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Mysterious Leak/Noise

Hey there. I noticed a mysterious leak and hissing/sucking kind of noise coming from behind the steering wheel/on the firewall in my 76 lincoln mark iv. I think I tracked it down to what seems to be called a vacuum supply manifold based on the part number I got from a sticker (I can’t find an exact match, but some results come up for products that look similar). There seems to be an orange fluid leaking from it (my first thought is brake fluid, but I can’t tell for sure) along with a hissing/sucking noise. My car has been harder to start recently and has a tendency to almost stall out when it is first started as well as stuttering when trying to accelerate.

The problem is that I can’t find an exact match to the part number I found (d5sa-9c490-aa), and I want to make sure I replace it with the right part.


These cars often used engine vacuum to control the heater/AC settings, moving little doors to direct air to the upper vents vs floor etc.
Also possibly to automatically release the parking brake when the car is put in gear.
As a stop-gap until (if) you find the right part I would track down the vacuum supply hose (going to the intake manifold or carburetor plate) and cap or plug it off to stop the leak.
You’ll lose some features but it’ll run better and not make funny sounds.

I am leaning towards a dual problem, 1 the vacuum hose for climate controls has a leak, 2 the speedometer cable has a leak.

Can you post a photo of the leaking part?

This is the best picture I have right now. I can try to get a better one tomorrow if it would help.

hmmm … yours seems to be bolted to the firewall, but my Ford truck has something similar, only it is bolted (or screws into ) the intake manifold at the rear of the engine. But otherwise it looks very similar. It just offers up a bevy of places you can plug a vacuum hose to, with various diameter of fittings is all. Are you sure the manifold is broken? Usually what happens is a vacuum hose will break right at the fitting, and produce a hissing noise and make the car idle and otherwise perform poorly. Orange fluid sort of makes me think auto transmission fluid. You got a c4 in that beast? If so you might have a problem with its vacuum modulator.

It’s a C6. I checked the fluid levels for everything (including the transmission), and they all seem fine, but the leak seems to be pretty gradual so I probably wouldn’t notice right away. The trans fluid is a pretty dark red color though, where this seems to be an orange color, though there is very little so it’s hard to tell the color for sure.

I’m not sure that the manifold itself is broken, I just know that’s the area where the leak and sound are coming from. I’ll check to see if it looks like it is any of the hoses rather than the manifold that seem broken/leaking.

In playing around under the hood, I found that this vacuum hose was disconnected from the carb, so I tried connecting it and starting the car.

Upon trying to start the car with this hose connected, there was a loud pop and some smoke coming from around the carb. I took the air cleaner off, and the smoke seemed to be coming from around the vicinity of that hose, but I couldn’t tell for sure. Upon further investigation, this same line seems to possibly be the one that is leaking the mystery fluid, so maybe whatever that fluid is upset the carb. This hose comes out of the intake and up to the carb before going to the supply hose (though that order could be backwards, I don’t know the flow of air).

This may be unrelated, but it also seems a lot of the carburetor is wet with some liquid, especially near the front.

I’ve taken a couple more pictures. The orange-stripe hose (which is wet near the “hub”) is the one that splits off the connector that was disconnected from the carb in the previous picture, and the large diameter hose (also wet at each end) is the one that goes to the vacuum manifold. The blue “hub” is bolted into the intake.

That vacuum hose on the carb you re-installed if I’m interpreting your photo correctly, it appears to be a ported vacuum source, from above the throttle valve in other words. That signal would usually go to the distributor to advance the timing on accelerations, to the EGR (often via some coolant temperature switches), and possibly to the transmission. Ford had both one input and two input vacuum modulator designs in that era. If you have the two input type then the above would apply. It might be when you re-connected it, it advanced the ignition timing and that is what caused the pop you heard. Maybe you got a little ping or backfire into the intake manifold or something. Normally that wouldn’t do anything at idle tho, as there is little to no vacuum from a ported source at idle. You could measure it I suppose. I expect you’ll find there’s no vacuum there at idle, but significant vacuum when you bump the accelerator.

Suggest you also connect a hand held vacuum pump to the line(s) which supply the transmission vac modulator valve. They should both hold vacuum to 20 inches. If not, you got a leak in those lines or the modulator itself.

It appears you got some cyphering to do there OP. On that era Ford 302 the vacuum line configuration is very important to have it all connected correctly. Until that bunch of spaghetti is all correctly configured, it doesn’t make much sense to try to solve other engine performance problems. I suggest as your first task that you post a diagram of how it is configured now. Look around in the engine compartment, see if you see a vacuum hose emissions diagram somewhere there.

The experts here helped me w/a similar vacuum line configuration problem on my own 302 equipped truck, here’s the link. At the time I included a line-drawing of the configuration that I updated a couple of times with corrections, but for some reason that diagram has been deleted. Perhaps uploaded diagrams are only stored for a certain amount of time on the Car Talk server. At least I don’t see it when I look at that thread. If anybody here knows how to resurrect that diagram, the Lincoln owner OP of this thread would probably like to see it.

Upon further investigation, the line leading to the transmission is full of transmission fluid. Does this mean that the vacuum modulator valve is bad?

Yes, I expect the diaphragm has a leak in it. That will allow transmission fluid into the vacuum line and get sucked into the engine, not a good thing. Replace the modulator isn’t a big job tho, and I doubt it costs much, so that’s the upside. The way to tell for certain is the hand held vacuum pump test I mentioned above.

Got it. Looking at parts at a couple different parts shops I see options for purple stripe, black stripe, green stripe, etc). How can I tell which one I need?

You’ve got me there. I’ve never had to replace my truck’s vacuum modulator. But I know there is more than one version. First off, does yours have one vacuum input, or two?

The vacuum modulator, in case you are interested, is part of how the transmission decides what gear it should shift too at that mph. It has some flexibility there, b/c you might prefer it to be in 2nd rather than 3rd to get more power, so that’s is part of the reason there is more than one design. It is more important I expect to make sure you use the correct number of vacuum inputs, and what exact version you use beyond that probably won’t make much difference unless you are entering a drag race.

It appears to just have one, there is only one line going from the engine area down as far as I can tell. I was curious as to what it did, and that may explain some shifting weirdness I’ve had recently, too. Should I just go with whatever is cheapest (that seems reliable)?

A failing vacuum modulator will result in unusual shifting behavior. Usually for the type of VM with only one vacuum input, I think that’s connected to the intake manifold vacuum. Not the ported carb above the throttle plate vacuum. My truck has the two input type. Didn’t you mentioned it was connected to a vacuum port on the carb, or am I misinterpreting?

The transmission can only be in one gear, so in that sense it is a digital device. The way it decides which gear to use, there’s pump inside the transmission who’s output is a pump pressure that is proportional to the vehicle speed. That’s one input to a decision making hydraulic valve located in the valve body. It’s a push/pull gadget, with hydraulic pressures pushing against a piston from both ends. The other side of the piston’s input is the output pressure from the vacuum modulator, which is a function of the vacuum input level. Those two pressures push on opposite sides of the decision making valve’s piston. Whichever pushes harder causes the piston to move in that direction, and that’s the direction the transmission will shift.

Visit your local Ford dealership when they aren’t busy. Look for a tech there with not too many teeth, an older guy in other words, and ask them.

Wipe off the old modulator, there may be some identifying paint left on it.

It does come from the manifold vacuum supply, sorry for the confusion.

Thanks for the explanation!