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 The
following is info from the all-knowing all-seeing Arlene.
It is a very handy list of what different vacuum readings
mean. For reference, my car reads 21 inHG at idle and
the needle on my gauge moves just a hair. When I take
a measurement I pull the vacuum hose going to the boost
gauge pressure sensor and plug that into the vacuum gauge.
I have also added some information to the table from the
makers of my vacuum gauge, Mityvac,
that was included when I bought it.
---
For those that have asked for additional
information for using a vacuum gauge as a diagnostic tool
(ie, for those of you who have read my long dissertation
on Buying a Used Supra and wanted additional information),
I have compiled the synopsis below. If you have an after-market
boost gauge, then you can use it as the diagnostic tool.
Note: an abnormal vacuum is only an indication of a problem
and further testing (by other methods, such as compression
test, reset timing, etc.) will need to occur to verify
the true cause of a “problem.” Any reference
to “idle” means the factory idle speed of 650
rpm; if your engine is not at this setting, then the following
comments are invalid (heck, due to the individually of
each car, they may be invalid :) - a vacuum gauge is but
a tool!!).
| Condition |
Behavior |
| Normal |
Steady 17-21 inHG, at idle. This
number will decrease 1" for every 1000 feet
above sea level. Drops to 2 inHG, then rises
to 25 inHG when accelerator is rapidly depressed
and released. |
| Leaking Valves |
Regular drop about 2 inHG from
highest level. |
| Burned Valves |
Regular, evenly spaced down-scale
flick about 5 inHG from highest level. |
| Sticking Valves |
Rapid intermittently flicks downward
about 1-4 inHG from highest level. |
| Weak Valve Springs |
Violent oscillation (about 10 inHG)
increasing as rpm increases. Often steady at
idle. |
| Worn Valve Guides |
The gauge reading will be lower
than normal and will fluctuate rapidly over 3
inHG. The needle will become steady as RPM increase. |
| Leaking Piston Rings |
Low but steady vacuum reading at
idle 12inHG-16inHG. As the throttle is opened
then released, the vacuum reading should drop
to 0 then recover to approximately 21inHG as
engine decelerates, normal at this point would
be 23inHG-25inHG. |
| Blown Head Gasket |
Floats between 5-20 inHG. A sharp
drop might be seen each time the affected cylinder
fires. |
| Exhaust Restriction Test |
Normal when first started. Raise
and hold engine speed at 2,500. The needle will
drop when you first open the throttle, but should
then rise and stabilize. If the vacuum reading
starts to drop, pressure may be backing up in
the exhaust system. If reading increases over
10inHG the system is not restricted. |
| Intake Manifold Leaks |
Low steady reading (5-6 inHG). |
| Late Ignition Timing |
Steady 14-17 inHG at idle, normal
cam. |
| Late Valve Timing |
Steady 8-18 inHG, at idle. |
| Incorrect Idle Air-Fuel Mixture |
Floats slowly between 13-17 inHG. |
| Small Spark Gap |
Slight float between 14-16 inHG. |
| Worn Rings, Diluted Oil |
Drops to 0, then rises to 22 inHG
when accelerator is rapidly depressed and released. |
Again, vacuum readings alone are not conclusive and additional
testing verification is required.
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