Here's a large, hi-res diagram that is color coded to make it a little easier to distinguish between different vacuum sources. We'll dissect this thing piece by piece to make it easier to figure out.
|Color-coded FSJ Vacuum Diagram for V8|
Let's step through the diagram to understand it better.
Vacuum Spark AdvanceSpark advance is based on engine temperature as follows:
- Under 160°F: mixture of manifold and vacuum advance
- Over 220°F: manifold vacuum
- Otherwise: ported vacuum
Knowing how the system behaves, let's trace the vacuum hoses from the distributor backwards.
The distributor vacuum hose traces back to the middle "output" port of the HDC CTO (Coolant Temperature Override) vacuum switch; that's the CTO on the thermostat housing. The middle of these type of CTO's is the "output" and based on temp, it switches between one of the two "inputs".
|Spark advance vacuum routing|
If the engine is too hot, above 220°F, it draws from manifold vacuum. Why? Because the theory is that this will bump up the idle and thus cool the engine down. Does this work? I doubt it.
|Hot (>220°F) spark advance vacuum routing|
|Normal spark advance vacuum routing|
|Normal temp, spark vacuum advance routing|
|Cold temp, NLVR vacuum routing|
Simplify Vacuum Advance Routing
|Simple FSJ Vacuum Routing|
What you end up with is the Spark CTO gets Manifold vacuum when cold, and ported when hot. This is how the earlier model FSJs were "wired".
You can also ditch the high temp (HDC) CTO. Run the Vac advance line direct to the Spark CTO "output" (middle). Remove the T fitting for the HDC CTO that goes to manifold and replace with straight or run new line.
Make sure you preserve a manifold vac signal going to the power valve!! (or else it will always be on!) This greatly simplifies the spaghetti. You now have one CTO where you used to have a 2 CTOs and an NLVR.
EGR Vacuum Control RoutingLet's move on to the the EGR system. It's operating is also based on temperature. If the EGR is enabled, it only operates on part throttle load. Not full throttle, and not lightly loaded part throttle or idle. The EGR is only enabled when both the engine coolant temperature and incoming air charge are warm.
Starting at the EGR itself, it is ultimately sent a ported EGR signal from the carburetor. But that signal only makes to the EGR if the air drawn into the motor is warm, as determined by a sensor on the side of the air cleaner housing called the TVS (Thermal Vacuum Switch). When it's warm, it lets vacuum through. When it's cold, it blocks vacuum signal, disabling the EGR.
|EGR Vacuum Control|
Also, there's often a delay valve inline between the EGR and the TVS so that the EGR comes on gradually after a short delay. However, one service bulletin fix for pinging is to remove this. So you may not have it. That's the EGR system.
AIR System Vacuum RoutingLet's look at the AIR system. AIR stands for Air Injection Reaction and it injects air into the exhaust manifolds under certain conditions to oxidize unburned fuel and at other times into the catalytic converter to aid its operation.
The source of this air is the AIR pump. The air goes to a diverter valve (AIR Control Valve) that sends air to the manifolds or to the catalytic converter, based on engine temp as well as engine load.
Under acceleration the signal goes away which diverts air to the cat (to address the vehicle running rich). When cold or under cruise, the air is sent to the manifolds to further oxidize the outputs of combustion.
On 86+ FSJs (see below) the AIR Control Valve also includes a valve to allow or shut off the flow entirely; when the vehicle starts up, it turns on the valve with manifold vacuum. During acceleration, when manifold vacuum drops, the reverse delay keeps the system on.
|86+ FSJ Air Control Valve Vacuum Routing|
|AIR system vacuum routing|
And that's it for the AIR system.
Thermal Air ControlThe Thermal Air Control (TAC) system operates the valves on your air cleaner snorkle to regulate the intake air temperature.
When the engine is on, manifold vacuum is held and applied to the main valve in the snorkle; when the engine is shut off, the valve closes, helping to prevent evaporation of fuel, presumably.
A TAC sensor, in essence a thermal vacuum switch, tells the second valve to select hot air drawn across the exhaust manifold through the silvery corrugated downpipe that you probably lost years ago or, once the engine is warm, from cooler outside air, through the corrugated black tube going to the front of the engine bay near the battery if you didn't lose that too.
|TAC vacuum routing|
Vacuum Brake BoosterAnd that's pretty much it. You also have a giant vacuum tube going from the front manifold brass fitting to the brake booster.
Heater and 4x4 ControlThere's supposed to be a vacuum signal going to the round ball -- a vacuum reservoir for your 4x4 and heater.
Cruise ControlAnd also vacuum to the black coffee can on the passenger side-- your cruise control reservoir. Those aren't shown on the diagram.
PCV, Bowl Vent, Gas Tank VentThe diagram above shows your PCV (Positive Crankcase Ventilation) routing, bowl vent and purge valve routing, and gas tank vent routing. The carbon canister opens up to allow the carburetor to draw fuel vapors in by way of the purge valve, operated by ported vacuum. Under part throttle heavy load, the carburetor sucks some fuel vapor out of the canister.
|PCV, Bowl Vent, Carbon Canister|
Thanks to Strider on IFSJA BBS for providing the graphic of a simplified spark advance. I did this on my rig some time ago for testing, and it seems to work fine. Basically you're eliminating the 2nd CTO and the NLVR. This really cleans up the spaghetti.