What's This Engine Bit?

[MrT]

Has anyone any idea what this bit is on the front left hand side of my engine? It looks as if the left bit is coming away from the right bit leaving a gap marked by the two arrows. It seems tight though with some sort of plastic between the two pieces. Also there is a clean bolt hole with no bolt in it, the cleanliness makes me think something is missing.

 

There is a chain like rattle coming from this thing and when the air-con is on, the sound changes from a rattle to a whirr. The sound is definitely from this, not nearby as can be told by putting a rod on it and listening to the other end. You have to have the bonnet open to hear it, but all rattles to me sound like future expenditure.

[Guest nimrod]

That is the auto tensioner for the aux drive belt, the threaded hole is used to

de-tension the belt, by sticking an M8 bolt in it and tightening it up the tensioner is lifted away from the belt giving enough slack to remove it

[MrT]

Thanks for the fast reply. So is that how it should look with that gap?

[Guest nimrod]

IIRC thats how mine looks, well not as clean but yes there is a slight gap

my mechanic has a T bar thats threaded on the end and he winds that into the hole to release the tension, it took him less than 15 minutes to change the aux drive belt last time he did it!

[Richmond]

FWIW, the two inserts in the gap appear to be engaging each other at the top of the gap, which is presumably what they're supposed to do.

 

After last week's picture of an unidentified and apprently broken bit in someone's engine bay which turned out to be perfectly OK, the fact that it looks as though it's about to fall apart should not worry you.

[MrT]

My concern with it possibly moving out and making a slight rattling noise when under no load worried me in case it was anything to do with the cam chain. That should not rattle and if it broke disaster, if an aux belt breaks, it's bad but a little cheaper to fix.