Intake Vacuum Leak

[70F250CamperSpecial]

I have been trouble shooting a vacuum leak on my '70 360. It seems to be at the back of the intake manifold. Is this a common place for a vacuum leak?

[Jake11]

Can you take a pic?

[FORDification]

Sort of...the end seals on both ends of the intake can leak, but it's been my experience that the rear seal tends to let go more often. Those leaks seem to occur more often with intakes that have been removed/replaced by a previous owner, who didn't do a very good job sealing them up.

These days it seems most 'experts' suggest that when replacing an intake to not use the factory-style end seals, and instead use a thick bead of silicone. Getting the factory-style pieces to seal is sometimes difficult and/or short-lived, whereas the silicone almost always seals the first time and is longer-lasting. If the factory seals are used, then you at least need a dab of silicone at each end.

Sounds like an intake manifold remove/replace is in your future. (Might as well throw a nice aluminum 4-barrel piece on there...as long as it's apart.)

[Hawkrod]

Keith, leaky end seals do not cause or are related to vacuum leaks, unless you also have significant water leaks as well. The intake ports are well inboard of the water ports so vacuum leaks almost never have anything to do with end seals (even if the end seals are leaking oil). The most common reason for intake gasket vacuum leaks are slipped or damaged gaskets from lowering the heavy iron intake onto them wrong and Felpro Print O Seal Gaskets (on the FE forums they are often reffered to as Print O Leaks!). Hawkrod

[Jake11]

Is it loud enough to hear a hiss or whistle? Easy to find then with
a piece of heater hose held to ear. The intake gaskets on a 360 are
buried. It's probably a hose or something attached to the motor.
A guess. Jake

[Dragon]

Intake vacuum leaks from the manifold to head surface also are oil leaks into the intake. Unlike all other engines the FE is in an oil bath on top and oil vapor and spray on the bottom.

[FORDification]

Keith, leaky end seals do not cause or are related to vacuum leaks, unless you also have significant water leaks as well. ... Hawkrod

Whoops, you're absolutely right. Sorry about that....I was confusing a rear oil leak with a rear vacuum leak.

[70F250CamperSpecial]

I am with you Hawkrod. It seemed to me to be an unlikely place for a vacuum leak. I sprayed some carb cleaner in that area and the idle speed increases. Just to make sure it wasn't the line to the transmission, I checked it after plugging it off. I have replaced all of the vacuum lines except the pvc to carb hose, and checked all of the plugged connections, replaced the carburator gasket, replaced pcv valve, checked the carb shafts for leakage, etc. My vacuum bounces 8-9 at idle and is very steady when revved up.

[LUB69F100]

My info suggests a burnt valve or combustion chamber leak.
Maybe do a compression test?


my 2 pennies
Lub