[Joe37]

I am hooking up vacuum lines on a '79 W72, going off the emissions sticker. I don't see where the air cleaner hooks up to vacuum. I have the Choke figured out, but not the preheater / stove pipe switch. I am running out of ports to hook to!

One more question, did the exhaust manifold EFE valve use hose the whole length, or was there a section of steel tubing around the back of the engine?

Ok one more question, What does the DS-VMV valve do? I assume it has check valves that close off one vacuum line when the other is active since the distributor has both ported and manifold vacuum plumbed to it. Is there any replacements for it?

Joe

[burd]

EFE had a metal pipe w/ a high heat rubber note on it.
Blue and yellow I think is the color for the hoses, blue valve and the triangular choke port on the Aircleaner. Look in the 78 new products manual.

[nUcLeArEnVoY]

Originally Posted by Joe37

I am hooking up vacuum lines on a '79 W72, going off the emissions sticker. I don't see where the air cleaner hooks up to vacuum. I have the Choke figured out, but not the preheater / stove pipe switch. I am running out of ports to hook to!

One more question, did the exhaust manifold EFE valve use hose the whole length, or was there a section of steel tubing around the back of the engine?

Ok one more question, What does the DS-VMV valve do? I assume it has check valves that close off one vacuum line when the other is active since the distributor has both ported and manifold vacuum plumbed to it. Is there any replacements for it?

Joe

There is a small air temp sensitive vacuum switch with two ports on the underside of the air cleaner that the vacuum line for the air cleaner door hooks to. Looks like this:



The other port is where the vacuum line goes to the actual manifold vacuum source on the carburetor itself. On the carb, It's the skinny port with the letter "B" above it, located to above and to the left of the fuel inlet when looking at the carburetor head-on.

The triangular sensor on the underside of the air cleaner with three holes in it that Burd is talking about is the fresh air modulator which regulates the amount of air the hot air choke gets during startup.

Finally, the EFE (aka heat riser) valve hooks to the second port (from the top down) on the big blue 4-port vacuum switch at the front of the intake. The first topmost port hooks to a metal manifold vacuum fitting located behind the water crossover on the intake, with a check-valve in line. That metal fitting, if original, should have two ports on it: one skinny port going straight up, and a larger auxiliary port jutting out at an angle, and it's threaded into the intake. They're often replaced by fittings that have just the skinny one going straight up. You want to hook it to the skinny port, while the angled auxiliary port is capped unless you want to tap it for something else.


Finally, the DS-VMV valve gives sort of a "best-of-both-worlds" scenario to the distributor vacuum advance in terms of manifold vacuum and port vacuum. It's literally the only emissions device I still use on my W72. When hooked up correctly, it permits *EXACTLY* 9" of MANIFOLD vacuum to the distributor, which helps the car keep a consistent, smoother, and cooler idle. The stock W72 vacuum advance can adds a lot of timing, which it can afford to do since the car has a lazy centrifugal ignition curve, low compression, as well as EGR that reduces combustion temps. The stock vacuum can adds up to 25 degrees advance, all in by as low as 11" of vacuum. So with the DS-VMV allowing 9" of manifold vacuum to the distributor, it's adding nearly all of the vacuum advance (around 17 degrees worth) at idle. With the DS-VMV hooked up, you'll see you're making nearly 35 degrees or so of total advance at idle if you're at the stock base timing setting of 18 degrees BTDC. This is normal.

Then, once you rev up the engine, port vacuum increases, and once the port vacuum exceeds the calibrated 9", the DS-VMV just switches the vacuum going to the distributor to port vacuum until port vacuum falls below 9" again. All in all, the DS-VMV is basically just a check-valve that gives a precise, pre-calibrated amount of manifold vacuum to the distributor without giving it FULL manifold vacuum, which for a stock W72 camshaft would be over 18" at idle. It was a compromise between consistent, smooth, and cool idle versus more emissions, since the cooler, smoother idle that higher timing provides creates more emissions. A lot of cars of the era didn't have features like this - instead, they didn't have any vacuum going to the distributor at all during idle, and therefore the timing was very retarded and they idled super hot, which while it reduces NOX emissions, was not good for the longevity of the motors.

There are no NEW replacements for it, but the 1978 W72 unit is calibrated and looks/functions exactly the same as the 1979's, it's just got a different part number and the plastic is a darkish green color instead of black like for 1979. I also heard that there are a few 80's Chevy trucks and vans that use the same essential part, just a different number and slightly diffrent appearance if all you care about is functionality. For instance, this one on eBay right here is for Chevy 350's and looks slightly different, but I bet it functions and is calibrated exactly the same: https://www.ebay.com/itm/27578780436...Bk9SR5zoqZ-EYg

I replaced my original with a NOS 1978 unit since the diaphragm in my original was weakening, and so the pre-calibrated 9" became 8" after 45+ years. That one inch of vacuum loss makes a big difference considering the large amount of timing the stock vacuum advance provides, and my engine has a mild aftermarket cam, so I need that extra little bit of timing for a smooth 750 RPM idle. I do realize I can just nix the DS-VMV altogether and hook full manifold vacuum to my distributor while dialing back my initial timing, but for some reason I actually like using the stupid little doohicky lol

Confused yet? Lol

It's a sh*t-show with the emissions junk. I know so much about the emissions system simply because I researched painstakingly about it, since I was SURE some emissions crap was the reason for a supposed vacuum leak I thought I had, since every time I would go on a drive, my idle on the tach was showing higher and higher when I'd come to a stop from cruising - the entire time, it was because my tach was inaccurate. There never was a vacuum leak, lol. In the process, though, I learned everything about the emissions system on these cars. Let me know if you have any other questions.

[Joe37]

Thanks for all that, it really helps. Finding anything about emissions is hard, let alone parts. This engine has been modified, it's now 468 ci with a aftermarket 068 type camshaft. I have rebuilt and modified the distributor, limited the advance, and speed up the curve, I need to dig out the paper work to see just how I have set. I can see how it would help idle and cooling with the valve in place, I know I have the vacuum limited to 10" .

My plan for the other vacuum stuff is place ball bearings in he lines, it will look stock but not function, other then the choke. I couldn't find a '79 screw in vacuum fitting with the one bigger outlet off to an angle, it has two smaller straight fittings. Not sure too many will know the difference and once all together, you can't see much anyway.

Thanks again,
Joe

[Trevor78]

Do you have the basic diagrams by GM, emissions and vacuum, often separate?

[Joe37]

I have the '79 emissions decal someone on this site posted, and a '77 service manual. '77 shows color codes, '79 does not, neither show the air cleaner hookup or choke.

[Trevor78]

Yeah, what I have seen was probably a larger version of the emissions label. Here's a quick googled one, probably what your have.

https://photos.classiccars.com/cc-te...s-am-thumb.jpg

[Joe37]

That's not what I have, but it shows more then the one out of the manual. Thanks for posting it.

[nUcLeArEnVoY]

This is probably the best vacuum diagram, out of the service manual:

Another thing I forgot to mention about the DS-VMV is that it works in tandem with the DS-TVS vacuum switch located on the passenger side cylinder head. Once the engine coolant temp exceeds I think around 210 degrees, that vacuum switch then switches full manifold vacuum to the distributor, bypassing the DS-VMV's pre-calibrated amount. This causes the idle to speed up due to the increased timing, which not only cools the motor in and of itself, but it also obviously makes the water pump and fan spin quicker to cool the motor off. Sort of a primitive version of an electric fan turning on full blast when the ECU detect the engine passing a set temp.

[Joe37]

Thats the picture I am going off. For now, I am going with all time manifold vacuum, with the canister limited, it should do OK. I'm still looking for the numbers for the distributor, I may have to pull it out and run it again on my distributor machine just to verify what it has for a curve.

Joe

[nUcLeArEnVoY]

Originally Posted by Joe37

Thats the picture I am going off. For now, I am going with all time manifold vacuum, with the canister limited, it should do OK. I'm still looking for the numbers for the distributor, I may have to pull it out and run it again on my distributor machine just to verify what it has for a curve.

Joe

Distributor part # is 1103315

Stock mechanical advance as seen on crankshaft: 0 degrees @ 1000 RPM, 7 degrees @ 2000 RPM, 17 degrees @ 4600 RPM. Like I said, a very lazy curve due to the stiff, heavy springs used in the distributor advance. If your springs aren't original or are worn out, you may see different numbers.

Original weights: #389 center cam, #139 weights

Original vacuum advance #1973636: starts adding vacuum advance @ 6" vacuum, fully deployed by 12" of vacuum - adds 25 degrees total advance, so roughly 4.15 degrees advance per inch of vacuum.

^^^This is from John Witzke's W72 history book.

[Joe37]

I changed it when I checked and rebuilt it. I like to get the advance in quickly, all in by 3000 rpm, with a limiter on the vacuum advance set to 10 degrees. If it needs more, its easy to change once the car gets on the road. The trick is to keep it steady up to about 1500-1800 so when setting the timing, its stable and not trying to advance. They also start easier if you can keep the initial around 10 but still advance enough for a total around 34.

Joe

[burd]

I’ve got all 4 PCs in the Aircleaner if you need the part #’s. Plus the one in the pass side head