[T3] '72 FI won't idle down

[Jim Adney]

Dave,

I think I've got it!

I'll bet you a beer that it's the decel valve.

The decel valve is supposed to admit extra air on engine overrun to limit 
intake manifold vacuum. It's got a spring loaded diaphram that is backed by 
a vacuum sensor line from the IAD and that spring can weaken slowly over 
time, by heat and corrosion, letting the valve open under less and less 
vacuum, until it's still open when the car is stopped and just idling.

Only '72s had the decel valves, and I took mine off at some point, possibly 
because of leakage like you are now experiencing. I really don't remember.

This is an easy theory to test. Just disconnect the large inlet hose to the 
decel valve and close it off with your thumb. If the idle drops, you've got your 
answer.

As a followup, with the large hoses both back in place, simply disconnect the 
small sensor hose. If the idle drops, it means that the decel valve is still 
working, it's just lost its calibration. Recalibration is done by turning the 
adjustment in until the idle goes back to normal. Then I'd adjust it one more 
turn.

Or completely remove it. If you do that, you'll have to cap off its connection 
to the Y between the AAR and the IAD. That's what I did. The end result is 
slightly more pollution on decel, but better driveability with more engine 
braking.

An alternative to removal is to stuff one of the large hoses with a suitable 
ball bearing or marble. That disables it but keeps it looking stock.

Hope this works. Let us know,


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