[T3] Question about D jet idle issues 73 T3

[Jim Adney]

Will,

Re: the intake runners and the hoses that connect to the IAD.

Misalignment there is common, and always the result of tightening the 
IAD into position before installing the intake runners. Here's what 
should be done.

Mount the IAD and both intake runners loosely and then move each of 
them to get the best possible fit to each other. Do this with the 
connecting hoses backed off, onto the runners. Tighten these 3 parts 
into this best possible place. Grab any misaligned intake runner and 
bend it until it aligns perfectly. Slide the connecting hoses into 
place.

Personally, I hate to see hose clamps on those hoses, because they 
actually need to be able to slip a bit as the engine expands and 
contracts as it warms and cools. If you really feel that your hoses 
are so distorted that they need help, snug up some nylon cable ties 
around them.

I also have new hoses if yours are too badly distorted, but in most 
cases, if you get everything lined up nicely and then use the cable 
ties for a year or so, the hoses will straighten out and the cable 
ties can be removed.

Re: Air leaks and richness/leanness

Since the D-jet system uses the pressure (MAP) sensor to determine 
the need for gas, leaks into the IAD are always compensated for. 
Think about it this way: The throttle valve, idle adjustment, and AAR 
all amount to nothing  more than "leaks", yet the pressure sensor 
adjusts for all of these. Any other leaks into the IAD are 
indistinguishable from those "legitimate" leaks, so the brain has no 
reason and no need to compensate for them differently. All such 
"leaks" are treated the same.

This is different from most later FI systems, where leaks which occur 
after the air measuring device will lean out the mixture.

So, the leaky vac adv can is not a problem. The lack of that advance 
will mean that you get 1-2 mi/gal less fuel economy, but only 
cruising at highway speeds, where it is active. The port for that vac 
adv is only active at partial throttle.

Re: Missing at idle

If the engine runs just fine in traffic and going uphill, the fuel 
pressure must be okay. It sounds like your only concern is the 
missing at idle and popping when you take your foot off the throttle.

You're probably right about the popping being due to exhaust leaks, 
but missing will contribute unburned gas to the exhaust stream. That 
only makes sense, however, if you also have missing under load.

Is the engine missing while you're driving?

For the missing at idle, I'd look very closely at the ignition 
components. Use an inductive timing light on each cylinder in turn, 
to see if one of them fails to flash occasionally. That might point 
you in the right direction.

If it's not one particular cylinder, consider trying a different 
rotor, especially if the one you have in there is not Bosch. Check 
the resistance of the rotor; correct ones measure ~5000 Ohms, but 
some aftermarket ones are zero, which usually works just fine. I've 
had Asian rotors fail by puncturing thru the Bakelite in such a way 
that the spark could ground occasionally to the dist shaft. Such 
punctures are invisible and you'll only figure them out by 
substitution.

Clean the inside and outside of the cap and the nose of the coil. OG 
Bosch parts are your best choice here, so be wary of aftermarket 
parts, including Bosch Blue coils.

Finally, reread what you post. I often have trouble following what 
you write, as it tends to wander and run several topics together. 
Stream of consciousness is not what we're after here.  ;-)

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