The Decel Valve Hose Diagram for the 2.0L Djet is Wrong, It doesn't work unless it's hooked up differently Options

[pbanders]

I made some postings before the "great split" about the decel valve where I asked questions on how it worked. I figured it out, and now I understand why I was confused.

If you look at the hose diagram Dave Darling made for the 2.0L, and I also believe it's the same way in Jeff Bowlsby's diagram, they show the "side" port to the decel valve connected to manifold vacuum, and the larger "end" port connected to the air box. George Hussey of Automobile Atlanta posted a diagram in a thread on the '74 hose configuration that showed it connected oppositely. Which one is right?

Turns out George's diagram is correct. As I complained in the earlier thread, I couldn't see how the valve worked when hooked up the way Dave and Jeff's diagrams showed. Not their fault - it's shown that way in the Factory Workshop Manual (see the FI manual, page 0.1-1/3, page 0.1-2/1) and all the other references I've seen. The problem is that if you pull the same manifold vacuum on both the side port (which is on one side of the internal diaphragm) and the skinny end port (which is on the other side of the internal diaphragm), there's no pressure differential to open the valve.

It didn't occur to me that the majority of diagrams might be wrong until I saw George's diagram. Now it makes sense - the valve works to limit vacuum if connected so that the large end port is on manifold vacuum and the side port is connected to the air box. Why? Because in this configuration, only the control side of the diaphragm is initially connected to vacuum (actually, it's slightly different from that - a very small area on the other side of the diaphragm is under vacuum where the valve seat for the large end port is located). Once the vacuum is high enough to overcome the internal spring resistance, the valve opens and the intake manifold vacuum is limited.

I tested three different decel valves on the bench, two used 914 valves, and a NOS valve I picked up on Ebay for a Volvo D-Jet application. I used a universal plastic tee connector to connect my hand vacuum pump to both the control port and either the large end or side ports of the valves. In each case, if I connected vacuum to both the control port and the side port (same vacuum across both sides of the diaphragm), the valve never opened up to 25 inHg of vacuum. If I connected the vacuum to both the control port and the end port (vacuum differential across the diaphragm), then at an onset vacuum, the valve opened, and I could not pump to a higher vacuum level - the vacuum was limited, as the valve is supposed to do.

Both of my 914 decel valves had been "adjusted", so I had no idea of what the correct onset vacuum should be. I recently had my motor rebuilt and did some manifold vacuum level testing. Fully warmed up, at idle, I am running about 10 to 12 inHg of manifold vacuum. If I rev the motor to 3500 rpm and snap the throttle shut, I see a maximum of about 22 inHg of manifold vacuum. The decel valve for the Volvo still has the factory paint mark on it and hadn't been tampered with. Measurements on that valve showed an onset of about 15 inHg. Seemed like a good number, so I adjusted my valve to that level and reinstalled it.

Now, when I test it as shown in the Factory Workshop Manual, where you open the throttle to 3500 rpm and snap it shut, you can feel it pull vacuum when the valve opens. I haven't had time to drive it around much since then (working on other problems), but as soon as I do, I'll report on any issues. I may increase the onset vacuum to 18 inHg, as when my motor is cold, it develops about 15 inHg at idle, about the same as the decel valve, and I don't want it to act as a leak.

[JeffBowlsby]

Kudos Dr. Anders. (IMG:style_emoticons/default/smile.gif)

Well I studied those decel valve cadaver photos on the train ride home tonight...1 hour...still doesn't make complete sense to me, need better pics. I can make some sense of the upper half of the valve, but the lower end and small spring and other parts are still a mystery. Scalpel please.

Did you notice the small dark needle valve part lurking in the shadows of the opened valve? Does the red flex diaphragm have a small metering hole in its center or is that just a dirt smudge? Somehow I imagine that the flex diaphragm and the metal stop plate create a pressure differentially-operated valve that operates the needle valve, regulating intake air entering the side port, which then exits the big end port to feed the air plenum on decel, but I do not understand the mechanics yet.

I also swapped the big end and side hoses for my 20 minute cross-town jaunt home from the train station tonight. I noticed that the engine decel rate slowed down some over what I have always known and personally liked it. Was not a huge difference, only slightly noticeable, the decel rate was maybe 10-20% slower at most. I have not tested what my activation pressure differential calibration is yet.

I think I will keep it this way and change my hose diagram. How could all those diagrams be wrong for so long?

[pbanders]

Kudos Dr. Anders. (IMG:style_emoticons/default/smile.gif)

Well I studied those decel valve cadaver photos on the train ride home tonight...1 hour...still doesn't make complete sense to me, need better pics. I can make some sense of the upper half of the valve, but the lower end and small spring and other parts are still a mystery. Scalpel please.
(other comments snipped...)

The pics Jeff is talking about are at:

http://www.914world.com/bbs2/index.php?sho...ic=8803&hl=

Thanks to Rod for posting these. They're good, but not enough to figure out how the thing works exactly. I spent about 20 minutes last night using a small light and looking in the side port of a valve I had as I opened and closed it, and as you say, there appears to be more going on at the valve seat. The basic operation is pretty simple - you pull a vacuum on the skinny control port, and something inside that's snugged up against the top port tube moves and the valve is open. Exactly what's moving is not clear, I'll need to open up a valve to look at it more closely. I'm still waiting to get a sacrificial valve to do this with.

I also swapped the big end and side hoses for my 20 minute cross-town jaunt home from the train station tonight. I noticed that the engine decel rate slowed down some over what I have always known and personally liked it. Was not a huge difference, only slightly noticeable, the decel rate was maybe 10-20% slower at most. I have not tested what my activation pressure differential calibration is yet.

So, it looks like your decel valve started working when you reconfigured it, too. And you have the branched vacuum elbow, so that's one more item out of the picture.

I think I will keep it this way and change my hose diagram. How could all those diagrams be wrong for so long?

Give it just a bit more time before you change your diagram. I'd like to take a valve apart and fully understand its operation, and get some more owners to weigh in with results from their cars.