Testing AB Valve (anti-backfire valve) - weird results.

[spddm0n]

I was pretty sure my valve was shot, as it seemed like the engine had a very large vacuum leak.  I tested the valve on the truck and found it was passing air straight in from the air cleaner, with no vacuum present on the vacuum line on the side of the valve.  I decided to remove the valve, since it was in my way for getting to some vacuum lines I wanted to inspect.  I decided to test it further and found something interesting...

 

When there is no vacuum applied to the small port (nipple) on the side of the valve, it (correctly) does NOT allow air to pass though the valve.

When vacuum is applied, it correctly opens the valve and allows air to pass through the valve.

When the vacuum is REMOVED from the port on the side of the valve, it does NOT close immediately.  It takes 15 seconds to close the valve again. This is repeatable.  Each time I remove the vacuum source, it takes exactly 15 seconds before the valve closes again.  ??

 

I thought I read somewhere (cannot figure out where I read this) that it takes several seconds for the valve to close off again.

Anyone know if this is CORRECT behavior of the valve?  Should the valve close immediately, when vacuum is REMOVED, or is it normal behavior for the valve to take sometime to close off again??

 

Thanks!!

[datzenmike]

I wouldn't think so. If activated, and you were to shift into neutral or put the clutch in, the engine would stall.

[spddm0n]

Ok, after further testing, I figured out the weird behavior.  Turns out...when the vacuum levels rise quickly (0-25 in-hg), it is very difficult to determine what the valve is doing.

I connected it to a vacuum pump and slowly allowed the vacuum to increase.  This valve is opening at about 4 in-hg, rather than the FSM required 24.86 in-hg.  It's opening way to soon.  Then, it stays open until the valve reaches 4 in-hg again, as the vacuum levels are reduced.  Clearly, the valve is bad. 🙂

 

I can't find anywhere that sells the valve either.  I'm beginning to find similar cases with other vehicles, in which you simply cannot buy new replacement parts for items that ideally require newly manufactured parts (not good used parts), such as emission equipment.  Unfortunately, our laws are not allowing for the gap between age of these vehicles, emission requirements, and availability of parts.

 

I removed the valve and I'll visit the emission testing center soon and see if I can pass the visual inspection without it.  I imagine it won't affect normal driving conditions, but they may still be interested in monitoring the hydrocarbon output during deceleration.  I'll find out. 🙂

[Charlie69]

I live in Phoenix and am running a Weber DGEV 32/36 on my 86 720 feedback truck.  I am running the Weber rectangular air cleaner with the valve cover vent connected to it.  I am running the EGR but deleted both AB valves and all the shwitches and gadgets associated with the air cleaner. 

 

I replace all the vacuum lines a couple months ago and crossed up the EGR lines.  I failed emissions last Saturday on NOX and bad gas cap.  I rerouted the lines for the EGR and replaced the exhaust manifold gaskets.  On my way to the emmisions I stopped and purchase a new gas cap and a can of Seafoam.  I went to the gas station and put the Seafoam in the tank and topped off my tank.  I always buy premium gas for my Datsuns/Nissans.  Went to emisiion and passed with all the areas at about 50% lower than maximum aloowed figures.

[datzenmike]

On 6/21/2019 at 8:41 AM, spddm0n said:

 

I can't find anywhere that sells the valve either. 

 

Wrecking yard. Get a used one. It's original equipment looking. No one will question it.

[spddm0n]

12 hours ago, datzenmike said:

Wrecking yard. Get a used one. It's original equipment looking. No one will question it.

 

Thanks Mike.  I did just find the valve part number information on nissan4u.com (somehow missed it before), so I can cross-reference 8 different part numbers for the the AB valve - for model compatibility and see if I can find another similar model vehicle in the salvage yard that has one.  It looks like compatibility might go as far back as 1979.

 

There are zero of these trucks (or Z24's for that matter around here).  I had to "buy" a replacement carburetor from another customer in the yard who was already taking the engine out when I arrived to get it, while also paying the yard itself - just to get out with anything for this engine. 🙂  They are non-existent.  I also fear a used one (like all the used EGR's I've ever tried) would not work anymore.  Thank goodness it's not a bad EGR valve (+$200 now-a-days).

 

Thanks!

 

 

Edited June 27, 2019 by spddm0n

[spddm0n]

12 hours ago, Charlie69 said:

I live in Phoenix and am running a Weber DGEV 32/36 on my 86 720 feedback truck.  I am running the Weber rectangular air cleaner with the valve cover vent connected to it.  I am running the EGR but deleted both AB valves and all the shwitches and gadgets associated with the air cleaner. 

 

I replace all the vacuum lines a couple months ago and crossed up the EGR lines.  I failed emissions last Saturday on NOX and bad gas cap.  I rerouted the lines for the EGR and replaced the exhaust manifold gaskets.  On my way to the emmisions I stopped and purchase a new gas cap and a can of Seafoam.  I went to the gas station and put the Seafoam in the tank and topped off my tank.  I always buy premium gas for my Datsuns/Nissans.  Went to emisiion and passed with all the areas at about 50% lower than maximum aloowed figures.

 

Thanks Charlie - Yeah, I simply may be outmatched by this carburetor and control system.  I'm realizing just how complex it is,when I didn't understand how the clutch switch was playing a role in how the engine was running!  I almost posted a response about how I was sure when I disconnected the clutch switch (it was just hanging down from the dash when I bought it - and doesn't quite fit in the pedal assembly), the engine began to run badly.  I figured people would suggest I was wrong, so I triple checked it's behavior and then went into the wiring diagrams.  I learned the ECC is using input from the clutch switch and the neutral safety switch to adjust the mixture control solenoid when the vehicle is idling.  One must have the correct clutch switch connected (or modified) to ensure proper engine idling performance on this engine with the Hitachi carb! 🙂

 

I didn't make it to emissions yet, since it simply isn't running that well while driving.  Low on power and irregular idle.  I tested the coolant temp sensor the other day and it does not reflect the correct resistance figures outlined in the FSM.  I'll replace that.

 

I also know the altitude compensation solenoid doesn't work (there is no clicking noise when voltage is applied).  I tried simply connecting the vacuum lines together to simulate it operating at altitude (I'm at altitude all the time 🙂), but it appears the ECC may also be using the input from the altitude compensation "switch" (which I think could be working - though I could not find a valid test for it), so I don't know if this is causing a problem.

 

I'm not prepared to buy another carburetor at this point, so I'll keep at this, but I'm truly humbled by the complex nature of this design. 🙂

[spddm0n]

12 hours ago, datzenmike said:

 

Wrecking yard. Get a used one. It's original equipment looking. No one will question it.

 

I also just realized (after studying the shape of the AB valve from the images I found with the proper part numbers for the AB valve) I think the AB valve I have is NOT the correct valve anyway! 🙂  One of the previous owners must have attempted to replace it with a "similar" one. 🙂  Mine has one large hose entering/exiting on the bottom of the valve and one on the side, rather than two hoses on opposite sides of the valve.  That could be part of the problem. 🙂

[Charlie69]

I will look this weekend and see if I still have the AB valves I took off my 1986 king cab.  If I do you pay for shipping and they are yours.

[spddm0n]

On 6/28/2019 at 7:42 AM, Charlie69 said:

I will look this weekend and see if I still have the AB valves I took off my 1986 king cab.  If I do you pay for shipping and they are yours.

 

Charlie - 

 

That would be great!  Let me know if you find them and I'll PM you my address.  Thank you for such a wonderful offer!

[Charlie69]

Is your truck a 5 speed or an automatic?  I believe they are different.

[spddm0n]

It's a 5 speed. I remember the FSM talking more specifically about the BCDD on the side of the carb, related to the automatic transmission, but according to the part numbers on the Nissan Parts Deal website, the 5-speed manual and auto trans versions of the AB valve are the same.  Either one should work.

[Charlie69]

I will look this after noon.

[Charlie69]

I did not find the valves.  I will be in the junkyard this next weekend if there are any there I will grab them for you.

 

Is your truck Federal or California emmissions?

Edited July 1, 2019 by Charlie69

[spddm0n]

41 minutes ago, Charlie69 said:

I did not find the valves.  I will be in the junkyard this next weekend if there are any there I will grab them for you.

 

Is your truck Federal or California emmissions?

Thanks Charlie!  No sweat.  Yeah, let me know if you find one. 🙂

 

I have a federal emission model.