I'm at a total loss for a cause if this X is parked nose up.

In the past I've heard of stored vehicles sustaining damage by unfixed cats marking territory. Their "spray" is caustic and strip paint/stain chrome & aluminum/rusts steel.

if you get this must corrosion in an area that is NOT noticed by most others in the Salt Belt capitals of the world, then I implore you to look at the brake lines behind the left front wheels. they are covered by plastic that also traps water/salt and attacks the metal brake lines, as there are a few good threads here about this exact problem adn a potential solution with an extra hole drilled into plastic and using a normal garden hose to push up in this recess and flush the area time to time... just a thought for you.

I am curious if leaving the splash shield off is an option. You would notice leaks faster if it was off. I have mine off on my manual to keep an eye on a leak and avoid driving it in any wet weather. I thought that if the splash shield was off and water hit the alternator while driving, it would fry it. Am I wrong?

I've driven many vehicles for tens of thousands of miles with no splash shields with no ill effects.

Unless you are fording streams or flooded roads frequently... you'd be fine.

Having worked in a few GM assembly plants, I'm reasonably sure the subframe arrives to the line already painted. There is the remote chance that this subframe was not adequately prepped before painting and still had some oil residue from the forming process on it when it was painted. After some time, the paint itself would disappear and leave the raw, mild steel to readily rust away in a salt air environment. Based upon some of the rust visible on other components in his "as repaired" photo, I'd say he's definitely in a salt air environment.

2002 X5 3.0 298,000 miles
2014 428i 16,900 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles

I don't recall ever hearing of this type BMW problem before. Even in the good ol' days when rust was a huge problem with American made vehicles something this bad would be very unusual.

Lots of things could have caused the rust to get started but once it did the chloride in the saltwater air and high humidity would be excellent growth hormones. I don't think the shield had anything to do with it. My guess is there is a list of extremely uncommon circumstances that are the culprit not even normal to a saltwater environment.

I certainly don't see the need for a worldwide rust alert. Drill holes in the shield it if it makes you feel better but I wouldn't remove it entirely--it is not window dressing it is functional.

Thanks for your input, everyone.

Just to draw a conclusion to his thread, my conclusions are:
1. As I live in a beachside suburb (about 500 metres from the ocean) there is regularly salt-laden air in the breeze.
2. That air contaminates the small pools of water that often occur (by rain flowing down the bumper and through the grill) on the topside of the splash-shield, right next to the sub-frame.
3. When the pools of water dry out, the salt crystalizes out and actually stays in the plastic wells on the shield.
4. The ongoing cycles of water/salt air/evaporation actually increases the salt content in the pools, many times over. The salt concentration, probably ends up higher that ocean water?
5. Once the rust starts in the immersed sub-frame metal, the porous rust draws the salty water up to the rest of the frame, with a wicking action.

I am sure my new drain holes, will stop this cycle happening again.

As far as the splash shield goes, I think it is really meant to have only 2 purposes: Prevent stones from entering the engine bay; and to slightly improve the high-speed aerodynamics/fuel economy.

In my case, it has caused the rust to occur in a vital structural component, and also prevented the visual discovery of the problem, until it was too late.

Thanks to Clockwork for suggesting I check the brake lines also.......I will definitely do that.

And thanks to this forum for helping me out over the years.
....Cheers.

Best of luck going forward.

Thanks. I want to keep the black plastic splash shield off until I get all leaks resolved on the 5mt.

I am pretty sure it is my OFHG. It turns out that Bav Auto sells a Viton OFHG gasket for 19.95 that "will" last the life of the engine. I am going to get one and replace that and hopefully with the very tough Viton, it will last 15+ years. Or forever.

I had a power steering fluid (ATF) leak to clean up today so I removed the under-shield - the subframe looks to be in good condition.

http://i49.photobucket.com/albums/f2...psyxk9hzy9.jpg

The under-shield itself has a thick urethane foam insulator attached to the interior - maybe this is a diesel thing. I drilled some new drain holes while I had it off the car - seemed prudent in light of this thread!

http://i49.photobucket.com/albums/f2...psjxftojnl.jpg

Also noticed the A/C belt tensioner/idler pulley bearing is shot - was wondering what was making a slight noise lately - will replace it once I figure out what the exact part number is - the OEM unit has a shaft as well as the pulley/idler inc. the bearing (visible in the first image above). :confused: