PARK the car, don't drive it until you do a cooling overhaul.

Info on 1998 528i:
ENTIRE COOLING SYSTEM REPLACEMENT Tips and Tricks (Very Long, Enjoy!)

Later models of E39:
Cooling System Overhaul: my diy experience - Bimmerfest - BMW Forums

The bottom line:
- Belts: Conti
- Pulleys x3. Check hydraulic vs mechanical. Search for INA pulleys on eeuroparts.com.


- Water Pump: HEPU (eeuroparts dot com)
- WP Pulley: BMW
- Radiator: Nissens
- Hoses x2: BMW

- Tstat: Wahler

- Fan Clutch: Sachs only. During install, use "Poultry Cord" trick I posted on bimmerfest E39 forum. Apply some anti-seize, it makes future removal much easier.
- Fan Blade: BMW only
- Reservoir: BMW only

- Coolant: Prestone green + distilled water (Walmart $12/g)


Option:
- Temp sensor on lower rad hose. Recommended.

---> This will set you back some $600 but you are good for another 100K miles


PS:
- When bleeding coolant, fill it above the KALT mark until it drips out of tsta housing bleed port.
- The alternator is AIR-cooled, not water-cooled.

The reason the belt comes off is that: the idler pulley has been neglected for so long so it is kaput.
I bet you have more than 160K miles.

See the photos posted by barbers528:
e39 Had to blow off some steam - Bimmerfest - BMW Forums

battery light - the diode bridge in the alternator is out of balance

bridge out of balance - a diode went kaput, or the alternator is not spinning when system is expecting it to spin

Alternator is watercooled on older V8 models, before the facelift, and as it was mentioned, the 3.0 is aircooled...

do not drive the car until you get your belts in order and have the cooling system pressure tested, as steamed up coolant might have created some escape routes (holes!!!)... It may happen so, that one you get the belt (and pulley) together, two days later you will have a major leak somewhere...

If you overheat engine, you may end up with coolant boiling over into the engine through a blown gasket - the extra pressure has to go somewhere, and will find the weakest spot possible...

is there WHITE smoke (steam ) from the exhaust?

IMO a complete overhaul of the cooling system is overkill. At this juncture there is no indication that anything, including the belt that is off, is in need of replacement.

I would inspect the pulleys, tensioners and hoses and get the existing belt back on or replace it if it is bad. If the belt is worn out I would replace both belts as they are probably the same age. Make sure the cooling system is full, start it up and go from there. I wouldn't be making any long trips until I had driven several trip of 50 miles or more.

Are you saying you are certain idler pulley is "kaput" because it has been neglected and implying there can be no other reason the belt came off and OP should have one on order?

So many E39 (1996-2003 528i, 525i, 530i) engines were destroyed because of this very issue.

First it started out as a broken idler pulley, then tstat housing cracks, then WP bearing goes, then reservoir cracks, then radiator blows.

Go to bimmerfest E39 forum and you will see so many overheat threads, it is not even funny.

Yes, the cooling overhaul may seem overkill, but most people would agree to it.
This is because the cooling system protects an expensive component: the engine.

I am a "if not broken don't fix it" kind of guy but the cooling system is the exception. Every 110K, I overhaul the whole cooling system.

Of course, people have the option of doing this (the belt) in bits and pieces, but these people usually end up spending $4000 for a head gasket job, if not worse.

So it is written in the temple of doom: The cooling system components are cursed, suicidal, consumed by demons. They must be eradicated, follow only the map of all new parts to ward off the imminent apocalypse of your engine.

Overstated or unintentionally inflammatory would have been much better than overkill. It is an overstatement to make a declaration that OP must park the X5 until the cooling system is overhauled or his X will more than likely die a horrible death.

It is not an overstatement to say it is best practice to overhaul the cooling system at some interval. If time and budget are of no concern there is no reason to offer an alternative to an overhaul but many of us have limitations of one or both.

I respect and acknowledge your point of view, even that it is best practice. It is clearly not that map or the apocalypse.

@bcredliner,

I respect your points too. But you are obviously not familiar with M52/M54 cooling issues. These components (reservoir, WP, radiator etc.) fail like clockwork.

Indeed, my cousin owns an exclusive-BMW indy shop in L.A. and he recommends the same thing every 110K (i.e. cooling overhaul), simply because he was tired fixing things in bits and pieces. Not to mention head gasket job, which he hates doing day after day. The items (reservoir, WP, radiator etc.) fail like clockwork.

I am a contributor writing technical DIYs on bimmerfest E39 section, and this is all I can offer: do a cooling overhaul before they regret. People of course have the option to listen to my advice or not: their car, their choice.

It is very easy math: cooling overhaul ~ $400-$600 vs $4000 for a blown head gasket. It does not take a PhD degree to figure this out.

Okay, so I had some time today to go through what was wrong.

I am still waiting on the tools I purchased on amazon to come, so I can open up the fan. (Amazon.com: Bmw Gm Fan Clutch Nut Wrench Water Hub Holding Tool Holder: Automotive)


Here is a status report:


It seems like the belt is in great condition, but you guys are the experts, so let me know what you think.
http://i.imgur.com/iBMIUd4l.jpg




The power steering fluid hose seemed to be wet, and there was so much gunk everywhere on the power steering reservoir housing and the hose below. (but the gunk was isolated just on those items). Is that normal? Does it seem like I need to replace my hose?

http://i.imgur.com/nUqjtbjl.jpg
http://i.imgur.com/OLq9bUsl.jpg
http://i.imgur.com/Rm1DFful.jpg
http://i.imgur.com/MZxDQLdl.jpg

Also the intake boot corner (the corner that was kinda touching the coolant reservoir) has a crack in it.
http://i.imgur.com/D9PR1SCl.jpg

Also I finally was able to see the coolant leak from my car. I squeezed the thick hose a few times, to create pressure in the coolant reservoir. I was able to see green coolant leaking from under my car, but was unable to find where the leak was coming from. Hopefully once I get the tools to remove the fan, I will be able to find out where it leaks from. But it seems like coolant did cause the belt to slip off!

How many miles in your car?

Couple things:

1. The Plastic Accordion Boot is easy.

2. The PS Cap has an O-ring, it is a few $ at dealer: Detail below:
What are the dimensions & material of the power steering fluid reservoir cap o-ring? - Bimmerfest - BMW Forums

3. The coolant leak: this is precisely what I mentioned above. You need a cooling overhaul.
There are very few reasons for the belt to come off:
a. Bad idler pulley
b. WP bearing going out, so you need to check the WP bearing.
c. Weak tensioner ---> loose belt (but it squeaks first)

4. You probably know already but the Fan Clutch Nut can be very stubborn.
It is LEFT-HAND threads.
Spray PB Blaster or WD-40 onto the nut a few days before you try to break it loose.
- During re-installation, it can be difficult to thread it back on.
I used the "poultry cord" (or dental floss) technique + some grease. It is a breeze to re-install the Fan Clutch. I wrote the detail below:

DIY - Fan Clutch Nut Installation
DIY - Fan Clutch Nut Installation - Bimmerfest - BMW Forums