[T3] AAR woes

[Jim Adney]

On 24 May 2017 at 23:32, Dave wrote:

> I disagree, Jim, as your next 7 paragraphs after this line are chock full of
> valuable information; WELL SAID!!!

Thanks for the attaboy, Dave, but I'd like to add one more comment about 
our FI systems.

The D-jetronic FI system is significantly different from all the later versions 
because it works on intake manifold PRESSURE. All the later systems 
measure air FLOW past a give point in the intake system. The difference 
may seem academic, but it gives the D-jet system a significant advantage 
once the cars start to age.

In order to measure air FLOW, we have to have some sort of measuring 
device placed in the intake path that can do this. L-jet and K-jet, and all the 
later manifestations of each, do this in different ways, but it's always a single 
device in the intake path. The weakness of this system is that leaks that 
occur after this device introduce air that didn't get measured, which leans out 
the mixture.

In our D-jet systems, leaks into the IAD ALL effect the vacuum in the IAD and 
all get measured. So leaks into the IAD don't "confuse" or mislead the MAP 
sensor or the ECU (brain.) The D-jet FI system continues to work normally in 
spite of the leaks.

This is not to say that leaks are never a problem. Tiny leaks, as around and 
thru the AAR, for example, will never be noticed. Larger leaks, however, will 
make it impossible to adjust the idle speed down to where it needs to be to 
set the ignition timing. Larger leaks need to be fixed.

It's unfortunately common for leaks to occur at the connection between the 
intake air runners and the heads. Such leaks occur downstream from the 
IAD, so they produce incorrect MAP sensor vacuum readings, confusing the 
FI. They need to be fixed. These leaks commonly happen when someone 
rebuilds a Type 3 FI engine and uses the Beetle intake gaskets instead of 
the correct Type 3 FI gaskets.

The incorrect Beetle gaskets are thinner than the correct gaskets, so they 
also lead to misalignment of the intake air runners with the ports on the IAD. 
This misalignment deforms the sealing tubes at those connections, leading 
to leaks there. These leaks are not directly into the IAD, but they are close. 
They probably result in slightly leaner operation, but their main annoyance is 
that they make it difficult/impossible to adjust the idle speed correctly. The 
tell-tale sign of this is a clutter of hose clamps on both sides of the IAD.

The Beetle gaskets will also lead to leaks at the head on '70-3 FI cars. (The 
end castings on the first years were different and will at least seal there, but 
it's still important to use the right gaskets.)

I occasionally see leaks where the steel cover on the front (front is front) of 
the IAD has come loose. This is uncommon, unless someone has taken that 
cover off to clean inside and not gotten it back on tightly. Such leaks are 
directly into the IAD, so they do not confuse the MAP sensor or upset the 
fuel mixture, but they can make the idle impossible to adjust. Those leaks 
can be large, but they are usually easy to fix.  

So, to summarize, the only leaks you need to worry about are at each end of 
the air runners, plus anything that keeps you from adjusting the idle and 
timing correctly.

Daniel's original complaint about his AAR falls into the category of keeping 
him from adjusting the idle AND timing correctly. I need to take a second 
look at his AARs and figure out why they didn't close as far as they should 
have.



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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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