How To Interpret Vacuum Gauge Readings
Contributed By: Gordon McKenzie
I'm using the Haynes Fuel Injection book as it seems to give the longest
description of the readings. I doubt this is a copyright infringement as
this is public knowledge, but I'll give them credit anyway as they put it
- Normal vacuum: All motors regardless of number of cylinders all have about
the same vacuum. 15 to 20 inches of mercury (in-Hg) At wide open throttle
(WOT) it will approach 0. On deceleration it can briefly go as high as 25 to
28 in-Hg. I think higher compression motors are at the high end while our
big lumbering 360;s are middle to low end with 454's being the lowest. Don't
quote me on that. For every 1000 feet above sea level, normal readings will
drop 1 in-Hg
- Cranking vacuum: 1-4 in-hg when cranking with the ignition disabled and
throttle at WOT.
- Operating vacuum: 15-20 in-Hg at idle. 19-21 in-Hg at steady 2500 rpm. About
0 at WOT. 25-28 while decelerating. In all cases the needle should be fairly
- Low stead reading: Leaking gasket carb base, leaky vacuum hose (biggest FSJ
problem), late ignition timing or late crankshaft timing (jumped timing
- Low fluctuating needle: Needle fluctuates three to eight inches below
normal. Suspect a leak in the intake manifold gasket.
- Regular drops: Two to four inch consistent drop. Likely a leaking valve.
Perform a leakdown and compression check to isolate the bad valve.
- Irregular drops: Two to four inch occasional drop. Sticking valve or
ignition misfire. Do a leakdown and compression check as well as inspecting
the plugs for fouling.
- Rapid vibration: A rapid 4 in variation at idle combined with exhaust smoke
indicate worn valve guide. Perform leakdown test to confirm. If rapid
vibration occurs with an increase in engine speed, check for a leaking
intake manifold gasket or head gasket, weak valve springs burned valve or
- Slight fluctuation: A slight 1 inch fluctuation may mean ignition problems
Check all tune up items and try an ignition analyzer.
- Large fluctuation: Perform compression or leakdown test to find a weak or
dead cylinder or blown head gasket.
- Slow Hunting: Needle moves slowly through a wide range. Check for clogged
PCV, incorrect idle mixture or carb to manifold gasket leaks.
- Slow return after revving: Snap throttle to WOT until engine at 2500 rpm.
then release. Vacuum should drop to 0 then go to 25-28 then back down to
normal. If vacuum returns slowly and doesn't peak the rings may be worn. If
there is a long delay, look for a restricted exhaust. If the exhaust seems
restricted, disconnect at manifold and repeat the now loud test.
Gord, Sue and Countess McKenzie
81 Jeep Wagoneer 360 2v all stock (Ox)