RE: questions

["Farmer" <pharmer@domain.elided>]

Neff, Matthew John - MBA wrote:
> what can one infer from high/low manifold vacuum?

Here's something I posted a while back:


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Date: Tue, 05 Aug 1997 10:12 -0600 (MDT)
From: Ken Farmer <ken.farmer@domain.elided>
To: ihc-digest@domain.elided
Subject: Interpreting Vacuum Readings

Here are a few notes from a highly scholarly automotive text book of
mine (Reader's Digest Complete Car Care Manual, 1981, used for $12):

Note: Readings will vary with altitude and atmospheric conditions
     - every 1000 feet of altitude lowers the reading by 1-2 in Hg

1.   Steady reading between 15 and 22 in Hg with the engine warmed
     up and idling is normal.  Snap the throttle plate open and closed
     suddenly.  The needle should drop to 5 in Hg or lower before
     stabilizing at the normal reading between 15 and 22.
2.   Low reading that holds steady around in Hg indicates a vacuum
     leak at intake manifold or carburator gaskets, or a disconnected
     or leaking vacuum hose.
3.   Low reading that holds steady between 8 and 14 in Hg suggests
     that ignition timing is off or that piston rings are leaking.
     Check timing and compression.
4.   Fluctuating reading that periodically drops 2 to 6 in Hg below
     normal indicates worn points or low compression.
5.   Regular fluctuation between a low reading of about 5 in Hg and
     a slightly lower-thannormal reading means the head gasket is
     leaking.
6.   Reading drifts back and forth over a range of 4-5 in Hg within
     the normal range indicates incorrect carburator adjustment.
7.   Rapid needle vibration between 14 and 19 in Hg indicates that
     worn valve guides are lettin intake valves chatter as they
     seat.
8.   Needle floats over a range of about 14 to 16 in Hg sugests that
     spark plugs may be gapped too close.
9.   Needle that swings erratically between about 10 and 20 in Hg
     when the engine is accelerated smoothly may indicate weak valve
     springs.
10.  Steady high reading that holds above 21 in Hg indicates a restricted
     air intake.  Check for clogged air filter or a stuck choke.
11.  Needle drops to near zero when the engine is accelerated, then
     climbs back almost to normal level.  Exhaust system may be blocked
     or kinked.

Hope that helps,

Ken Farmer