HELP - 69/70 Vacuum leak. What is this hose?

Top picture, what are those 2 hoses (one big and one little) right above where those electrical wires go through the firewall? Big one goes to top of vacuum tree. Small one goes to intake manifold. They come in about center under the dash and then run down on the far driver’s side under the dash to above a green electrical box and I can’t tell where they go from there. The issue is the big one is a vacuum leak and makes a sucking noise under the dash. If I pinch it shut, the sucking noise stops. I have no clue what those 2 go to. Can someone help me out???

I think it’s what I’ve got circled in red on this diagram. Except my big hose goes to the vacuum tree and the small straight to the intake manifold.

Mine is a non-A/C car.

My best guess is vent door / heater control a lot of things like headlight switch are all vacuum operated on a cougar compared to a mustang. Just unhook it and plug the port and see if anything changes. I have to replace my heater core this winter so I am in for some fun.

I believe that is for the Dist-O-Vac, which is the vacuum operated distributor control doo-dad thingamabob (can you tell I have no idea what it actually does?). It showed up on the '70’s, and I believe it has something to do with emissions. There should be a gadget that hooks into your speedometer / part of the speedo cable that goes along with it.

That’s all I got :frowning:

*I wonder if that might be part of your throttle response problems.

I missed the electrical box part, unhook it and see if your motor runs better then is it the hose leaking or is something still unhooked. My headlight switch did that if the hoses get hooked up wrong.

WOW, apparently it’s mystery hoses and the mystery Green box. I have no clue what any of it is but I’m capping those suckers off and getting back to tuning it.

Does everyone else have that little green box??? Those hoses disappear right above that box. Maybe they go into it? I’ll take the cover off that box tonight.

This what you have…

Ahhhhhhhhhhhhhhhh, thank you Don Rush. So obviously something is awry in my little bluish green box. I found more info on WCCC’s site about this setup and I’ll post it below for others to learn as I did.

The big question now is… Can I just plug off the two vacuum lines and everything be ok? The car is still running a little sluggish and I’m sure the vacuum leak at that box is at least part of the problem.


The purpose of the distributor modulator system was to help reduce carbon monoxide and hydrocarbons.

Distributor modulator system was standard equipment on some 1970 Ford Mercury Lincoln vehicles. We see it only on the 1970 Mercury Cougar and Ford Mustangs with the M code 351-4V Cleveland engine and FMX automatic transmission.

The purpose of the distributor modulator system was to help reduce carbon monoxide and hydrocarbons. Sometimes it is referred to as the Dist-o-vac system. The basic premise of the system was to use carburetor venturi vacuum to reduce engine emissions. This was performed by controlling distributor spark advance. A retarded spark provides hotter combustion chamber and exhaust manifold temperatures that cause more complete burning of the fuel.

The three main components are the Electronic Control Module, Speed Sensor, and Thermal Switch.

•Electronic Control Module is the black box which houses the PC board and solenoid valve. Ford part D0AF-9E718-S, in the manual it is described as having a black box and a blue cover, we also see them with black covers on the black box.

*Side note – There is also a white colored box along with black, blue, and white covers. This combination of boxes and lids make for 6 differently identified units for a range of other applications.

•Speed Sensor this actually is often confused with a cruise control part. It mounts inline on a two piece speedometer cable, it also has a two wire plug coming out of it. Ford part number D0AF-9E731-A.

•Thermal Switch this measures temperatures and only engages the modulator system on, at temperatures above 58°F. Ford part number D0AF—12A164-B.

For a 1970 Mercury Cougar or Ford Mustang the black module box will ‘cut in’ at speeds of approximately 23 mph and ‘cut out’ when speeds fall below 18 mph, provided that ambient temperatures are about 58°F.

Those 2 hoses I thought disappeared behind that little box actually go into the box…

Canted 393, I just done my heater core about a month ago and I truly think it’s easiest to do it with the dash pad and the entire lower dash out of the car. Mine is a non A/C car and it’s not too bad with everything out of the way. I have no clue if it’s worse on an A/C car.

Be very careful with the clips on the heater box. Don’t pry them off. They snap off a certain way. Go watch Don Rush’s video on it. He shows a ton of great tips.

Thanks for the input, I have watched it and that’s why I decided to make it a winter project along with the motor and trans rebuild.

Is that your little box? Did you remove it?

You might try just trimming some off of the end of the vac hose ~ past the leaky section, and reconnecting it.

I simply disconnected and plugged off all the vacuum hoses for now. Still working on getting it to run better. It was running decent before I put on the new brake booster and master cylinder but now it’s running rough as can be today. So I’ve plugged off every vacuum line but the brake booster and I’ll work on timing and tuning the carb today and then back track into hooking the vacuum back up to the other stuff one at a time.