[T3] Question about D jet idle issues 73 T3
Jim Adney
jadney at VWType3.org
Wed Jan 27 18:01:42 PST 2016
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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