Vehicle Emission Control Decal

[Gary Lewis]

You may be right, Bill.

Jeff - When you test please see if you can tell a difference in the total advance with the restricted vs unrestricted vacuum.  That should answer our question.

[81-F150-Explorer]

I've been wondering if he's got the right temperature PVS for his application?

Green is 125 to 131 degrees Fahrenheit.  

If the purpose of the PVS is to help cool an overheated engine, in this application, then the Green PVS is too cool isn't it?  When normal engine temp is 190F+

Wouldn't he need the 3 port Black (Original color RED) PVS marked "G1" that operates between 222F  and 228F?

Ported Vacuum Switch - Napa


Or am I missing something?

[Gary Lewis]

I think you are right about the switch not working to cool the engine.  So if that is the intent for that switch then he does have the wrong one.  But, how did we settle on the one he has, which is green?  I think maybe I've lost something?

The closest parts list I came up with is below, and your list is on the right.  How to we get from one to the other?  Both of the valve assemblies shown are said to be "black/green", but no such animal exists in your chart.    

[BigBrother-84]

Gary Lewis wrote

Clean both the timing pointer and the marks on the harmonic balancer, and put a but of chalk or white paint on the 10 degrees Before Top Dead Center on the balancer.

Why, oh why everything always seems so simple when done by others?

Went under the truck with a marker and… just don’t find any marks down there.

[BigBrother-84]

Am I supposed to see some marks there?

[Gary Lewis]

I see the timer and the notch that is probably TDC.  And if you sand the balancer either side of that notch you should see other marks.

[BigBrother-84]

Ok, I did a little archeology, soft sand paper and brake cleaner.

Found it.



There’s a big notch at 0 and a smaller at 8.

[Gary Lewis]

Ok, it looks like the big notch is TDC and there's a smaller notch at 8 degrees BTDC, which is probably factory timing.  I'd mark the 10 degree notch and set my timing to that were I you.

[BigBrother-84]

Gary Lewis wrote

Point the timing light at the balancer/pointer combo and you should see the white @ 10 BTDC.

It was setup at 12, I changed it to 10.

Gary Lewis wrote

Now put the vacuum line on the vacuum advance and the timing should advance into the 20's and the RPM will go up.

It is what happened, the advance jumped by about 20, I would say (gee, how do you do to get and exact reading?).

I reduced the idle to around 800rpm (Yeah! I installed a tach for other reason than just “nice in my cluster”! )

Is this a correct idle speed?

[Gary Lewis]

They had it at 12 probably to compensate for not having vacuum to the advance.  But going back to 10 will make starting easier and help prevent pinging.

And yes, when you connect the vacuum advance the timing will go up A BUNCH.  But you won't get an exact reading as it'll be jumping around some.

There is no "right" RPM.  At least not one everyone can agree on.  Right is what you like, something that sounds reasonably slow and yet isn't so slow it is easy to kill the engine when taking off.  I like 650 RPM, but that's on a 460 which has more torque at low R's than yours.  So you'll have to adjust it to your liking, although 800 might be a bit fast.

[BigBrother-84]

Gary Lewis wrote

Jeff - When you test please see if you can tell a difference in the total advance with the restricted vs unrestricted vacuum.  That should answer our question.

Gary, maybe the engine is too cold?
I clamped the unrestricted tube (port 3) in order to only have the restricted (port 1) working, I didn’t see any difference…


Big Bro is idling for at least 45 minutes…

[Gary Lewis]

If Ralph is right your valve/switch opens up at far less temp than your engine is already at.  So I'd pull the vacuum hose going to the vacuum advance off, put a vacuum gauge on if you have it, and then clamp first one input hose and then the other to see which one it is using.

But, it is possible the engine won't run w/o vacuum since you've changed the timing and idle stop setting.  In that case pull the vacuum line going into the valve one at a time to see which it is using.

But, just clamping off a closed circuit doesn't really do much as the vacuum remains in the system.

[81-F150-Explorer]

I've been doing some research on this and he indeed has the wrong ported vacuum switch.

He needs the NAPA one that I posted earlier that opens at 222F. I have not found a Ford Number for it yet. Although D7AZ-12A091-A might be it.

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I found a Hot Start / Drivability kit Part Number: E4TZ-12000-A in the MPC.

Fits: 1984+ F-E-U150/350 with a 351 4 barrel.  Sounds similar to what we are working on I'm thinking,

I also found this post at Vintage Mustang...

Ported Vacuum Switch Function.

So with the Green PVS it's going to open/close way too early and defeat the purpose.  

[81-F150-Explorer]

Gary Lewis wrote

I think you are right about the switch not working to cool the engine.  So if that is the intent for that switch then he does have the wrong one.  But, how did we settle on the one he has, which is green?  I think maybe I've lost something?

The closest parts list I came up with is below, and your list is on the right.  How to we get from one to the other?  Both of the valve assemblies shown are said to be "black/green", but no such animal exists in your chart.    

That chart I posted is for Ported Vacuum Switches, and I do not see one listed in the Calibration Chart.
Ford made it really confusing, but I'll try to explain.

The Basic Part Number for a Ported Vacuum Switch that controls the EGR is 9D473
The Basic Part Number for a Ported Vacuum Switch (TYPE 2) that controls the DIST is 12A091
The Basic Number For A PVS that controls Coolant temperature controls is 8A564
  ... and so on.

To make it more confusing 12A091 is the basic number for ALL Distributor Vacuum controls. There are several different types.

D5TZ-12A091-A for instance is a (TYPE 3) vacuum switch. Which is not a PVS. Picture below.



But D7TZ-12A091-A is a type 2 vacuum switch and IS a PVS.   Picture below.



Both Distributor Vacuum switches in the calibration list you posted are for type 3, which are not PVS switches.


Confused yet?   

Why couldn't Ford group all the PVS switches together in the catalog? Instead of spreading them about all over the MPC?

[Gary Lewis]

81-F150-Explorer wrote

Confused yet?   

Why couldn't Ford group all the PVS switches together in the catalog? Instead of spreading them about all over the MPC?

Yes, I’m CONFUSED!

As for Ford spreading things out, perhaps you could create table that brings it all together?

[BigBrother-84]

LOL!  Gentlemen, imagine following your discussion from my newbie situation, what is my own confusion level?


So, do I conclude that I have to change my PVS for a 225ºF model (the black NAPA one that Ralph posted)?

BTW, I did a road test and Big Bro rides very well, no pinging, good acceleration, all perfect.
(Well, success for the engine test I would say...  I had another problem that I describe here).

I did my first timing today!  Thanks so much!

[81-F150-Explorer]

Yes I would change the green PVS with the one I posted earlier from NAPA that operates at 225-F

From what I understand, the vacuum advance gets normal ported vacuum from the Carb during normal operation.

When the vehicle is turned off and the temp heats up to overheating with it parked, or the truck overheats with the engine running the PVS will switch the Vacuum advance to full manifold vacuum. This increases the timing, thus increasing the RPMs, thus increasing the fan speed, thus cooling the engine. When the temps drop below 225F it switches back to ported carb vacuum.

With the much cooler green PVS the truck will switch to overheating mode way too soon.

What is interesting, is the E4TZ-12000-A  Hot Start / Drivability kit for the 351W with 4 barrel carb does the same thing that yours has from the factory.

You may have to adjust the idle, and timing again, if your adjustments compensated for the green PVS.

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Gary, I will try to make a list of PVS's but they are literally scattered throughout the MPC. It may be a working document that needs adding to as more info is found.

[BigBrother-84]

Thanks Ralph, so let’s get this PVS!

Will be the first client at NAPA’s door tomorrow!

And who cares for having to redo a timing again, I’m now a Pro!

[81-F150-Explorer]

Looking at your emission decal ....

When the truck is running normally, the distributor vacuum goes from ported carb vacuum through the blue vacuum resistor.

When it overheats it bypasses the blue restrictor, thus increases the vacuum advance.

Quite ingenious those Ford engineers.

[BigBrother-84]

81-F150-Explorer wrote

Looking at your emission decal ....

When the truck is running normally, the distributor vacuum goes from ported carb vacuum through the blue vacuum resistor.

When it overheats it bypasses the blue restrictor, thus increases the vacuum advance.

Quite ingenious those Ford engineers.

Ralph, I suppose that you are decrypting this part of the decal?


Or maybe this one?


Does this latter indicates an idle rpm that I should target?