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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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