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Old 07-03-2013, 05:21 PM   #1
Oz Striker
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Exclamation Belt Tensioner FAIL! - Importance of Dust Caps - A Lesson Learnt

Been a while since I've posted, been busy and all, but hope to get back into the swing of things

Anyway last October I was planning on a 450 mile trip to see the folks in Glasgow. A week or so before my departure I was doing some general checks under the bonnet, and I spotted something unusual with the mechanical belt tensioner which I had replaced approx 20k miles earlier. I was pretty sure that it was meant to have a plastic dust cap, as I could see the torx bolt underneath. I made a mental note to check realoem, but simply forgot before leaving for Glasgow.

So I was about 300+ miles into my trip on the highway, when suddenly I saw the red battery light come on , my mind started to do the math, either it was alternator/battery issue, or it was the first sign of a belt/pulley issue (the other being a heavy steering, but I was driving straight as an arrow on the highway). By the time my mind worked through this, perhaps 5 secs, I'd eased off the gas, but then the needle went straight from 12 'o'clock and into the red

I pulled over straight away, connected up my bluetooth OBD tool and coolant temp was reading 126 deg C (normal is 90-96)!! I opened the bonnet expecting to see a shattered water pump pulley, and torn belt, but instead it was the belt tensioner pulley that had come off instead :



Larger pic here: http://2.bp.blogspot.com/-VfQl-S7AoY...0/IMAG0504.jpg


In fact the pulley had come off completely, weeks later I found it still on the engine under tray intact. But the next few pics will show where the failure occurred.

Initially I thought it was a freak failure, but then by the time the tow truck got me home and I removed what remained of the old tensioner the following day and I got a closer look, I realised what part the missing dust cap played. Here are pics of the failed tensioner:






Larger pic here: http://3.bp.blogspot.com/-zA2RNDWEdQ...0/IMG_0179.JPG


Note in the first photo the black discolouration on the body of the tensioner, at the time I presumed this was from the (black) pulley literally disintegrating and exploding into the body of the tensioner. But like I mentioned, I found it intact on the engine under tray. I presume the discolouration is release of grease inside the tensioner?

But the most telling picture is the last one, which shows where that circular spring clip snapped. I presume that with the dust cap missing, water and dirt managed to work there way into the inner mechanism of the tensioner, and over time caused it to fail.

I installed new belts as well as replacing the tensioner, despite the old belts look totally undamaged, but best not to take a chance. Instead I keep the old belts in the boot, in case of a future emergency. And guess what? I now keep a spare set of dust caps in my glove box, and any time I open the bonnet to check the oil or fill the windscreen washer, I now have a glance down and check all dust caps are in place!!

On a side note, I was shocked at how quickly the temp needle shot into the red. Think about it, I wasn't haemorrhaging coolant. OK, so coolant wasn't flowing around the engine, but it was still in place around the engine. It would be reasonable to assume that in my case the needle would have gradually increased, but no such luck for us E46ers!

An lastly I was worried about potential cylinder head damage, as the we all know these M54 engines don't take too kindly to overheating. But I can report, that after a further 15k miles of driving (incl a return trip to Italy), all is well, no coolant use, no white smoke and no yellow sludge in the oil

I hope this info helps out any fellow fanatics.
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Old 07-03-2013, 06:47 PM   #2
jgold47
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Interesting. Having said that I think these pulleys have a habit of breaking exactly as you described. I had the same thing on my 36 but it thankfully was the idler pulley.
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Old 07-03-2013, 08:06 PM   #3
lszlszx
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I guess the lesson is, lack of dust cap can not cause premature wear of the pulley , but it can ruin your whole engine.
The engine cooling fan actually blows the (moist, or dusty) air directly on
the bearing, so it makes sense.
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