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ported vacuum switch


Mike8199

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The top one goes to the air filter so any air drawn in by the vacuum on the other two ports is filtered. The other two are for EGR and vacuum advance. The T V V (thermal vacuum valve) is an air leak when cold, to remove the vacuum signal. When warmed the air bleed closes and vacuum can now activate the EGR or the vacuum advance. EGR and vacuum advance are thus prevented from working on a cold engine. Just so you know how it works. I don't think a two hose will work for this. 

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The top one goes to the air filter so any air drawn in by the vacuum on the other two ports is filtered. The other two are for EGR and vacuum advance. The T V V (thermal vacuum valve) is an air leak when cold, to remove the vacuum signal. When warmed the air bleed closes and vacuum can now activate the EGR or the vacuum advance. EGR and vacuum advance are thus prevented from working on a cold engine. Just so you know how it works. I don't think a two hose will work for this.

really appreciate the info on this, i just gotta find out where to get a 3 port, even rock auto shows 2 ports. The only 3 port i found on rock auto was for the 2.5 diesel, which im not sure if it will thread in.
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Found one on eBay that "says" it is the right one, but I'll let you decide.  It's for a z22, which I thought ended in the 1982 model year, but I'm not so knowledgeable about 720's below 1985.  I figured worst case it might help in a direction of the search.

 

s-l500.jpg

 

http://www.ebay.com/itm/Ported-Vacuum-Switch-Original-Eng-Mgmt-9410-fits-81-83-Nissan-720-2-2L-L4-/302167966885?fits=Make%3ANissan%7CModel%3A720&hash=item465a9d44a5:g:NqMAAOSwYudXGKZk&vxp=mtr

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  • 2 years later...
On 1/22/2017 at 10:25 PM, datzenmike said:

The top one goes to the air filter so any air drawn in by the vacuum on the other two ports is filtered. The other two are for EGR and vacuum advance. The T V V (thermal vacuum valve) is an air leak when cold, to remove the vacuum signal. When warmed the air bleed closes and vacuum can now activate the EGR or the vacuum advance. EGR and vacuum advance are thus prevented from working on a cold engine. Just so you know how it works. I don't think a two hose will work for this. 

Hey Mike - 

 

When you say "cold", would you expect this thermal valve to be working properly if it's open when the water temperature is about 70degF?  I would consider that a "cold" starting condition, but I noticed the FSM says it should be "closed" below 95degF.  Mine is open at 70degF (morning temperature here this time of year).

 

I need to get the engine warmed up a bit to further test it, but in the "cold" (1st start of the day at 70degF) condition, mine is open.  Probably part of my problem. 

 

Of course, looking further at the table in the FSM, it could be that this vehicle is equipped with a TVV for California.  Maybe that's part of the problem.  I guess I won't know until I put it back together and check it under the hot condition?  I'm currently in CO and the truck could have been made for California (I suppose), but later equipped with high altitude compensation equipment (mine has that).  Is there a way to tell if the vehicle was built for California (without pulling a carfax or something?)

 

What do you think?

hHVapXGh.jpg

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It's open to leak vacuum so it can't advance the distributor OR provide an EGR vacuum signal when the engine is cold. When the engine warms up the openings close and now vacuum is sealed in and works the distributor and EGR. The distributor and EGR are separate and come on at different temperatures.

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I understand the basic operation, but it would appear the the EGR and dist advance are both connected to this thermal valve (different ports), so I’d imagine they activate at the same temp, unless it’s just slightly different as the temp rises and the thermal valve activates the two different ports, but it would probably be negligible since the temp rises so quickly. 

 

My valve is open at 70°F.  Either it’s bad or it’s the California thermal valve. 😉

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They are connected to the TVV but don't operate at the same temperature but 'close'. EGR at 140F and vacuum advance at the mid position on the temp gauge. This is not specified but would be closer to when the thermostat opens.

 

5 hours ago, spddm0n said:

My valve is open at 70°F, soneither it’s bad or it’s the California thermal valve. 😉

 

Already explained that it will be open below 140F and closed above. When open it allows air from the air cleaner to destroy the vacuum signal to the EGR and vacuum advance. Open is a huge vacuum leak. Only above 140F will it close and seal the air leak. Same with the vacuum advance.

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Ok, but then why does the chart in the FSM suggest it is closed under 95F, open between 95F and 131F, and then closed again above 141f?  It looks like it's supposed to be a range, according to the book.  In testing (unit installed on the vehicle), it did not open again at operating temp.  Do you suppose the book is printed wrong?

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I don't have an '85 FSM. Keep in mind that the vacuum signal is ported from just above the throttle plate so at idle when the throttle is closed the port sees atmospheric pressure or no vacuum signal at all and the EGR will be off. Likewise, at full throttle the vacuum signal is extremely low so no EGR.

 

To compound things there is a back pressure transducer that senses exhaust pressure, a good indication of engine load and this varies the EGR, increasing it with increased load.

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Mike - yeah, it's a little complicated with the EGR, but it makes sense as you describe it.  I did test the BTP valve (another acronym I could not find explained in the FSM), and it is behaving correctly.  It's pretty cool how they did  that, actually.  

 

How does the dist vacuum advance part work with the vacuum source on the carb then, relative to temperature?  It would make sense for the TVV to be closed when the vehicle is cold, to allow for vacuum advance.  Mine is open at 70F, such that the vacuum advance will not work.  I imagine this is the reason for the "range" of temps.  You would want vacuum advance when it's cold out, thus you would need the TVV to be close, right?

 

I also noticed the port the dist advance is connected to on the carb (different port from the EGR, but also in the bottom section of the carb), accesses the same ported vacuum area of the carb chamber (above the butterfly valve), but it also has a small hole just below the primary butterfly valve (I was studying another carb I have).  This would suggest the vacuum advance is getting mostly a ported vacuum signal, but also some direct manifold vacuum signal. ?? 

 

Either way, I imagine part of my problem could be related to cold startup (70F) without any vacuum advance on the carb, since the TVV is open, and some could also be related to something else.  I may remove the valve and put it in cold water and see if it closes down again. 🙂

Edited by spddm0n
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