Very unusally low gas mileage. Check engine light came on and had it checked was told I had crack in upper intake manifold/plenum. That did not fix the problem???

Is the check engine light on, and if it is, get it read to find out what the codes are.

Sounds like the Idle air motor is not working , Possible throttle plates stuck , do you get a check engine light or service engine soon light if so pull the codes and let me know what they are and we can go from their , this will allow us to diagnose the problem better

Exactly what is the wrong? Need the part? Need to know torque adjust or porcedure to take off?

For 1994 Pontiac Grand Am 3.1L SFI OHV 6cyl check this procedure for Intake Manifold...
(see Figure 14)
The 3.1L engine is equipped with upper and lower intake manifolds. The upper intake manifold is also known as the intake manifold plenum.

CAUTION The fuel system is under pressure and must be properly relieved before disconnecting the fuel lines. Failure to properly relieve the fuel system pressure can lead to personal injury and component damage.

1. Relieve the fuel system pressure.
2. Disconnect the negative battery cable.
3. Remove top half of the air cleaner assembly and throttle body duct.
4. Drain and recycle the engine coolant.
5. Disconnect the EGR pipe from exhaust manifold.
6. Remove the serpentine belt.
7. Remove the brake vacuum pipe at the intake plenum.
8. Disconnect the control cables from the throttle body and intake plenum mounting bracket.
9. Remove the power steering lines at the alternator bracket.
10. Remove the alternator.
11. Label and disconnect the ignition wires from the spark plugs and wire retainers on the intake plenum.
    
12. Remove the ignition assembly and the EVAP canister purge solenoid together.

Fig. 14: View of the intake manifold and related components

1. Disconnect the upper engine wiring harness connectors at the following components:
   Throttle Position Sensor (TPS) Idle Air Control (IAC) Fuel Injectors Coolant temperature sensor Manifold Absolute Pressure (MAP) sensor Camshaft Position (CMP) sensor
2. Tag and disconnect the vacuum lines from the following components:
   Vacuum modulator Fuel pressure regulator PCV valve
3. Disconnect the MAP sensor from upper intake manifold.
4. Remove the upper intake plenum mounting bolts and lift off the plenum.
5. Disconnect the fuel lines from the fuel rail and bracket.
6. Install engine support fixture special tool J 28467-A or an equivalent.
7. Remove the right side engine mount.
8. Remove the power steering mounting bolts and support the pump out of the way without disconnecting the power steering lines.
9. Disconnect the coolant inlet pipe from the outlet housing.
10. Remove the coolant bypass hose from the water pump and the cylinder head.
11. Disconnect the upper radiator hose at thermostat housing.
12. Remove the thermostat housing.
13. Remove both rocker arm covers.
14. Remove the lower intake manifold bolts. Make sure the washers on the four center bolts are installed in their original locations.
    

When removing the valve train components they should be kept in order for installation the original locations.

1. Remove the rocker arm retaining nuts or bolts and extract the rocker arms and pushrods.
2. Remove the intake manifold from the engine. Remove and discard the gasket.
3. Using a suitable scraper, clean gasket material from all mating surfaces. Remove all excess RTV sealant from front and rear ridges of cylinder block
   

To install:

1. Place a 0.12 inch (3mm) bead of RTV, on each ridge, where the front and rear of the intake manifold contact the block.
2. Using a new gasket, place the intake manifold on the engine.
3. Install the pushrods in their original locations. Coat the pushrods with prelube.

The intake pushrods are marked yellow (5 3 /4 inch long) and the exhaust are green (6 inches long). Make sure the pushrods are properly seated in the valve lifters and rocker arms.

1. Position the rocker arms in there original locations and tighten to specifications. Refer to the procedure earlier in this section.
2. Install lower the intake manifold attaching bolts. Apply sealant 12345739 (or equivalent thread locking compound) to the threads of bolts. Tighten the vertical bolts first then the diagonal bolts to 115 inch lbs. (13 Nm).
3. Install the front rocker arm cover.
4. Install the thermostat housing.
5. Connect the upper radiator hose to the thermostat housing.
6. Fasten the coolant inlet pipe to thermostat housing.
7. Connect coolant bypass pipe at the water pump and cylinder head.
8. Install the power steering pump in the mounting bracket.
9. Loosely install the serpentine belt.
10. Connect the right side engine mount.
11. Remove the special engine support tool.
12. Fasten the fuel lines to fuel rail and bracket.
13. Install the upper intake manifold and tighten the mounting bolts to 18 ft. lbs. (25 Nm).
14. Install the MAP sensor.
15. Connect the upper engine wiring harness connectors to the related components removed earlier.
16. Connect the vacuum lines to the PCV, vacuum modulator and fuel pressure regulator.
17. Install the EVAP canister purge solenoid and ignition assembly.
18. Install the alternator assembly.
19. Connect the power steering line to the alternator bracket.
20. Install the serpentine belt.
21. Connect the spark plug wires to the spark plugs and intake plenum wire retainer.
22. Install the EGR pipe to the exhaust manifold.
23. Attach the control cables to the throttle body lever and upper intake plenum mounting bracket.
24. Install air intake assembly and top half of the air cleaner assembly.
25. Install the brake vacuum pipe.
26. Fill the cooling system.
27. Connect the negative battery cable, then start the vehicle and verify that there are no leaks.

For PARTS REPLACEMENT check in THIS LINK: carpartswholesale.com

Hope helps.

COULD BE GASKET ON MANIFOLD CRACK BUT ALSO TRY CLEANING YOUR AIR FILTER AND ALSO YOUR THROTTLE BODY THAT COULD HELP WITH THE GAS MILEAGE BUT IF YOUR LOSING COMPRESSION WHILE SITTING STILL IT COULD BE CRACK SOMEWHERE AND SUCKING IN AIR AT THAT POINT AND THATS WHY YOU STALL OUT

Bonneville, LeSabre, Park Avenue 2000-2005 Intake Manifold - Removal & Installation

3.8L Engine
VIN 1 Engine

1. Before servicing the vehicle, refer to the Precautions section.
2. Relieve the fuel system pressure.
3. Drain the cooling system.
4. Remove or disconnect the following:
   Negative battery cable Supercharger Thermostat housing Exhaust Gas Recirculation (EGR) tube at the intake manifold Engine Control Temperature (ECT) sensor Intake manifold

To install:

1. Install or connect the following:
   Intake manifold with new gaskets. Torque the bolts, working from the center out, to 11 ft. lbs. (15 Nm). ECT sensor connector EGR tube to the intake manifold Thermostat housing Supercharger Negative battery cable
2. Refill and bleed the cooling system.
3. Run the engine and check for leaks and proper engine operation.

VIN K Engine

1. Before servicing the vehicle, refer to the Precautions section.
2. Disconnect the negative battery cable.
3. Drain the cooling system.
4. Relieve the fuel system pressure.
5. Remove or disconnect the following:
   Fuel injector sight shield Air inlet duct Spark plug wires from the right side Manifold Absolute Pressure (MAP) sensor Vacuum lines from the intake manifold Fuel lines Fuel injector electrical connectors Fuel regulator vacuum line Fuel rail from the intake manifold Exhaust Gas Recirculation (EGR) heat shield Throttle cable bracket from the cylinder head mounting bracket and the throttle body cables Throttle body support bracket Upper intake plenum and gasket Thermostat housing Electrical connector from the Engine Coolant Temperature (ECT) sensor Drive belt tensioner assembly EGR valve outlet pipe Lower intake manifold
   
   
   Fig. Exploded view of the intake manifold and related components-3.8L (VIN K) engine
   

To install:



Fig. Upper intake manifold torque sequence-3.8L (VIN K) engine



Fig. Lower intake manifold torque sequence-3.8L (VIN K) engine

1. Install or connect the following:
   Intake manifold using new manifold gaskets. Torque the bolts in sequence to 11 ft. lbs. (15 Nm); then, re-torque to 11 ft. lbs. (15 Nm). EGR valve outlet pipe Drive belt tensioner assembly. Torque the tensioner bolts to 37 ft. lbs. (50 Nm). Electrical connector to the ECT sensor Thermostat housing Upper intake plenum. Torque the intake plenum bolts to 88 inch. lbs. (10 Nm). Throttle body support bracket Throttle cable bracket to the cylinder head mounting bracket and the cables to the throttle body lever EGR heat shield Fuel rail. Torque the fuel rail bolts to 88 inch. lbs. (10 Nm). Fuel lines Fuel regulator vacuum line Fuel injector electrical connectors Vacuum lines to the intake manifold MAP sensor Spark plug wires Fuel injector sight shield and air inlet duct Negative battery cable
2. Refill and bleed the cooling system.
3. Run the engine and check for leaks and proper engine operation.

Hope this help (remember rated this).

it is the intake manifold, actually called upper plenum the plastic crackes right behind the alt.autozone has one with aluminum water passages that wont do that.i bought one 3 years ago it was 266$ the dealer wanted 600$ for the same plastic one

mcdevito75 here, Vacumn lines broke or disconnected will truely effect gas mileage even with the best tune up, Leaking fuel injectors also, a leak in your fuel system also, you can usually smell the gas, also a crack in the intake manifold or loose bolts on the intake manifold will create bad gas mileage, have your repair shop check all these items for you.

a plenum is the top part of the intake manifold, modern fuel injected cars and trucks (gas engiones) have a upper and lower intake manifold, the upper section is called the plenum.

The most likely source of your problem is NOT a head gasket or cracked head, but the black plastic plenum or "upper intake manifold". These GM engines tend to melt, crack, or warp the plenum because of the hot EGR tube sticking up through the plastic.

If water is getting into your oil (milky looking), your lower intake manifold gasket is probably cracked. Once the plenum is off the engine, there are only about a dozen bolts holding the lower intake manifold on. When you replace the gasket, it is recommended to use an ALL METAL gasket with rubber seals to prevent this from happening again. The metal gaskets cost about $80 where as the plastic ones that are prone to breaking cost about $45. Better to fix it right once than to do it wrong several times.

Also, if you are still using Dexcool coolant (orangeish), be sure to change it every two years or so as it does bad things to seals when it gets old.

if the light is flashing it most like hasly some sort of misfire or converter failure, p0171 and p0174 is lean bank 1 and 2, replace you upper and lower plenum gaskets, while they have they off have they replace your ignition coil, plugs and wires, windstars are known for 2 things, lean codes,(plenum gaskets) misfire(bad ignition coil)