Navajo 4WD V6-4.0L OHV (1991)
Catalytic Converter: Testing and Inspection
NOTE: A blocked or restricted exhaust system usually results in loss or lack of power or popping through the intake. Verify that this condition is not
caused by ignition or timing problems, then proceed with diagnosis.
1.
Visually inspect exhaust system for collapsed pipes, holes and cracks.
Replace any damaged components.
2.
Attach vacuum gauge to intake manifold vacuum source.
3.
Connect a tachometer.
4.
START engine and observe vacuum gauge needle.
5.
Gradually increase the engine rpm to 2000 with the transmission in NEUTRAL.
NOTE: The vacuum gauge reading may be normal when the engine is first started and idled. However, restriction in the exhaust system will cause
the reading to drop to a low point and stay there when the engine speed is increased. Extremely restricted exhaust will cause a low reading
even when the engine is idling.
6.
Decrease rpm to idle speed.
With the engine rpm at 2000, did manifold vacuum reach above 16 inches?
If so, there is no restriction within the exhaust system.
Refer to Powertrain Management/Emission Control Systems/Testing and Inspection/Procedures.
If not, proceed to next step.
7.
With vacuum gauge and tachometer still in place.
8.
Gradually increase the engine rpm from base idle to 2000 rpm with the transmission in NEUTRAL.
9.
While maintaining increased engine rpm, observe the RATE of change in the vacuum reading.
a.
On a non-restricted system the needle will drop to zero and then quickly return to the normal setting without delay.
b.
On a restricted system, as the engine rpm is increased to 2000 the vacuum gauge needle will slowly drop to zero and slowly rise to normal.
c.
The rate at which the vacuum reading returns to the normal setting is much slower on a restricted system than on a non-restricted system.
10.
Decrease engine rpm to base idle.
11.
Was needle return rate much slower than that of a non-restricted system?
If not, there is no restriction within the exhaust system.
Refer to LACK OF POWER/DIAGNOSIS BY SYMPTOM.
If so, proceed to next step.
12.
Turn OFF engine.
13.
Disconnect exhaust system at the muffler(s).
14.
Repeat vacuum test.
15.
Did manifold vacuum reach above 16".
If so, replace muffler(s).
If not, proceed to next step.
16.
Turn OFF engine.
17.
Disconnect exhaust system at catalytic converter(s).
18.
Repeat vacuum test.
19.
Did manifold vacuum reach above 16".
If so, replace converter(s).
If not, proceed to next step.
20.
Turn OFF engine.
21.
Disconnect exhaust system at exhaust manifold(s).
22.
Repeat vacuum test.
23.
Did manifold vacuum reach above 16".
If so, restriction is located between exhaust manifold(s) and catalytic converter(s).
If not, proceed to next step.
24.
Remove the exhaust manifold(s) and inspect the ports for casting flash by dropping a length of chain into each port.
NOTE: Do not use a wire or a lamp to check ports. The restriction may be large enough for the wire or light to pass through but small enough to
cause excessive back pressure at high rpm.
25.
Is a restriction present?
If so, remove casting flash or replace exhaust manifold.
If not, refer to LACK OF POWER/DIAGNOSIS BY SYMPTOM.
