Engine Noise: Help with diagnosis

Greg · Post by **Greg** » Tue Apr 13, 2010 4:51 pm

Out here in the wilds of northern Japan, and not being in any significant way mechanically capable, I would appreciate any views on what might be going on with my engine (67 911S).

The carburettors were rebuilt late last year, but with delays in getting them back, the snow beat me to getting a chance to put them back on and try them. I have only now managed to get them in place and give the car its first spring drives.

Overall, the problems I was having before the rebuild appear to have been resolved, though a little bit of expert fine tuning will still be necessary when next I can get to my mechanic.

Now I am picking up a new noise though that I haven't heard before, and am not sure how alarmed I should be. It is difficult to describe, but is an irregular clunk/rattle noise that appears to me to be coming from under the red shroud pretty well immediately behind the fan housing.

It is hard to describe it in any more detail than that, but I am - in all my ignorance - wondering if it might have something to do with the alternator?? Is it located somewhere there?

If so, any thoughts on what it might be, and what might be entailed in getting it sorted. Parts are never easy to get here, but there are a couple of places that do good rebuild work. Doesn't appear to be affecting performance at this stage, but would hate something to go bang out in the middle of nowhere.

Sorry it is not a lot to go on, but would appreciate any thoughts at all.

Cheers
Greg

Lightweight_911 · Post by **Lightweight_911** » Tue Apr 13, 2010 5:21 pm

It's very difficult to identify the source of unusual noises over the internet but you're correct in thinking that the alternator is under the engine shroud - it's mounted on the back of the fan.

However, you say that the noise is 'irregular' - any noise associated with the alternator would almost certainly vary in direct relation with the engine speed.

Any other info you can give us ?

Dakota · Post by **Dakota** » Tue Apr 13, 2010 10:35 pm

Hi Greg
a quick way of checking your alternator would be to remove the fan belt and try to wiggle the fan up and down and spin it to listen to the bearings , there should be a kind of shoosh noise from the brushes but not a rumble from the bearings or with it idling use a long screwdriver as a stethoscope agianst the fan housing (the cast ring) obviously the if the bearings are really shot the fan will foul the housing which will be very obvious, another interesting noise is a kind of low clacking that a worn intermediate shaft can make at idle this is more obvious if you stick your head down by the silencer and only occurs at a fairly low idle, it can only be fixed with a full strip and rebuild ... Lastly it may be a clutch issue, tell you what fly me out to you and put me up etc and I will diagnose it free of charge, Bill.

stuby · Post by **stuby** » Tue Apr 13, 2010 11:18 pm

If you want a second opinion I'll gladly do the same, let me know the flight time and accommodation details....lol
Stu

911hillclimber · Post by **911hillclimber** » Wed Apr 14, 2010 6:56 am

Run the car briefly without the fan belt on. If noise dissapears then suspect a bearing or brushes in the back of the alternator.

Replacement 'rears' available from www.type911.com This is the brush electrical end, not the shaft bearing. My brushes had worn very unevenly.( 140K 3.2 engine)

Awkward job to do and everything fights you for space. Easy to nip those fingers!

If the noise is still there obviously it is coming from elsewhere.

This could be chain tensioner(s)? Low revs, intermittent.
When mine went (on my 3.2!) you did not need a a screwdriver to any engine part...

Greg · Post by **Greg** » Thu Apr 15, 2010 10:27 am

Many thanks for the helpful responses. I'll get the fan belt off over the weekend and see if the noise vanishes. When I am driving, I can't hear it at all - with the back window zipped out and a bit of weight on the accelerator and all those other wonderful P-car noises drown everything else out. It is only really when the car is idling that I can hear it at all.

I wouldn't describe it as a 'clacking' sound - so am hoping it is not the worst case scenario mentioned!

Anyway, I'll start with the alternator premise and see where we go from there.

Thanks again.

Greg

hot66 · Post by **hot66** » Thu Apr 15, 2010 11:58 am

rattling / clanking at low revs .... sound disappears as you increase revs, is usually a sign of tensioners being shot

Greg · Post by **Greg** » Mon May 17, 2010 1:19 pm

Apologies for not updating early in response to the helpful advice I received. I had to get back to Oz to sort out some business and haven't had a chance to get back on line. I'm still not back in Japan yet, but hopefully next week.

Before leaving however, I managed to get the fanbelt off and note the difference in engine noise.

Absolutely no doubt about it - the worrying noise disappeared entirely!! Must have been just my poor powers of description that raised some questions.

Got my mechanic mate in the remove the alternator, and it will be repaired by the time I get back, so minimal downtime, I hope. Will get to the final tuning the rebuilt Webers when we get the alternator back in and looking forward to seeing - and hearing - what she really can do for the first time since I found her two years ago.

Thanks again for the help. Much appreciated.

Greg