Function of Vacuum Delay Valve
Function of Vacuum Delay Valve
Vilas
Let me share some of my experiences and opinions about the EAC3349 for
the benefit of anyone who is interested in this curious plastic
cylinder. For those without a parts catalog who may not be clear on what
this thing is, it is a little plastic cylinder like a top hat, about an
inch and a half tall and about the same in diameter. It has a steel cap
crimped over one end of the cylinder and three small pipes, two on one
side and one on the other. It connects to various vacuum hoses, in
particular, one from the distributor. It normally lives under the intake
manifold, though I have relocated all my vacuum lines to a tangle that
resembles a bird's nest between the battery and the AFM. Not pretty, but
they are easy to access until I get everything sorted out. As far as I
know, which isn't far, this was used on 4.2 L Ser III American market
cars, not on Euro cars. don't know when they started or ended. Somebody
else can fill in these details if they're so inclined. I am of the
opinion that my car has a vacuum advance like most normal cars. Timing
light tests bear this out, as well as examination of the function of the
advance when the distributor cap is removed. I've been told that some
V12 engines have a vacuum retard, which is probably where I got the
incorrect idea that this is so on the 6 cylinder cars. The belief in
vacuum retard on the 4.2 is on it's way to becoming an urban legend.
EAC3349 connects to three objects: The pipe marked "dist" goes to the
vacuum advance. The pipe marked "delay" goes to manifold vacuum by way
of a T junction which supplies vacuum to control the Delanair climate
system. The pipe marked "carb" goes to a vacuum tap just upstream of the
throttle plate. On non-USA cars, there is no EAC3349, and a hose goes
directly from the throttle plate tap to the vacuum advance. The function
of this valve is quite simple. It prevents the advance from seeing any
vacuum until there is plenty of vacuum. that is, it creates a "step" in
the vacuum advance curve. No vacuum, then full vacuum. The connection to
the manifold, "delay" has a one way valve that allows only pressure from
the manifold, not vacuum, and removes all advance if the manifold vacuum
is ever less than the vacuum at the throttle plate, though I can't
imagine when this would occur. The object of this device in terms of
emissions or engine control is beyond me. Could be that more advance
gives more NOx. So they didn't want any vac advance at partial throttle.
This is a wild ass guess. Anyone who knows, or thinks they know, please
fill us in. To test your own EAC3349, you need a vacuum source like a
"Mighty Vac", a couple small scraps of vacuum hose and a cold beer to
rinse your mouth after you're done. 1) Attach "Mighty Vac" to "Carb" 2)
Stick yer tongue on "Dist" 3) Put yer finger over the end of "Delay" 4)
Now pull some vacuum with mighty vac. You should not feel any vacuum
with your tongue at first. then at a certain point, you will hear the
valve click and you will feel plenty of vacuum on your tongue. then take
your finger off "delay" and you should suddenly have no vacuum on your
tongue. 5) Now try to suck through "delay" it should not allow any air
through, but it should let you blow freely. 6) If it does this, then it
is fine, put it back in the car and drink some beer. If not, then you
can take the valve apart by prying open the steel cover at one end.
After the spring goes flying across the garage, and you spend half an
hour sweeping the floor to find all the parts, you'll see how it goes
together, and you may be able to clean or repair it. It is a
surprisingly costly part, so it may be worth trying to fix it....or you
could run a straight line to the throttle plate. I don't know the
details of the differences in distributor advance curves from US and
other markets, so I don't really know if you can get away with this. I
hope this sheds some light on the distributor vacuum ADVANCE. I welcome
your informed comments. It would be very nice to have access to some
male and female fittings of the type used in the FI sensors and
injectors, to make it easier to experiment. Alex suggested that. Anybody
know a source for those plugs? Yours, Vilis
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