QuestionI own a 1998 Pontiac Trans Am 5.7L, 6 Spd Manual Trans. I recently had the engine removed, rebuilt, and reinstalled. Here's my problem: With the fan switch in the off position and the car sitting at idle, no air is coming from the floor vents. This is normal. When I drive the vehicle at 25 mph or more, air starts entering the car through the floor vents along with noxious fumes. I've taken the vehicle back to the shop several times and they assured me that everything is connected properly. This can't be because as long as the fan switch is off, no air should come through either the upper or lower vents. Your thoughts would be greatly appriciated.
AnswerHi Ted
It sounds like you may have a vacuum problem.
Engine vacuum ir routed through a check valve (located on the passenger side around where the a/c lines go into vehicle).
I would REALLY suspect that check valve, it MAY be leaking and allowing lowered vacuum to enter the air distribution
system. (mode) The Modes work from vacvuum, the blower is electrical.
It tee's off this valve, and goes into cabin to the Control Vacuum selector valve (the mode selector knob).
The other tee off goes to the Vacuum storage tank located under front bumper fascia air deflector. It MAY be faulty, but usually you would have different symptoms.
I would suspect the check valve leaking or bypassing.
I will send along this diagnostic worksheet, directly from GM service info.
It may or may not help....but at least you'll have it if needed.
Mode Control Vacuum Valve Positions
1
OFF
2
MAX A/C
3
NORMAL A/C
4
BI-LEVEL A/C
5
VENT
6
HEATER
7
HEATER/DEFROST BLEND
8
DEFROSTER
A vacuum operates most of the air distribution system. Electrical power operates the blower motor.
The engine vacuum is routed through the following areas:
The vacuum check valve
The control vacuum selector
The vacuum tank
The vacuum check valve prevents vacuum loss. Heavy acceleration or steep uphill traveling may cause vacuum loss.
Vacuum applied to a vacuum actuator moves the air door to the air door's correct position.
Preliminary Inspection
Inspect the vacuum lines at the engine and vacuum check valve for a proper connection.
Disconnect the vacuum line that leads to the engine at the vacuum check valve.
Start the engine.
Inspect for vacuum presence. If no vacuum is present, repair as required.
Disconnect the vacuum line that leads to the vacuum tank at the vacuum check valve.
Use a hand vacuum pump in order to apply vacuum to the vacuum line that leads to the vacuum tank. If the vacuum does not hold, replace the vacuum line and/or the vacuum tank.
Disconnect the vacuum line that leads to the control at the vacuum check valve.
Apply vacuum to the check valve port. If the vacuum does not hold, replace the vacuum check valve.
Quickly rotate the temperature control from full COLD to full HOT several times.
Listen for the temperature valve to seat in both positions. If the sound indicates that the temperature valve is not fully seating or opening, inspect the temperature control cable for proper operation. If the problem persists, ensure that the temperature valve door operates freely with the blower off.
Ensure that the vacuum line seals are positioned correctly at the vacuum line multi-connector that is located above the right-hand sound insulator panel.
Perform the diagnosis.
Diagnostic Aid Checks
Action
Apply vacuum to the red vacuum line.
Air flow is diverted to the instrument panel outlets only.
Apply vacuum to the yellow vacuum line.
Air flow is diverted to the side window defogger and floor outlets.
Apply vacuum to the purple vacuum line.
Air flow is diverted to the side window defogger, floor and instrument panel outlets.
Apply vacuum to the green vacuum line.
Air flow is diverted to the windshield and side window defogger outlets.
Apply vacuum to the orange vacuum line.
Air flow is diverted to the blower motor assembly air noise increase.
Apply vacuum to no vacuum lines.
Air flow is diverted to all outlets except the instrument panel.
Improper Air Delivery Diagnosis Step
Action
Value(s)
Yes
No
1
Perform the preliminary inspection.
Set the mode selector valve in the OFF position.
Disconnect the vacuum line at the check valve.
Use a hand vacuum pump in order to apply vacuum to the vacuum line that leads to the mode selector valve.
Does the vacuum hold?
--
Go to Step 3
Go to Step 2
2
Inspect the black vacuum line that leads to the mode selector valve.
Replace the mode selector valve if the line is connected and working properly.
Is the replacement complete?
--
System OK
--
3
Apply vacuum to the vacuum line that leads to the mode selector valve.
Inspect each setting (MAX A/C to DEFROST).
Does vacuum hold in every setting?
--
Go to Step 4
Go to Step 8
4
Disconnect the vacuum line multi-connector under the right side sound insulator.
Turn the ignition switch to the RUN position (engine not running).
Select the HIGH blower motor assembly speed.
Apply vacuum to each vacuum line individually (except for the black and orange lines).
Inspect the lines for air flow at the proper outlets. Refer to the Diagnostic Aid Chart.
Does air flow at the correct outlets?
--
Go to Step 6
Go to Step 5
5
Repair or replace the actuator linkage and/or air valve (door).
Is the repair complete
--
System OK
--
6
Apply vacuum to the orange vacuum line.
Does the blower motor air flow noise increase?
--
System OK
Go to Step 7
7
Repair or replace the air inlet vacuum actuator linkage and/or air inlet valve.
Is the repair complete?
--
System OK
--
8
Disconnect the vacuum line multi-connector at the mode selector valve.
Apply vacuum to each line at the multi-connector that is controlled by the mode selector valve at a setting where vacuum did not hold.
Is a vacuum leak present at one or more lines?
--
Go to Step 9
Go to Step 10
9
Repair or replace the vacuum line and/or the vacuum actuator.
Is the repair complete?
--
System OK
--
10
Replace the mode selector valve.
Is the replacement complete?
--
System OK
--
I Hope this helps
Good Luck
Pawl
