Thoughts and advice on the care and feeding of fine automobiles from Machine Aficionado and bestselling author John Elder Robison, owner of JE Robison Service in Springfield, Massachusetts

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belt failurebmw engine failureBMW engine repairengine ingests belt

Engine failures when broken belts are sucked through the front seal

Most automobile engines have what’s called a harmonic balancer on the end of the crankshaft. The balancer often looks like a large steel disc inboard of the fan belt pulley.  The photo below shows a balancer on a Rolls-Royce V8.

One unrecognized benefit of a large balancer disc was that it acted as a shield when fan belts broke. A broken belt tended to stay on the outside where it was easily removed and did no harm

In the past year we have seen three six-cylinder BMW engines that failed after belt breakage.  Those engines don’t have large balancer discs; they don’t really have any disc at all.  The shredded belts wrapped themselves around the crankshaft, where they passed through the seal to be ingested in the engine.

Once inside the engine the belt strands can derail the timing chain, or get sucked into the oil pump intake.  Either is disastrous for the engine.  The photos below show the most recent failure in our shop, where the belt went into the oil pump inlet and the engine seized.

The first photo shows the crankshaft protruding from the timing cover.  You'll note the crank end is about flush with the cover, so the crank pulley is right up against the crankcase.  Any belt that gets in between is likely to go right through the seal.

Once the belt threads get sucked through the seal it's just seconds to disaster.  

The moral of this story:  pay attention to the condition of your belts.  Don't drive with belts that are cracking and beginning to shred.  If a belt breaks you should know, and you should stop the car immediately.  As with overheating, the decision to "drive a mile to the exit" may well cost you an engine.

While the manufacturer shares some blame for an engine design that is vulnerable to belt ingestion, there is a large element of driver negligence at play here.  Serpentine belts in newer cars are very long-lived, and there should be plenty of chances to spot cracks and breakdowns before they happen.  

This is one more reason you need more than a minimum wage worker draining and refilling your oil.   A skilled mechanic will look for issues like this, but owners must be willing to pay for such service, and in fact they must insist on it, if they can't do it themselves.

John Elder Robison

(c) 2018 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Bentley, BMW/MINI, Mercedes, Land Rover, and Rolls-Royce restoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the car clubs, and he’s owned and restored many fine British and German motorcars.  Find him online at or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.  

1 comment:

spiffy said...

Your article opened my mind this morning when I couldn't imagine a way for this kind of part failure to happen on a car, and at first I thought you were full of it when I was told about the article.
After reading it, I was astonished and understood just how legit this was.
Then later in my day I was faced with a challenge. A proposed job to replace the pads on an old crusty Jeep, it was leaking brake fluid badly out of the caliper.
Before all this I couldn't fathom a way for the brake system to fail either, with it leaking fluid so substantially without having a failed caliper.
But as it turned out, the pads were so far worn that they had eaten halfway through the rotor turning it into a jagged faced nightmare. This undue ease of space in front of the piston allowed the piston to pop out of its bore, causing of course a massive leak.
It turned out that it was possible for a non damaged caliper to be profusely leaking oil.
Hey I guess you learn something new every day. And today I learned 2 new things.
Keep up the good work!

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