Equus 3660 - Hand Vacuum Pump & Brake Bleeding Kit - Manual (Page 17)

3660 15
4. Leaking head gasket, worn
valve guides or weak/broken
valve springs (Figure 9)
If the gauge needle fluctuates
regularly between a high and a
low reading, the cylinder head
gasket may have a leak between
two adjacent cylinders. If the
needle vibrates rapidly at idle
speed, the intake valve guides are
probably worn.If the needle
vibrates rapidly when the engine
is accelerated, the valve springs
are weak.
X. EXHAUST SYSTEM RESTRICTION TEST
The vacuum gauge is also a valuable tool to help in the troubleshooting
of a restricted exhaust system. An exhaust restriction usually allows
normal engine performance at idle, but causes poor performance at
higher engine speed or with the engine under load.
Use the following procedure to test for a restricted exhaust system:
NOTE: In order for this test to be accurate, make sure other engine
problems (as described in “DIAGNOSING MECHANICAL ENGINE
CONDITIONS") are repaired first, before the Exhaust System
Restriction Test is performed.
1. Connect the vacuum pump to an intake port and start the engine.
2. With the engine at idle, note the vacuum reading on the vacuum
gauge.
• The vacuum reading should be within manufacturer's
specifications.
3. While observing the vacuum gauge, slowly increase engine speed to
2500 ~ 3000 RPM. At high RPM, engine vacuum should be equal to
the vacuum reading at idle ± 3 in./8 cm Hg.
• If the vacuum decreases to zero or close to zero at high RPM
there is probably a restriction on the exhaust system.
The most probable cause of a restricted exhaust system is a defective
Catalytic Converter or muffler. Consult the vehicle's service manual for
further testing instructions.
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Figure 9