TOYOTA PREVIA 91-97 REPAIR MANUAL

3-74 ENGINE AND ENGINE Q\L

USING

White needle - steady

A VACUUM GAUGE

needle Dark needle = drifting needle

The vacuum gauge is one of the most useful and

provides valuable information about the conditiorl

Indication: Normal engine

condition

i good

Gauge reading: Steady, from 17-22 in.IHg.

Indication: Late ignition o

timing, low compression

throttle valve, leaking car

or manifold gasket.

easy-to-use diagnostic tools. It is inexpensive, easy to hook up, and

of your engine.

Gauge reading: Low (15-20 in.IHg.) but steady

Indication: Weak valve springs,

worn valve stem guides, o leaky

cylinder head gasket (vt rating

excessively at all speeds).

NOTE: A plugged catal tic

converter may also cause

this reading.

Gauge reading: Needle fluctuates as engine speed im reases

Indication: Choked mutflei or ob-

struction in system. Speed up the

engine. Choked muffler \ ex-

hibit a slow drop of vac um to

zero.

valve

stuck

•u re tor

Gauge reading: Gradual drop in reading at idle

Indication: Sticking valve or igni-

tion miss

Gauge reading: Needle fluctuates from 15-20 in.IHg. at idle

Indication: Improper carburetor

adjustment, or minor intake leak

at carburetor or manifold

NOTE: Bad fuel injector

O-rings may also cause

this reading.

Gauge reading: Drifting needle

Indication: Burnt valve or im-

proper valve clearance. The nee-

dle will drop when the defective

valve operates.

Gauge reading: Steady needle, but drops regularly

Indication: Worn valve guides

Gauge reading: Needle vibrates excessively at idle, but

steadies as engine speed increases