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.

[Mid_Engine_914]

And that, sir, is why you’re the king of D-Jet.

[reverie]

[QUOTE] 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.


Could you add George's diagram to this thread, please?

Thank you. (IMG:style_emoticons/default/smile.gif)

[computers4kids]

Hmm!

[StratPlayer]

This should be posted in the classic thread

[bperry]

Why not put a direct link to the diagram in the "Info" section,
since posts about hose routing pop up every now and then.

--- bill

[type4org]

With the decel valve connected the wrong way, what would be the symptoms? Does it change anything apart from lowering emissions?

[pbanders]

With the decel valve connected the wrong way, what would be the symptoms? Does it change anything apart from lowering emissions?

When the decel valve doesn't work, your intake manifold vacuum level is unrestricted during overrun (throttle closed, car moving) conditions. With the clutch released, you'd get stronger engine braking, with the clutch engaged, your engine would drop to the idle condition more rapidly. Enhanced engine braking is one of the reasons many people removed their decel valves. Having the engine drop to the idle condition more slowly seems to help with idle stability in some cars.

[type4org]

When the decel valve doesn't work, your intake manifold vacuum level is unrestricted during overrun (throttle closed, car moving) conditions. With the clutch released, you'd get stronger engine braking, with the clutch engaged, your engine would drop to the idle condition more rapidly. Enhanced engine braking is one of the reasons many people removed their decel valves. Having the engine drop to the idle condition more slowly seems to help with idle stability in some cars.

Thanks Brad. I'll take a closer look at mine (a '76) to make sure it's correct. I have a symptom where engine speed falls very quickly with the clutch depressed, it falls way below the idle level and then recovers. It's a bit of an annoyance, makes me worry that the engine may just quit every time I coast towards a stop.

[JeffBowlsby]

So I get back from vacation to find this Brad... (IMG:style_emoticons/default/biggrin.gif)

Actually I have been thinking about this too, lately. I was only recently given the Elfrink book and it has lots of great text and photos about early D-jet systems, info I have not seen elsewhere. It also includes the following page on the decel valve...with an explanation...see the text for how they say the decel valve works and notice their hose layout and the airrows marking the airflow path.

Reading through the recent testing I am a little confused by terminology and I am sure I am misunderstanding some things. Can you describe the purpose of each port? Which is the 'control' port? Is there a diagram you could post showing the internal workings of the valve? All three of my 74 2.0L 914s have been/are connected per the 'other' diagrams and my idles have been rock steady. Willing to try swapping hoses for the new configuration for grins, but cannot get to it until this weekend.

Some observations and thoughts:

Point 1 - Regardless of the correct hose layout, 2 of the three hoses are connected to the intake air plenum and I think they both read the same vacuum pressure of the intake manifold system at all times (which varies with engine speed). These two hoses are of different IDs, yet the vacuum pressure they transmit to the decel valve is the same. What is the significance of the different ID tubings, in the actual pressure seen by the valve? Could it be that while the pressure is the same, the air VOLUME transmitted through the smaller and larger tubing is different, resulting in a short time pressure difference at the valve diaphram before equilibrium is reached? This pressure difference would reduce to zero as the engine decelerates to idle speed.

Point 2 - The third hose supplies filtered intake air at a reference 0 vacuum pressure (atmospheric) at all times, this hose could be left off, except that it provides filtered air which is better for the engine than non-filtered air, the vacuum signature and other effects should be the same.

[pbanders]

So I get back from vacation to find this Brad... (IMG:style_emoticons/default/biggrin.gif)

Actually I have been thinking about this too, lately. I was only recently given the Elfrink book and it has lots of great text and photos about early D-jet systems, info I have not seen elsewhere. It also includes the following page on the decel valve...with an explanation...see the text for how they say the decel valve works and notice their hose layout and the airrows marking the airflow path.

Hey, hope your vacation was good, and thanks again for sending me that alternator harness so quickly!

Actually, Elfrink does not say how the decel valve WORKS - instead, he simply says "it opens" - he doesn't tell us WHY it opens, based on its internal construction and the vacuum signals it sees. This is the crux of the problem, because once you understand how the valve actually works, then it is clear that connected as is shown in Elfrink's diagram (which is lifted from either a VW or the 914 factory workshop manual), it cannot do what it is supposed to do.

Reading through the recent testing I am a little confused by terminology and I am sure I am misunderstanding some things. Can you describe the purpose of each port? Which is the 'control' port? Is there a diagram you could post showing the internal workings of the valve? All three of my 74 2.0L 914s have been/are connected per the 'other' diagrams and my idles have been rock steady. Willing to try swapping hoses for the new configuration for grins, but cannot get to it until this weekend.

I don't have a diagram showing the internal workings of the valve, someone was supposed to send me one so I could cut it open, but I never received the valve. The three I have are in perfect condition, I don't want to sacrifice them. The internals are similar (but not the same) as the fuel pressure regulator, with the difference being that the chamber behind the diaphragm is ported in the decel valve.

Here's a diagram with the ports labeled as I describe above:

(IMG:http://members.rennlist.com/pbanders/Decel%20-%20labeled.jpg)

Regarding the points you make about how you think the valve works in the configuration you show in Elfrink's diagram, rather than debate them point by point, I suggest you first do the experiment, which is the procedure described in the factory workshop manual (and copied in the Clymer and Haynes books) to verify the decel valve is working. Connect it as shown in Elfrink's diagram, pull the hose off of the air box, put your thumb over the open end of the hose, and open the throttle to 3500 rpm, then abruptly let it snap shut. If your decel valve is working, you will DEFINITELY feel vacuum when the valve opens. I don't think you'll feel a thing. Now, reverse the large hose connections that go to the "end" and "side" ports, and repeat. Assuming your valve hasn't been jiggered with so that the onset point is too high, you'll feel the vacuum pull when the valve opens. If not, check the valve's adjustment by removing it from the car, and using a hand vacuum pump to pull an increasing vacuum on the "control" port while blowing through the valve. It should open at about 16 to 18 inHg, if not, use the adjuster to set it.

I've done this with three different decel valves (2 used, 1 new) on both my car and on a bench setup I made, and they act just as I described above. If you find yours acts differently, we'll have to figure out why. One difference between our setups is that my plenum does not have the stacked vacuum elbow, as my plenum is physically identical to the 2.0 plenum but has different ports, a 5 mm and a 9 mm port, instead of a 9 mm and 12 mm port. I've got the "control" line of the decel valve connected to the 5 mm port, and I've got a tee connector on the 9 mm port that supplies vacuum to both the "end" port of the decel valve and the PCV valve. I will note that the stacked vacuum elbow is a part that is specific to the 914, and other T3 and T4 engines with decel valves lack the stacked vacuum elbow configuration (e.g. look at the Elfrink diagram for an example), indicating it is not essential to the functioning of the valve.

Let us know what you find out. If what I've discussed here is wrong, I want to understand why so we can figure this thing out.

[John]

I've been reading this thread and I have a question.

Please correct me if I am wrong in my understanding of what you wrote.

The Decel Valve is a pneumatically controlled one-way check valve.

Flow is only permitted one direction when the signal line goes high (16-18" Hg).

The flow through the valve would introduce ambient air (from the air cleaner) into the manifold (below the throttle valve) any time that the manifold has a vacuum of 16-18" HG. Flow direction is in the SIDE and out the END.

This would limit the vacuum that the MPS would ever see, and introduce a controlled vacuum leak until 16-18" Hg vacuum is reached.



Am I understanding this correctly?

[sean_v8_914]

this is a highjack

I would like to publicly thank Jeff and Brad for their efforts. the availability of their info has converted me and many others to FI preservation. although I do not understand its function as well as they do, I am fairly proficient at troubleshootong 914 FI. Im always willing to help the San diego guys troubleshoot their FI cars (with the help of Brad and Jeff

thanks guys!!!

[pbanders]

I've been reading this thread and I have a question.

Please correct me if I am wrong in my understanding of what you wrote.

The Decel Valve is a pneumatically controlled one-way check valve.

Flow is only permitted one direction when the signal line goes high (16-18" Hg).

The flow through the valve would introduce ambient air (from the air cleaner) into the manifold (below the throttle valve) any time that the manifold has a vacuum of 16-18" HG. Flow direction is in the SIDE and out the END.

This would limit the vacuum that the MPS would ever see, and introduce a controlled vacuum leak until 16-18" Hg vacuum is reached.



Am I understanding this correctly?

Yes. The point of debate is "what is the flow direction?". I say it's as you put it, in from the "side" port, and out the "end" port. I've verified on a bench setup that the valve operates this way, limiting vacuum to a setpoint value when vacuum is simultaneously applied to the "end" and "control" ports. I've also verified that if I simultaneously apply vacuum to the "side" and "control" ports, the valve never opens up to the maximum vacuum level I could attain with my hand pump (about 25 inHg). These results held true for three different decel valves I have. I also have verified that when I install the decel valve in my car, and connect it such that manifold vacuum is applied to the "end" and "control" ports, that when I snap the throttle shut from 3500 rpm, a strong vacuum signal can be felt on a hose connected to the "side" port, which agrees with the testing procedure for the valve from the factory workshop manual. I've also verified that if I connect manifold vacuum to the "side" port and repeat the same test, no vacuum signal can be felt on a hose connected to the "end" port. I also described that I'm not using a stacked vacuum elbow as per the factory configuration.

In an earlier thread, I asked others to try this test on their cars, one person responded and said that when manifold vacuum was connected to the "side" port, like me, they didn't feel a vacuum signal. I'd like for others to try this test, especially those with a stacked vacuum elbow in place. These cars are tricky - there's still a chance that I'm wrong about the "correct" vacuum hose configuration.

[pbanders]

this is a highjack

I would like to publicly thank Jeff and Brad for their efforts. the availability of their info has converted me and many others to FI preservation. although I do not understand its function as well as they do, I am fairly proficient at troubleshootong 914 FI. Im always willing to help the San diego guys troubleshoot their FI cars (with the help of Brad and Jeff

thanks guys!!!

Thank you - I hope the discussions here aren't too confusing!! We're all just trying to figure out this crazy car and solve those annoying idle and drivability problems that clog the forum. I'm sure in another 50 years we'll get it all right.

[pbanders]

Short update: I have a branched vacuum elbow in my parts box. I did a bench test where I used the elbow to supply vacuum to the decel valve under test. Using the branched elbow made no difference in the results - the valve only limited vacuum when the connection was to the "control" port and the "end" port, not the "side" port as most vacuum hose diagrams show.

When I get my car back from the shop (who knows when), I'll rig up a setup so that I can do the same test on the car to see if the results are the same under dynamic vacuum conditions.

[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?

[John]

How could all those diagrams be wrong for so long?

Ummm, because nobody ever checked?

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.

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

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

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.

[Bleyseng]

around 15hg at idle on a stockish motor IIRC

[John]

around 15hg at idle on a stockish motor IIRC

Then it would make sense to have the decel valve set for 16-18"Hg. It would stay closed at idle and only open during over-run conditions (deceleration).