miles. And, of course, it failed while we were on a family road trip in Montana - about 1,000 miles from home. Gotta love an adventure!

Initially, it looked like it was the o-ring leaking at the lower hose connection. I was hopeful it would just be an o-ring replacement and I would be on my way. However, after removing the tank and doing a quick pressure test on it, the leak was a failure of the seam between the top and bottom halves of the tank. The way the coolant was coming out of the tank caused it to pool right on top of the hose connector and look like a leak at that location. Examination of the o-ring showed it was in great shape.

After reading many online recommendations, (including some here and some on bimmerfest) I was somehow persuaded I should only buy a genuine BMW expansion tank - brainwashed, apparently, and blind to the fact it was a genuine BMW part which had just failed at a most inopportune moment. As fate would have it, there are no BMW dealers in Montana and no quick way to get a genuine BMW part to Bozeman at anything close to a reasonable cost. So, that reality had me surfing Amazon for anything I could get delivered the following day.

The only tank available for next day delivery was this product http://www.amazon.com/gp/product/B00P9MSO5M. I ordered the part, figuring it was good enough to get me home and thinking I would exchange it for a genuine BMW part at the first opportunity. That was the plan when I clicked BUY.

Today, the part arrived, on time and well packaged. It came in "Hamman" brand packaging - which is at least a parts manufacturer I had heard of before. So far so good. Examining the part, it was truly indistinguishable from the OEM BMW part (the photo on Amazon is not of the part I received). The part I received actually looked like it was made in the same tooling as the OEM part. The labels, mold lines and injection points were all identical to the original. The fit as well as the material color and finish were perfect. The part shipped with a new coolant level sensor, which I tested and it worked perfectly. This is handy, as it is the same sensor that is used in the washer fluid reservoir - so now I own a spare.

I also ordered a new expansion tank cap while I was at it - an insurance policy against the possibility it was a failed pressure relieving cap that had caused the tank to fail. I ordered the URO cap on Amazon for pretty much the same reason I ordered the Hamman tank - because I could get it quickly. This cap also turned out to be an excellent replacement part, identical in every visual way to the OEM cap. You can find it here: http://www.amazon.com/gp/product/B0066RZKII

I installed the new X-tank with no difficulties, flushed and filled the cooling system per the crazy BMW procedure (using Pentofrost NF, as no BMW antifreeze was available) - and tested the new level sensor in the process. I sealed everything off with the new cap, then tested it with some hard driving in Montana summer weather with the AC on full bore. No issues.

The way I went about this was out of necessity, based on no availability of BMW parts. The outcome, I believe, is as good as it would have been with OEM equipment. I guess the fact this happened on the road where I had no access to a BMW dealership ended up saving me quite a bit of money - I completed everything (including new tank, cap and 6 liters of new coolant) for under $120.

Obviously, only time will tell if these parts will hold up as well as the original BMW parts. If not, I will return with an update. For now, I'd recommend this path for anyone facing this all-to-common problem with all late-model BMWs and looking to do it for less than BMW pricing.

[juthale]

And a photo of the replacement parts...

[Sam617]

How did you pressure test the old tank? I'm just curious to see where it leaked on my old tank. Also, How do you flush the coolant in the 4.8? Thanks, Sam

[juthale]

I have tested expansion tanks in he past by plugging the return line and using a regulated air source at 10-15 PSI in the outlet. Preferably, this is done under water in a bucket, making small leaks easy to spot and giving some safety against the tank rupturing. Being away from home in Montana, I had limited tools and no access to an adequate shop or an air compressor. In this case it didn't matter. With my thumb over the return line, I blew in the outlet and I watched air and coolant bubbling at the seam inside the cap drain hole. A low-budget pressure test for sure, but my leak was bad enough for this method to be effective and reveal the leak.

On the 4.8 liter engine, there is a drain in the bottom tank of the radiator and another in the engine block. With the key in the ignition and set to the second click (engine light on, but engine off), set the climate control to max temp (84 degrees) and the fan on low. This will open the valves to the heater core and permit the full system to drain.

Refilling the system is a 50/50 mix of coolant and distilled water - about 12 liters in the 4.8i.

[Sam617]

Thanks juthale for replying back. How did you bleed air out of the system?

[juthale]

The E70 cooling system is "self-bleeding". That is BMW's claim, and why the E70 does not have a bleed screw like other (earlier) BMW models. Unlike conventional cooling systems, BMW's "self-bleeding" systems continuously cycle fluid through the reservoir (the expansion tank). By doing this, any air in the system gets returned to the top of the tank and is replaced by fluid from the bottom of the tank.

Instead of a bleeding process, the filling process also has a step requiring the climate control to be set to hot and the fan set to low. Even following these directions, I had to add another 0.5 liters of coolant after the first drive.

I have 200 miles on the SAV since adding that last 0.5 liter and the coolant level has not changed since. The heater works fine, which is a pretty good indicator the system does not have air in it.