For all interested parties, I have to disagree with some of the information contained in the link provided in the earlier post. So, taking each condition separately:
Normal engine: Information given is good- no question there.
Steady low: Ok, but there are other causes for that condition.
Steady very low: Ok, but other indications will be far more prevalent than just low vacuum- sluggishness, backfiring (intake or exhaust), hard starting, etc. If I saw that indication in the shop, I would use it only to substantiate the suspicion of valve timing- but if valve timing is off enough to cause that low level of vacuum, most often the ignition timing will be off as well- distributors are camshaft-driven.
Steady, extremely low: If there is an intake leak large enough to cause that level of manifold vacuum, the fuel mixture will be too lean for the engine to continue running. If by chance an engine were still running with that large an intake leak, no matter where, the speed would be so erratic that the vacuum gauge would fluctuating severely- half-scale or so. Not good information.
Needle rhythmically drops: Ok, but you will seldom see more than 1 1/2 inches of mercury drop with a compression misfire, and the drop depends on the compression loss and engine speed. An ignition misfire causes the condition described, but the drop is seldom more than 1 "Hg. An intake manifold leak affecting one cylinder more than others will give a similar vacuum indication, but will diminish as engine speed increases. Ok with the sticking valve idea.
Drops to 0, then rises: The indication is correctly described, but the stated causes are pretty shaky. With a sudden full-throttle acceleration, the vacuum shown by a gauge will reach zero, if for no other reasons than momentum of the gauge mechanism, spring tension of the internal tube, and slow acceleration of the engine- some accelerate faster than others when the throttle is snapped fully open. Also, some gauge movements are heavily damped to prevent immediate responses to large changes, so gauges from two manufacturers may give different readings. The possibilities given are technically valid, but you will not see the results with an engine that is not loaded. If you run an unloaded engine long enough at full throttle to reach 4-5 "Hg, you risk catastrophic failure. Those indications would be valid if the engine/vehicle is loaded with a dynamometer, but don't try that in your driveway with no load on the engine. An exhaust restriction doesn't show up with that check.
Needle wavers rapidly, 10-20 "Hg: Not likely for the causes given; that level of fluctuation is showing the "death thoes" of and engine that is only a few revolutions from stalling. The possibilities cited will cause the same indications as compression loss/compression misfire, although the indications may be intermittent or erratic. The slow valve opening cited is incorrect- remember valves open under camshaft/lifter/rocker arm force, then close under the lesser spring force, so slow closing, not slow opening. The irregular wavering described with that same indication is somewhat all right, but higher speed above 1200-1400 RPM will diminish the indications to the point of being invisible.
Drifts at idle, stabilizes at high speed: No. Drifting at idle speed represents a fuel mixture situation- rich or lean are similar. The burned valve causes a compression misfire that does, in fact, diminish as speed increases; at low speed, compression misfire will appear as the regular, rythmic drop. See the condition described in "Needle rhythmically drops." It can't be both ways- and it isn't.
Needle jumps 2-5...: That is a normal indication. The return to "normal" is dependent on the speed of deceleration- a loose engine decelerates slower than does a tight engine. An exhaust restriction does in fact cause back pressure. In that state, air doesn't exit the engine properly, so air doesn't enter the engine in the proper quantity, therefore, lower manifold vacuum rather than higher vacuum. And under deceleration, there is less air going into the engine, so there is less air exiting the engine, so exhaust restriction becomes less of an effect. High air flow, large effect from restriction; low air flow, low effect from restriction.
Needle does not jump...: Technically correct, but if piston rings are worn to that extent, the likely indication will be a heavy cloud of exhaust smoke that is a much better indicator of ring/cylinder wear than is a vacuum reading. But this is technically correct, just don't overhaul your engine based on that indication. A wet compression check is a much better indication of ring/cylinder wear.
Not mentioned: Exhaust restriction will show up as a declining vacuum level as engine speed increases. Holding the engine at a given speed will stabilize the vacuum at what ever level exists for the throttle position and the degree of restriction: wider throttle, more restriction, lower vacuum level.
There can be more said about vacuum checks.
Harvester
