Ad0911

On the back of the engine there is a tube running from one cylinder head to the other cylinder head. It's name is "air tube" 92811313503" Illustration 108-00 See picture I stole on FLICKR. I cannot imagine that it really is an air tube. There must be cooling liquid running through it. Can somebody explain?

Speedtoys

The air tube connected to the air pump via a valve for emissions?

Ya, its an air tube.

Mrmerlin

it carries air for injection into the exhaust ports

Landseer

There are crush washers at the heads where the pipe screws in.

Mine on 84 came off easier than expectexd.

Ad0911

Sorry for my misunderstanding. If this is an air tube, why is it connected to the waterside of the cylinderhead. I appears to me that the tube is a cooling liquid connection between the cylinderheads, no water running through it. The checkvalve at the right side appears to block liquid going out but allow air coming in. Can anyone confirm this?

Dave928S

It doesn't connect to the water passages. The drilling goes from that back connection right through to the front where it's fitted with a threaded plug. At each exhaust port there's a small drilling from the port to that front to back air passage. The first pic is looking down the air passage, and you can see the light from a torch through from the nearest exhaust port drilling.


Air passage from rear to front. Light coming from exhaust port drilling.



Exhaust port drilling through to air passage.

belgiumbarry

Ad, just get rid of that tube and plug the holes with a matching threaded plug.
Discard the air pump junk.....

Gage

There is a similar drilling in each of the intake ports. Are those also connected to air injection system?

Speedtoys

??

Then why would they call it an 'air tube'?

Even the drawings show it works with only air...the page is called 'air injection'..

http://www.autoatlanta.com/porsche-p...section=108-00

hacker-pschorr

This is one of the differences between Euro / ROW and US heads.

US cars these are connected to the intake ports and used for the cold start injector and idle / air motor.

On the Euro cars they are connected to the exhaust ports.

Here is the cold start plumbing for US heads:

Gage

I wondered if it might be a convenient place to deliver a little wet nitrous

Speedtoys

Seems you would spend too much energy cooling the head in the liquid to vapor phase, than the cylinder.

Landseer

For ref, building on Hacker's photo,here's a picture of the air tube and the fuel system.

84 fuel system, typical LJet.
​Air tube is in center of pic.

Driver side connector is hidden under the fuel rail in picture.

you can see the 9th injector, the cold start injector in the picture just the upper left of the air tube

Landseer

If you look closely, maybe zoom in, you can see that the cold start injector gets input fuel from rail. It injects fuel into that air tube, see the mating flanges?

In Hacker's picture, the flange that accepts the cold start injector has two allen head screws sticking out of it.

So, to eliminate the air tube seems, unless I'm missing something, to require eliminating cold start methodology. Make sense?

Rob Edwards

To be clear, starting at post #10, the L-jet discussion above is for '80-84. Ad0911's motor, if I remember correctly, is for his '79. CIS, not L-jet, so none of this cold start discussion applies.

From the 1979 service info guide RE: secondary air injection: