One of the most confusing aspects to tuning up an engine from the
late 1970's and 1980's is all the early attempts manufacturers made
to improve emissions with various vacuum-operated devices. In some
cases (especially early-mid 1980's), it almost looks like they
designed the system by simply throwing a handful of spaghetti at
the engine and saying, "Route the hoses like that."
The best way to understand where everything goes is to get a copy
of a factory service manual (FSM/TSM).
Two good sources of these manuals are
and Z&M Jeeps. Since these are
pretty expensive (but worth it!), I've scanned a couple vacuum diagrams
and my 1974-80 parts manuals. I'm sure this collection isn't complete.
EGR (Exhaust Gas Recirculation)
The purpose of the EGR port is to feed exhaust gas from the exhaust
system back into the intake air flow whenever the coolant temperature
is high enough and the ported vacuum is also high (ie, not during
startup and only during heavy loads or hard acceleration). This
decreases the engine temperature, which in turn decreases the emission
of harmful nitrous oxide (NOx) gases. Reduced temps will reduce pinging,
which will in turn allow you to advance your timing farther for better
power throughout most of the RPM range. By diluting your intake charge
with exhaust gas, the EGR will reduce your peak horsepower at higher
RPM's, but it will not adversely affect power under normal driving
conditions. In fact, with the base ignition timing properly set, the
EGR will allow for increased power at low- and mid-range RPM's. If you
use the engine for drag racing, then disabling the EGR is probably a
good idea. For any other use, you're better off making sure it works.
Performer intake manifolds
that come either
an EGR port.
Here is the vacuum diagram
for both the I6 and V8 engines. This was the same throughout the 1974-80
model runs for all Jeep models. If your EGR vacuum line has a red & blue
delay valve in it, the blue side should point toward the EGR valve and
the red side should point toward the CTO switch.
PCV (Positive Crankcase Ventilation)
The crankcase on an engine will slowly build up positive pressure.
This pressure must be released in a control manner to prevent the
seals (most likely the oil pan seal) from blowing out. Some engines
simply vent this pressure to the atmosphere using a breather cap on
one or both valve covers. Since the vapor that's vented contains
nasty chemicals in addition to air, the environmentally-polite method
is to vent these vapors back into the intake air flow. Jeep engines
do this via a large (3/8" or larger) vacuum fitting at the base of
the carburetor. There is also a similar-sized hose that runs from
the air filter to the oil fill cap (V8) or the front of the valve
Here is the vacuum diagram
for both the I6 and V8 engines. This was the same for all Jeep models,
at least in 1977.
Fuel Vapor Storage Canister
When it gets hot outside, the and and fuel vapor inside your gas tank
expands. This excess pressure must go somewhere to prevent your tank
from rupturing. Most older vehicles simply vent these vapors into the
atmosphere through the filler cap. These caps use a valve that only
opens when the pressure reaches a certain level. If the valve in your
cap seizes shut, it's time for a new cap. Newer vehicles (starting
first in California, and later in all states) instead vent these vapors
into -- you guessed it -- the intake air flow via a charcoal canister.
Here is the vacuum
diagram for both the 1977 I6 engine. This system first arrived in
California FSJ and all CJ models, and was later added to 49-state FSJ's.
Distributor Vacuum Advance
Most distributors from this era have several simultaneous methods of
advancing the ignition timing. Base timing (the lowest amount of
advance ever achieved) is usually set around 8-12 degrees BTDC. In
addition to this, spring-loaded centrifugal weights will further
advance the timing (as much as a dozen or two degrees) as the engine
speeds up. The third, and most complicated method, is the vacuum
advance. There are several different methods that Jeep employed to
implement the vacuum advance feature, but it essentially works like
this: A CTO (coolant temperature override) switch uses engine coolant
to sense when the engine has warmed up. When it's cold, the distributor
is advanced using manifold vacuum, so the ignition is advanced farther
when idling than it is under load. When the engine warms up, the
distributor is advanced using ported vacuum, so the ignition is advanced
farther under load than it is at idle. When using a 3-port CTO switch,
the center port is common; the outer port is open when cold, and the
inner port is open when hot.
Here is the vacuum
diagram for both the 1974-80 I6 engine.
here are the vacuum
diagram for both the 1974-80 V8 engines of various configurations.
In these V8 diagrams, the vacuum line to the base of the carb on the
manifold actually indicates ported vacuum, not manifold vacuum. Yes,
it's confusing. Also, in these diagrams "10-20-40" and "10-25-45"
indicate FSJ's, while "80" indicates a CJ. "E/CALIF" means "except
California," while "CALIF" indicates a California-only model. "LHDC"
indicates "less (without) heavy duty cooling" package, while "WHDC"
indicates "with heavy duty cooling."
There are a few other diagrams out on the Net that are also useful.
sells factory service manuals for lots of different Jeep models at great prices.
These two are for a 49-state 6-cyl 258, auto tranny,
heavy duty cooling. Judging from all the smog stuff, I'd say it's
probably for an early-1980's Jeep.
is another scan from a similar manual detailing the vacuum diagram for
a 1979 California V8 FSJ with auto tranny and heavy duty cooling.
another set of scanned pages outlining the vacuum routing for mid-1970's
is a poor-quality scan of the V8 vacuum diagram from a 1984-85 FSJ manual.
Here are two more poor-quality scans of the V8 vacuum diagram from
a 1989 V8 Grand Wagoneer with
heavy duty and
is another scan detailing the 1978 EVAP (charcoal) canister ports.
is a scan detailing the AIR injection pump setup.
And of course, there's always
Tom Collins' site,
which contains scans from TSM's from several different years. It's
geared toward FSJ's, but will mostly apply to CJ's, too.
In the lower right corner of
Joe Gorfinkle's site,
you'll find lower-resolution scans from a 1976 TSM of much of the info
that I scanned from my '77 TSM above.