Vacuum lines Troubleshoting
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New User
Joined: Jan 2011
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Vacuum lines Troubleshoting
The previous owner of my truck appeared to care little about whether or not lines were connected. i need help figuring out where to attach some new lines. Does anyone know where i could find a diagram for a 302 vacuum line set up?
Senior User
Joined: Mar 2006
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You'll need to post a little more info. What year truck? What options does it have? AC, Power Brakes, carb type, etc. I may have "a" diagram for my truck that I'll have to find first. It may not be helpful though if the trucks aren't similar.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
There should be a vacuum tree on the back of the intake, or on the firewall with a line running to it. This should be sourced from manifold vacuum.
If you have power brakes, the booster runs from manifold vacuum off the tree. If you have an automatic transmission, then the modulator valve on the transmission runs to manifold vacuum as well.
The distributor vacuum advance typically runs from ported vacuum. There should be a small 3/16" vacuum port on the passenger side of the carburetor under the choke, often referred to as the "spark port," which sources ported vacuum for the vacuum advance.
If you have EGR, then the EGR valve is typically sourced from ported vacuum, through a ported vacuum switch (PVS). There should be another source of ported vacuum on the front of the carburetor, near the driver side idle mixture screw. There were a handful of different setups and types of PVS's used throughout the years, but what I've just mentioned will get the job done. The PVS threads into a coolant passage and is typically found on the intake (for wet manifolds) or threaded into the thermostat housing.
If you still have the factory air cleaner with the hot-air intake door at the end of the snorkel, then manifold vacuum goes to the temperature switch in the air cleaner, and the other port on the switch goes to the vacuum motor on the snorkel.
Last but not least, the heater controls come from a 3-port check valve. Manifold vacuum goes to one side of the check valve; the heater controls and vacuum reservoir (looks like a coffee can) run to the other side of the valve.
If you have power brakes, the booster runs from manifold vacuum off the tree. If you have an automatic transmission, then the modulator valve on the transmission runs to manifold vacuum as well.
The distributor vacuum advance typically runs from ported vacuum. There should be a small 3/16" vacuum port on the passenger side of the carburetor under the choke, often referred to as the "spark port," which sources ported vacuum for the vacuum advance.
If you have EGR, then the EGR valve is typically sourced from ported vacuum, through a ported vacuum switch (PVS). There should be another source of ported vacuum on the front of the carburetor, near the driver side idle mixture screw. There were a handful of different setups and types of PVS's used throughout the years, but what I've just mentioned will get the job done. The PVS threads into a coolant passage and is typically found on the intake (for wet manifolds) or threaded into the thermostat housing.
If you still have the factory air cleaner with the hot-air intake door at the end of the snorkel, then manifold vacuum goes to the temperature switch in the air cleaner, and the other port on the switch goes to the vacuum motor on the snorkel.
Last but not least, the heater controls come from a 3-port check valve. Manifold vacuum goes to one side of the check valve; the heater controls and vacuum reservoir (looks like a coffee can) run to the other side of the valve.
