on the back of the carb spacer I have 2 units:
D6AE-9D475-B1A 1D6
D8AE-9F452-TA DD8

One unit goes in front of the other so that when you bolt them on you have to use 2 EGR gaskets. They're connected together by a vacuum hose and the other hose goes to the ported vac on the rear of the intake.

If need be I can post a pic.

 

Let's see the picture.

PS - no ported vacuum on the intake; only manifold vacuum. Ported vacuum has to come from the carburetor, since it's taken above the throttle plates.

 

 

I thought the unit screwed into the rear of the intake was called a Ported Vacuum Switch....it has 3 nipples to connect vacuum hoses to.

Anyway, in the picture, the unit on the rear is the EGR. The vac hose from the EGR goes into the front diaphram mounted first on the carb spacer (the one I don't know what it is) and then exits that unit to the PVS on the intake via a 2nd hose.

Is it some sort of thermostat or vacuum regulator for the EGR?

When looking for replacement EGR's, I only see one unit available and I'm think of replacing the EGR since I think this one is 30 years old PLUS when you look into the 2 holes it's all gunked up with black carbon deposits.

I just didn't know if I needed to replace both units or discard the one on the front.

 

The unit on the back of the intake is called a vacuum tree. It is used to distribute manifold vacuum to multiple items - such as power brakes, automatic transmission modulator, heater controls, etc. There's no switch element - all ports are active 100% of the time.

A ported vacuum switch (PVS) mounts in a coolant path and opens and closes vacuum to one or more ports based on coolant temperature. Most often, a PVS passes ported vacuum, but some dual-function PVS's can switch between manifold vacuum and ported vacuum based on the configuration.

I've got to say, I'm stumped. I've never seen the item in your picture before. You learn something new every day. Obviously though, it's some sort of switch because all it can do is shut off or pass vacuum to the EGR valve, downstream of all the other EGR hardware. I doubt it's a thermostat as you've guessed (good guess though) because EGR systems use a temperature-controlled PVS upstream of this mystery item. If I had to make a wild guess, I'd tend to think it's a barometric sensor - maybe something to shut off EGR above certain altitudes. There aren't any other wires or hoses running to it - so it obviously senses something about the environment. Any idea where the truck was sold new?

When you look up new EGR valves, they're going to be a replacement for the valve on the back (with only one port). You can test the EGR valve easily with a vacuum pump, but they often have failed by 30 years. The carbon in the EGR spacer plate will need to be cleaned out.

I highly doubt you can find a replacement for the other piece. Since I don't know what it is, I'm not sure how you'd test it. Regardless, it should pass vacuum to the EGR at warm idle. Once you replace the EGR valve (if it's tested to be bad), run the engine at warm idle and open the throttle. You should see the spindle of the EGR valve move inward, meaning the valve is opening. If you don't see it, then the mystery valve is broken, or the PVS upstream of the mystery valve is bad too (those fail with great regularity as well).

Because of how rare the mystery valve is and the fact that it very likely is some sort of ambient environment sensor, chances are it'd be safe to remove if you find it to be causing problems. Note, however, that these pieces alone aren't the only pieces of interest - the PVS upstream needs to be inspected as well.

Hope all that helps.

 

Thanks fmc,

I have some suspicions. The truck has a manufacture date of 9/77. However, the carb spacer only has one stamped mark on it: 2 dots with the number 75 inside a circle...1975?

The EGR part# D6AE-9D475-B1A 1D6 comes back to a '76 LTD, Marquis, or Cougar w/5.8L 351M.

I'm guessing it's possible the carb spacer could have come from a '76 car...possibly the whole motor since the engine mysteriously has 2 oil dipsticks. One dipstick is on the left by the valve cover and then there's a shorter dipstick directly behind the alternator.

I probably won't know for certain until I find a block code.

The other issue that furthers my suspicions is that no EGR gaskets fit this carb spacer exactly. The EGR gaskets for a '78 F150 have larger holes and the stud bolt holes are farther apart. I had to cut the gasket to make it fit. I could find anyone local that had gaskets for the 76 Mercurys.

Anyway...I could be wrong, but I have a feeling that's the direction this is going. Heck, I might have a 351M and not a 400!

 

D8AE-9F452-TA = D6AZ9F452T .. EGR Back Pressure Transducer Valve

Fits: 1976 Mustang II & Maverick/Comet 302 exc Canada / 1976 Granada/Monarch 302/351W CA only / 1977 & 1979 Granada/Monarch 302 Canada only.

D6AE-9D475-B1A = D6AZ9D475B .. EGR Valve

Fits: 1976 LTD/Torino/Mercury Marquis/Montego/Cougar 400 (351M = D3AZ9D475F).
---------------------------------------------------------------------------------
D7PZ9A589R replaced D5AZ9A589A .. Carburetor Spacer/EGR Plate (Motorcraft CM3036)

Marked: D5AE-9A589-AB or AC / D7PE-9A589-RA // Fits: 1975/76 351M/400 LTD/Torino/Mercury Marquis/Montego/Cougar

D5AZ9D476A .. EGR Valve to Carb Spacer Plate Mounting Gasket (Motorcraft CG560)

Fits ALL: 1975/76 I-6 & V8 Passenger Cars/Trucks/Bronco's/ Econolines.

D5AZ9447B .. Gasket-Spacer Plate to Manifold (Motorcraft CG466)

Fits: 1975/76 351M/400

E5HZ9C477A .. Gasket-Carb to Spacer (Motorcraft CG539A)

Fits: 1975/76 351M/400 / 1977/96 Misc vehicles/engine sizes.

 

So what is that part supposed to accomplish on the cars?

 

Cannot recall, I haven't sold any in prolly 25 years.

Retired, I haven't worked a day in 13 years.

 

I wish I could say that.

The EGR plate in the pic looks exactley like the plate on my 302. Last fall went through and cleaned my EGR and plate and needed a new gasket. Went to Napa to get the gasket and they couldn't find one. The guy told me the trucks in 74' didn't have EGR. Ended up getting an EGR gasket for an early 80s 302. The holes were oversized and it wasn't a perfect fit but it looks like it sealed ok.

 

Thanks, Numberdummy!

That's some interesting info. It looks like at least the carb spacer is a transplant from a car. I'll look at trying to get a new EGR for one of those models listed.

As for the Back Pressure Tansducer....since MOST vehicles only have the EGR valve itself, I'm wondering if I could just ditch this transducer thingee and slap a new EGR on by itself.

I suppose it would be easy enough to test drive and watch mpg and idle condition without it?

 

The transducer changes the amount of vacuum to the EGR valve based on engine load (vacuum). Thats what they do in general. I've never seen one on a truck like this.

 

Hmmm...it may be worth a trip to the junkyard to see if I can scarf the right carb spacer. If not, I may try the LTD/Cougar EGR.

 

That should work fine. My EGR valve bolts directly to the plate. Like I said before that plate is identical to the one on my 302 but your EGR is bigger all around than mine. Looks like they kept the 351/400 egr valve and swiped the spacer and ducer dohiggy from a car.

 

The problem with NAPA, and other parts stores...is, their computers do not have listings for older models, but their paper parts catalogs do.

The other problem is, getting these beetle-brained parts clucks to actually look in their paper parts catalogs.

From the 1973/79 Ford Light Truck Parts Catalog:

There are F-O-U-R different EGR valves listed for 1974 F100's with a 302.

D3BZ9D475F / C4 / Marked: D3BZ-AA or AB / D4TE-A1A or A2A.

D3OZ9D475D / C4 / Marked: D3DE-CA / D4DE-B1A or B2A.

D4TZ9D475E / M/T / Marked: D4TE-E1A or E2A.

D4TZ9D475K / M/T / Marked: D4TE-K1A or K2A.

D5AZ9D476A .. Gasket-EGR Valve to Carb Spacer Plate.

Fits ALL 1973/76: F100/350 240/300/302/351M/360/390/400/460 // Econoline 240/300/302/351W/460 // Bronco 302.

Fits ALL 1973/76 Passenger Cars with I-4's, I-6's, V8's.