What's a Heat Exchanger?

[supersportyspice]

Hi all,

I gather the heat exchanger is part of the exhaust system, but what exactly does it do, and what happens when it goes wrong?

What are the symptoms of a faulty one?

Cheers

 

[Chris_911]

It's effectively the exhaust manifold except on a 3.2 it's a manifold with an additional metal jacket.

In the 3.2 it also is the source of heat for the interior heating system, being ducted through the sills. The source of this heat is the air jacket around the manifold - this air is heated (by the exhaust manifold sitting within the heat exchanger) and drawn into the heating system (assuming the flapper boxes are set for interior heat).

What symptoms does your car have?

 

[supersportyspice]

Many thanks for the detailed reply Chris.

My car (hopefully) doesn't have any symptoms, but I was wondering what they would be so I could look out for them, as I have seen this issue being discussed on many forums.

I guess one of them would be insufficient interior heating?

Thanks again.

 

[Chris_911]

You'll know pretty quickly if you have an exhaust blow - systems the same as any other car.

I would guess that you could have a pretty large hole in the exchanger before you'd see a real difference in interior heat - something you definitely wouldn't miss on a visual inspection.

The flapper boxes that control the flow of hot air are more a source of interior heat problems than the exchangers themselves. They are only mild steel and corrode leading to eventual sticking of the boxes - open or closed.

 

[Big_nige]

Only problem if the h/e rots from within the 'metal jacket' is that exhaust gases will enter the cabin making you a bit on the sleepy side. Its common problem on aircooled VWs (similar heating arrangement) for this to happen but I've not heard it being a big problem on 911s.

If they do fail it tends to be on the flanges/joints to the other exhaust sections. I replaced mine a couple of years ago and the flanges where blowing and were barely recognisable other than for a rusty fossil. I cut the heat exchanger part open the steel was almost like new under the metal jacket, the rest was just a mess.

New ones aren't that bank breaking but if they've never been off, the studs that hold them onto the heads are usually b*ggered which then becomes a time consuming/costly job just to remove them.

 

[Chris_911]

Good point Nige.