GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1996 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 4.3L (Caprice Only) and 5.7L 🢒 Distributor Vent System Check

Object Number: 21178 Size: SH
(1)Distributor
(2)Vacuum Port
(3)Vacuum Hose
(4)Vent System Harness
(5)Air Intake Duct
(6)Clean Air Supply Hose

Circuit Description

The distributor is equipped with a fresh air vent system. When the engine is started, vacuum that the engine produces pulls fresh air in from the air intake duct. A constant supply of fresh air is needed within the distributor. An inoperative distributor vent system may cause premature distributor failure.

Diagnostic Aids

If the check valve/filter or the check valve is installed backwards the system will not function. Verify flow direction is toward the manifold using a hand held vacuum pump at the check valve and the check valve/filter's ports. Vacuum should hold in one direction and not the other. If vacuum holds or does not hold in both direction, for either component, that component is faulty.

Test Description

Number(s) below refer to step number(s) on the diagnostic table.

  1. Check for a blocked or restricted vacuum source.

    2.

  2. Check for a blocked, restricted or collapsed hoses. Check the connections at the distributor cap for a blockage. Check for a faulty check valve or check valve/filter.

    3.

  3. Check for damaged or leaking hoses or connections. Check the seal between the distributor cap and the distributor. Also, check the distributor cap for cracks or other damage.

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Disconnect the distributor vacuum hose at the intake manifold.
  2. Connect a hand held vacuum pump to the distributor vacuum hose.
  3. Apply vacuum and observe vacuum pump gauge.

Does the vacuum not build up or bleed off quickly?

--

Go to Step 3

Go to Step 5

3
  1. With the vacuum pump still connected, disconnect the distributor clean air supply hose from the air intake duct and plug it with a suitable stopper.
  2. Apply 34 kPa (10 in. Hg) of vacuum and observe the vacuum pump gauge.

Does the vacuum hold?

--

Go to Step 4

Go to Step 6

4
  1. Remove the stopper from the clean air supply hose.
  2. Disconnect the vacuum pump.
  3. Connect the vacuum pump to the clean air supply hose.
  4. Apply 34 kPa (10 in. Hg) of vacuum and observe the vacuum pump gauge.

Does the vacuum hold?

--

System OK

Go to Step 7

5

Repair the distributor vent system for a blockage or restrictions.

Is the action complete?

--

System OK

--

6

Repair the distributor vent system for vacuum leaks.

Is the action complete?

--

System OK

--

7

Replace the faulty check valve.

Is the action complete?

--

System OK

--