In digest #296, Al Hamilton posted
"That's my understanding also. High vacuum readings indicate an efficient
running engine. Also, keeping the vacuum higher than 6 Hg or so keeps the
carburator power piston from raising the primary rods and enriching the
fuel mixture. That's about the limit of my knowledge of it. I think it is
important to moniter vacuum and have just put in a new gauge because of
questionable readings from the old one. Can someone explain the importance
of good vacuum readings?"
Al, your auestion reminded me of Emery Stora's most informative piece from
a year or so ago describing the various conditions indicated by certain
vacuum guage readings. It's part of my permanent files. Here 'tiz again:
VACUUM GUAGE DIAGNOSIS:
The vacuum gauge gives indications of possible mechanical problems, but
they are not foolproof. Follow the vacuum tests with further tests, where
possible, to confirm your diagnosis. Do not expect specific (numerical)
amounts of vacuum. More important is the range of the vacuum readings and
the movement of the needle. Notice HOW the needle moves (in a smooth or
jerky manner, erratic, regular, etc.) What direction it moves, regular or
varying and how far it moves.
Normal Engine: Warm engine to operating temperature. At idle the reading
should be steady
and 16"-22" Hg (inches of mercury).
Burned or Leaking Valve: At idle the pointer will drop to a low reading and
return to normal at a
regular interval. It will drop 1" to 7" Hg at regular intervals whenever
the defective valve attempts to close.
Sticking Valve: A rapid, intermittent drop from normal. This is unlike the
regular drop that characterizes a burned or leaking valve. You can
pinpoint this by removing the valve cover and applying a lightweight oil to
each valve guide. It will temporally improve when the sticking valve guide
is reached.
Weak or Broken Valve Spring: The gauge fluctuates rapidly between 10" and
21" Hg at idle. Fluctuations increase with engine speed. A broken valve
spring will cause rapid
fluctuation at a regular interval. Again, every time the valve tries to close.
Worn Valve Guides: Worn guides admit air which upsets the air/fuel mixture.
The gauge reading
will be lower than normal and will fluctuate rapidly in a range of about 3"
Hg. As the speed of the engine increases the needle will steady.
Leaking Piston Rings: Vacuum at idle will be low but steady at about 12" to
16" Hg. Open throttle
to about 2000 rpm. Then close throttle rapidly. The needle should jump 2"
to 5" Hg above the low steady reading. A lesser jump may indicate faulty
rings. A complete cylinder leakage or compression test should be done.
Blown Cylinder Head Gasket: At idle the gauge will fluctuate between normal
and a low reading. The needle will drop sharply about 10" Hg from normal
and return each time the defective
cylinder or cylinders reach firing position.
Exhaust Restriction Test: Check the idle vacuum. Increase engine speed to
about 2500 rpm. An increase in vacuum over the idle amount indicates
exhaust free of restrictions. If the vacuum drops there is either a
restriction or, for cars that use EGR (exhaust gas recirculation) the EGR
valve might be bad.
Incorrect Idle Air/Fuel Mixture: When the gauge drifts slowly back and
forth at idle, over a range of 4" to 5" Hg. the fuel mixture is too rich.
A lean mixture will cause an irregular drop of the needle over about the
same range.
Intake Manifold or Air Induction Leaks: The gauge will be about 3" to 9" Hg
below normal but will be steady.
Late Ignition or Valve Timing: An extremely low but stead reading at idle
indicates late ignition or valve timing; or too tight setting of the valve
lash.
In my own use I found that my idle was fairly smooth but the vacuum was
about 15" Hg. When I did the procedure under Leaking Piston Rings I found
that the vacuum only jumped about 1" Hg. I then pulled the spark plugs and
tested all cylinders. Two were about 20 psi below the others. I squirted
some oil into the plug holes and retested. The pressure came up to the
range of the other pistons which indicates that the rings are leaking. The
valves are then judged to be the same as the others. If the pressure had
not changed it would indicate a leaking valve.
HTH (Thanks Emery)
Dave (& Dege), '76 Royale, Santa Barbara, CA
"That's my understanding also. High vacuum readings indicate an efficient
running engine. Also, keeping the vacuum higher than 6 Hg or so keeps the
carburator power piston from raising the primary rods and enriching the
fuel mixture. That's about the limit of my knowledge of it. I think it is
important to moniter vacuum and have just put in a new gauge because of
questionable readings from the old one. Can someone explain the importance
of good vacuum readings?"
Al, your auestion reminded me of Emery Stora's most informative piece from
a year or so ago describing the various conditions indicated by certain
vacuum guage readings. It's part of my permanent files. Here 'tiz again:
VACUUM GUAGE DIAGNOSIS:
The vacuum gauge gives indications of possible mechanical problems, but
they are not foolproof. Follow the vacuum tests with further tests, where
possible, to confirm your diagnosis. Do not expect specific (numerical)
amounts of vacuum. More important is the range of the vacuum readings and
the movement of the needle. Notice HOW the needle moves (in a smooth or
jerky manner, erratic, regular, etc.) What direction it moves, regular or
varying and how far it moves.
Normal Engine: Warm engine to operating temperature. At idle the reading
should be steady
and 16"-22" Hg (inches of mercury).
Burned or Leaking Valve: At idle the pointer will drop to a low reading and
return to normal at a
regular interval. It will drop 1" to 7" Hg at regular intervals whenever
the defective valve attempts to close.
Sticking Valve: A rapid, intermittent drop from normal. This is unlike the
regular drop that characterizes a burned or leaking valve. You can
pinpoint this by removing the valve cover and applying a lightweight oil to
each valve guide. It will temporally improve when the sticking valve guide
is reached.
Weak or Broken Valve Spring: The gauge fluctuates rapidly between 10" and
21" Hg at idle. Fluctuations increase with engine speed. A broken valve
spring will cause rapid
fluctuation at a regular interval. Again, every time the valve tries to close.
Worn Valve Guides: Worn guides admit air which upsets the air/fuel mixture.
The gauge reading
will be lower than normal and will fluctuate rapidly in a range of about 3"
Hg. As the speed of the engine increases the needle will steady.
Leaking Piston Rings: Vacuum at idle will be low but steady at about 12" to
16" Hg. Open throttle
to about 2000 rpm. Then close throttle rapidly. The needle should jump 2"
to 5" Hg above the low steady reading. A lesser jump may indicate faulty
rings. A complete cylinder leakage or compression test should be done.
Blown Cylinder Head Gasket: At idle the gauge will fluctuate between normal
and a low reading. The needle will drop sharply about 10" Hg from normal
and return each time the defective
cylinder or cylinders reach firing position.
Exhaust Restriction Test: Check the idle vacuum. Increase engine speed to
about 2500 rpm. An increase in vacuum over the idle amount indicates
exhaust free of restrictions. If the vacuum drops there is either a
restriction or, for cars that use EGR (exhaust gas recirculation) the EGR
valve might be bad.
Incorrect Idle Air/Fuel Mixture: When the gauge drifts slowly back and
forth at idle, over a range of 4" to 5" Hg. the fuel mixture is too rich.
A lean mixture will cause an irregular drop of the needle over about the
same range.
Intake Manifold or Air Induction Leaks: The gauge will be about 3" to 9" Hg
below normal but will be steady.
Late Ignition or Valve Timing: An extremely low but stead reading at idle
indicates late ignition or valve timing; or too tight setting of the valve
lash.
In my own use I found that my idle was fairly smooth but the vacuum was
about 15" Hg. When I did the procedure under Leaking Piston Rings I found
that the vacuum only jumped about 1" Hg. I then pulled the spark plugs and
tested all cylinders. Two were about 20 psi below the others. I squirted
some oil into the plug holes and retested. The pressure came up to the
range of the other pistons which indicates that the rings are leaking. The
valves are then judged to be the same as the others. If the pressure had
not changed it would indicate a leaking valve.
HTH (Thanks Emery)
Dave (& Dege), '76 Royale, Santa Barbara, CA
