911 sounds like a diesel at idle

[tomswa]

I was driving my 1978 targa to the gym tonight and heard a disturbing noise when I was stopped at a light. The engine sounds fine when you increase the rpm's but at idle it sounds like there a parts rattling around. The car has 105000 miles and the original chain tensioners. It also has the air pump.
I was concerned that it could be a chain so I had it towed home. Now the question is how do I safely diagnose the problem. If it is a chain, I do not want to lose the engine doing startups to demonstrate the noise to a mechanic. I will pull the pully off of the air pump and see if it is loose or other.

[twistoffat]

It definitely sounds like it the chain tensioners. Here´s a video of a 79 with a similar problem but in this video the sound pretty far gone. It was one of those must have upgrades and much cheaper than an engine rebuild
Diagnose rattle on 79 Porsche 911 engine - YouTube

[BrokeMyCar]

That's exactly how I described my tensioners failing in this thread:

Idle Now Sounds like a Diesel Truck!

[Bill Douglas]

Either way, now might be a good time to do the tensioners.

And good luck Tom, it's a nice looking car.

[tomswa]

So how safe is it to drive 30 to 150 miles to get it repaired. Also how difficult is it to replace yourself. I do brakes and all normal maintenance for the last 40 years (69 911t and this one)?
Wish I still had the 911t!

[Bob Kontak]

Originally Posted by tomswa

So how safe is it to drive 30 to 150 miles to get it repaired. Also how difficult is it to replace yourself. I do brakes and all normal maintenance for the last 40 years (69 911t and this one)?
Wish I still had the 911t!

Do the long screwdriver or stethescope (sp?) check in the chain tensioner area when at idle to confirm noise is from the chain box.

If so, I would not drive it.

[The cap'n]

Absolutely NOT safe to drive it even a few feet! If the cam timing jumps, you're in a world of hurt. This job can be done with the engine in the car, and any experienced air cooled 911 tech, or even advanced hobbyist, can do it. Care must be taken, but it's NOT a difficult job Your tensioners can be rebuilt and have a great life expectancy. Note there are 2 basic varieties, and the later type can be fitted to earlier engines with a cheap mod. Tensioner guards are always a good idea, too.

[Trackrash]

Originally Posted by tomswa

So how safe is it to drive 30 to 150 miles to get it repaired. Also how difficult is it to replace yourself. I do brakes and all normal maintenance for the last 40 years (69 911t and this one)?
Wish I still had the 911t!

It is not that difficult to rebuild, replace, or upgrade your tensioners. I have rebuilt the ones on my '71. It might be a little more involved on an SC, depending on engine accessories like smog pumps and AC.

Some background:

911SC Chain tensioner safety collars!
Pelican Technical Article: 911 Carrera Chain Tensioner Upgrade