You guys should have read this article before going to the track!
Photography by J.G. Pasterjak
Our latest BMW project car? The one that’s just off warranty? It broke after just a lap and a half.
While cruising around a damp-but-drying Florida International Rally and Motorsport Park in our 2015 BMW 435i, just warming up the oil, the dash let us know that our charging system was no longer a charging system and just a “sitting there doing nothing” system.
We pulled in, popped the hood, and immediately diagnosed the issue as a broken serpentine belt.
No biggie, right?
Not so fast, tough guy. This is a high-end European car we’re talking about. Things can get weird in a hurry.
See, on some BMW sixes, like our N55, as well as the B-Series engines, a rare but devastating failure mode can occur when the serpentine belt shreds. Drive around with the serpentine belt flopping around on the front of the engine, and it’ll start to shred.
Then those shreds can wind around the crank pulley.
And, from there, they can get pushed through the main crank seal and into the oil pan.
Read that again and just let your brain marinate in how insane that failure mode is.
Now, fortunately, we did not experience this potentially devastating twist of fate. The fact that we could easily remove our belt–which had been fairly neatly bisected along its entire length–meant it wasn’t wrapped around the crank pulley and being forced into the dark recesses of our engine.
So back on the trailer it went, and we turned to BimmerWorld for the answers.
They recommended a few parts–part of the “as long as you’re in there” nature of any BMW service–plus a new Continental belt, a new tensioner and a new idler pulley.
They also recommended a belt pulley crankshaft seal guard–a $79.99 part–that bolts behind the crank pulley and prevents intrusion of foreign material into the seal should another failure occur.
Serpentine belt failure on these N55 engines is not uncommon, and premature failure can usually be traced to the belt being contaminated with engine oil.
N55 engines frequently leak oil from two spots above the belt: the valve cover gasket and the oil filter housing seal.
The dealer that sold the car to us replaced the valve cover gasket but, even so, we didn’t see any leaks. Ditto on the oil filter housing seal. After 70,000-plus miles, it seems to have been doing its job.
So maybe this was just a random failure, or an older belt that was ready to go.
After removing the idler and tensioner pulley, however, we spotted the issue. The idler pulley’s shaft had begun to walk out of the bearing by nearly a half inch. This would place the idler further from the engine, and the belt would only be riding on half the pulley.
This all squares very neatly with the fact that the belt was cleanly sliced lengthwise along its entire length. It failed right at the point where it wasn’t supported by the idler pulley.
What we didn’t anticipate, though, was the lack of access needed to remove the crank pulley so we could install that belt pulley crankshaft seal guard.
We watched a few YouTube videos describing the process, all of which said, “Oh, yeah, you can toooootally do this with the radiator fan in place” before coming to the conclusion that they’re all filthy liars, or have hands so tiny they can barely operate their video cameras.
Our large, meaty paws were simply not going to be the ideal tools for that part of the job without further disassembly. And the block of time we had mapped out to complete the job wouldn’t be enough to do the whole thing.
So we popped on the new belt along with the new pulleys, and we’ll open things up again in another week or so when we can block out a larger slice of time to devote to removing the fan and getting human hands holding tools to the proper locations easier.
Our BMW 435i’s first test day ended shortly after it began, with a sliced serpentine belt that, thankfully, didn’t get stuffed into the oil pan.
The culprit here was a failed idler pulley. Its bearing failed, allowing the pulley itself to roll off the hub. The pulley wasn’t even supporting but half the belt.
This is what the pulley is supposed to look like. A new was ordered from BimmerWorld on Friday and arrived Tuesday. (They’re in Virginia, and we’re down in Florida.)
A new belt, naturally, and a new tensioner were also in order.
You guys should have read this article before going to the track!
Interesting! Does anyone make a steel pulley to replace the plastic one?
Wait! You guys have a lathe.
Glass half full - you made it all the way from Cincinnati without issues.
Slippery said:You guys should have read this article before going to the track!
I know, right!?
Jerry said:Glass half full - you made it all the way from Cincinnati without issues.
On balance, shredding a belt an hour away from home with a trailer is better than a day away from home on I-26.
Perfect example of why that era of turbo BMWs is not great. When mine started making a noise even the specailist couldn't diagnose, drove it down the street and traded it for a one year old Mazda 3.
$10k DCT? $7k for a used engine? Nope.
calteg said:
Uh, me?
Actually they got off easy. I'm glad the project update didn't begin with, "It ate the belt."
On my F25 chassis you can pull the whole fan assembly out the top and then have plenty of room.
If you see any sign of a leaking front seal I have the primer, glue and tools to do the job. The primer expires this month so let me know.
Every time I think about getting a BMW, I see/hear/read about something wild happening to them.
This doesn't seem toooooo bad, but still.
Duckzero said:Every time I think about getting a BMW, I see/hear/read about something wild happening to them.
This doesn't seem toooooo bad, but still.
Exactly this. And I spent half my life driving Subaru's and Rotaries and these BMW stories still scare me.
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