Ну, вроде разобрался. Вот что пишет Haynes (переводить влом, сорри):
a. testing at various speeds
- engine starting vacuum should be 1 to 4 in Hg. To test disable ignition (ground wire from coil), hold throttle wide open, crank engine slowly with starting motor.
- healthy engine at idle should read steady 15 to 20 in Hg.
- healthy engine at 2000 rpm should read steady 19 to 21 in Hg.
- healthy engine at open throttle should read close to 0 in Hg.
- healthy decelerating engine reading should jump to 21 to 27 in Hg as open throttle released.
b. testing at idle speed
- low steady reading usually indicates leaking gasket between intake manifold and carburetor or throttle body, leaky vacuum hose, or incorrect camshaft timing.
- low fluctuating (3 to 8 in Hg below normal) reading may indicate intake manifold gasket leak at an intake port or faulty injectors on port-injected engines.
- regular drops (2 to 4 in Hg) in reading at a steady rate indicates probable leaking valves.
- irregular drops in reading indicates possible sticking valve or ignition misfire.
- rapid vibration (4 in Hg) in reading combined with exhaust smoke indicates worn valve guides.
- slight fluctuation (1 in Hg) in reading indicates possible ignition problems.
- large fluctuation (10 in Hg) in reading indicates likely weak or dead cylinder or blown head gasket.
- slow movement through wide range in reading indicates possible clogged PCV system, incorrect idle fuel mixture, or gasket leak between carburetor, throttle body, or intake manifold.
c. testing at higher speeds
- rapid vibration (4 in Hg) in reading at increased engine speed indicates leaking intake manifold gasket or head gasket, weak valve springs, burned valves, or ignition misfire.
- reading returns slowly to normal and didn't peak above normal (5 in Hg) after dropping to zero when throttle quickly snapped open (2500 rpm) suspect worn rings.
- reading returns to normal after long delay when throttle quickly snapped open (2500 rpm) suspect blocked exhaust.
Похоже с MAP-датчиком у меня всё в порядке

При случае ещё и манометром проверю для пущей важности
