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

[Dave_Darling]

Did all the D-Jet setups (on other vehicles/makes) have the decel valve?

Not even all of the 914 D-jet setups had one. If you check an early 70, such as the one the 1.7 diagram on our site was made from, you'll find there is no Decel Valve.

--DD

[RustyWa]

I wrote this info down long ago, but the deceleration valve in my old car, which was a '75 2.0L, would start to open at 18" Hg vacuum as well. If I remember correctly it opened quite fast.

[pbanders]

How could all those diagrams be wrong for so long?

Ummm, because nobody ever checked?

Even though I've got a some data that indicates that the diagrams are wrong, I'm still not 100% convinced, until I hear from a few more people that their decel valve works when connected as I've described. I've been burned before.

If it turns out to be true, my guess is that it's probably traceable to a single diagram being wrong 35 years ago. It got copied, that diagram got copied, etc. As you say, nobody went back and checked.

When people hooked it up as shown, and actually checked to see if a vacuum was present at the hose when snapping the throttle shut, they didn't feel a vacuum signal. Instead of tracking down the reason, they said "it must be bad" or "that stupid thing isn't needed, anyway", and didn't investigate further.

It seems to me that if this valve does what I am hearing, that it limits the maximum vacuum of the intake plenum/manifold to the set pressure of the decel valve.

Another name for the decel valve is "vacuum limiter". It prevents the manifold vacuum from exceeding a setpoint value. As others in the past have suggested, it also may contribute to the longevity of the MPS, by preventing high vacuum stress on the full-load diaphragm and the bellows.

Did all the D-Jet setups (on other vehicles/makes) have the decel valve?

No, it was an emissions add-on in later years. D-Jet always was a work in progress.

What is manifold pressure (I know it's a vaccuum) at idle?

I just got my engine rebuilt, and took some measurements recently. When cold, manifold idle vacuum is about 15 inHg, dropping to about 12 inHg when fully warmed up.

I would assume that the decel valve would be set to activate at a greater vacuum than you would have at idle. Hence the term decel valve.

Exactly. When the throttle is closed, and the car is moving in gear with the clutch out, the condition is known as "overrun". Manifold vacuum can exceed 20 inHg in overrun. The decel valve limits the vacuum to a value a few inHg more than the idle condition.

[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.

[pbanders]

Jeff, after reading your comment about the "needle valve", I looked at the picture and I think I know how this thing works. If you look at the small spring, on one end, you'll notice there appears to be a disc. That's the valve seat. The small spring goes over the tube from the "end" port, and the disk covers the end of the tube when the small spring is compressed. It gets compressed by the diaphragm, which pushes on the back of the disk with the dark pin you can barely see in the picture. That pin goes into the hole in the plastic part in the middle of the large plate, and is pushed by the diaphragm. The purpose of the large plate is to relieve stress on the diaphragm when the valve is closed. The large spring pushes the back of the diaphragm up against the plate when there is no vacuum applied to the valve. When vacuum greater than the setpoint value is applied, the diaphragm moves, the pin slides back, and the valve seat moves away from the end of the tube, opening the valve.

When I open one up, I'll verify this.

[John]

There must also be a mechanism to prevent flow in one direction, or it would work regardless of big hose termination.

[pbanders]

There must also be a mechanism to prevent flow in one direction, or it would work regardless of big hose termination.

True, and that's the very issue we're discussing. From what I can see of the internal construction of the decel valve, it would be impossible for there to be air flowing from the large "end" port, to the "side" port. Why? Because for this to happen, you would need to connect the intake manifold hose to the side port, which would mean that for all values of manifold vacuum, there would be no pressure differential across the diaphragm, so it would never move. If you reverse the connections, then only the small annular area of the valve seat would be under vacuum, the majority of the diaphragm would be at atmospheric pressure, and the valve would open when it reaches the setpoint value.

[type47]

Another name for the decel valve is "vacuum limiter". It prevents the manifold vacuum from exceeding a setpoint value. As others in the past have suggested, it also may contribute to the longevity of the MPS, by preventing high vacuum stress on the full-load diaphragm and the bellows.

i'd like to hear more about this. if this is so, i'll re-install the decel valve, following the corrected diagram of course (IMG:style_emoticons/default/biggrin.gif)

[rhodyguy]

jeff, when you say the rate was 10-15% slower, do you mean for the engine to come to a steady idle at a stop? we're talking seconds right?

k

[JeffBowlsby]

Yes. The decel rate is noticeable but very subtle. I like the part about it reducing stresses on the MPS diaphragm.

Good to see you posting here Kevin.

[rhodyguy]

i'm wondering if this might help with the pesky idle fall off when the various electrical circuits are energized. were the lights on when you drove home? hi.

k

[JeffBowlsby]

No I don't think so. I drove it again today during the day for about an hour with no lights on, and it was the same as last night with the lights on. I think the lighting issue may be a weak grounding, loose connection or charging system issue

[pbanders]

OK, here's what I think the guts of this thing look like, open and closed - if you want to see the picture more clearly, click on it to open it in a separate window:

[pbanders]

i'm wondering if this might help with the pesky idle fall off when the various electrical circuits are energized. were the lights on when you drove home? hi.

k

I'm working on that problem, too, and I think I'm getting closer to a solution. I think it's due to the poor nature of the alternator ground. I should have more on this in a couple of weeks. I don't think the decel valve will help this problem.

[John]

I like the diagram, but I still don't see anything that would make this a check valve (one direction flow).

Have you been able to cut one open yet?

I'm guessing there is a poppet in there somewhere which would cut off flow in the wrong direction.

just my $0.02

[pbanders]

I like the diagram, but I still don't see anything that would make this a check valve (one direction flow).

Have you been able to cut one open yet?

I'm guessing there is a poppet in there somewhere which would cut off flow in the wrong direction.

just my $0.02

The reason the valve is one way is that when manifold vacuum is applied to the side port, there's no pressure differential across the diaphragm because the same vacuum level is present in BOTH chambers, and the valve never opens.

[Bleyseng]

My testing of a decel valve shows that it opens at 20-21hg pulled on the small end. At overrun the adding of fresh air into the manifold would lean out the AFR as the diaphram is already against the stopplate.

[pbanders]

My testing of a decel valve shows that it opens at 20-21hg pulled on the small end. At overrun the adding of fresh air into the manifold would lean out the AFR as the diaphram is already against the stopplate.

Geoff, is that on the "unmolested" decel valve you have? The NOS decel valve I bought off of eBay (identical in construction to the 914's decel valve) is for a '77 Rabbit, and it's set to 15 inHg. I wouldn't be surprised if the 914's decel valve's setpoint is different.

[Bleyseng]

Yeah, I am sending it to you along with the MPS's to look at.

[John]

The reason the valve is one way is that when manifold vacuum is applied to the side port, there's no pressure differential across the diaphragm because the same vacuum level is present in BOTH chambers, and the valve never opens.

I'll buy that.