Welcome to 2CarPros.
My first concern that needs attention is the intake vacuum leak. If the engine is getting unmetered air into the combustion chamber, it is going to cause a lean mixture at the O2 sensors. As a result, the computer is going to try to make the air fuel mixture richer. And yes, this will cause excessive fuel consumption.
To confirm there is a leak, here is a link that shows how to do it:
As far as the decrease in MPG when you stop, that is normal. The engine is running and consuming fuel, but the vehicle isn't moving. Therefore, the MPG will drop.
The other concern is the coolant temp. Follow the diagnostic flow chart in pictures 1 and 2 to determine if the fan clutch is bad. If may be fully engaged all the time which can cause the engine not to warm up properly.
One more thought. If you have access to a live data scanner, I need you to check the engine coolant temperature sensor reading when the engine is cold and at normal operating temp.
See if it seems accurate.
Of you confirm the intake is leaking, here are the directions for replacement of the gasket. The remaining pictures correlate with these directions (starting with number 3).
INTAKE MANIFOLD REPLACEMENT
Intake Manifold Replacement
Important:The intake manifold, throttle body, fuel rail, and injectors may be removed as a complete assembly. If not servicing the individual components, remove the manifold as a complete assembly.
1. Remove the throttle body.
2. Remove the fuel injectors.
3. Disconnect the following electrical connectors:
- The Manifold absolute pressure (MAP) sensor (1)
- The knock sensor (2)
4. Remove the knock sensor harness electrical connector from the intake manifold.
5. Set the electrical harness aside.
6. If equipped with vacuum assisted brakes, remove the vacuum brake booster hose from the rear of the intake manifold.
7. Remove the Positive Crankcase Ventilation (PCV) hose with valve.
8. Remove the MAP sensor (1) from the intake manifold.
9. Remove the Evaporative Emission (EVAP) purge solenoid vent tube by performing the following:
9.1. Remove the EVAP tube end from the solenoid (1).
9.2. Remove the EVAP tube end from the vapor pipe (2).
10. Remove the EVAP purge solenoid bolt (2), solenoid (3), and isolator (1) from the intake manifold.
11. Loosen the intake manifold bolts.
12. Remove the intake manifold.
13. Remove the intake manifold gaskets (1) from the intake manifold.
14. Discard the old intake manifold gaskets.
15. If required, clean and inspect the intake manifold.
1. Install NEW intake manifold gaskets (1) to the intake manifold.
2. Install the intake manifold.
3. Apply a 5 mm (0.20 inch) band of threadlock GM P/N 12345382 (Canadian P/N 10953489), or equivalent to the threads of the intake manifold bolts.
Notice: Refer to Fastener Notice in Service Precautions.
See: Engine, Cooling and Exhaust > Vehicle Damage Warnings > Fastener Notice
Intake Manifold Bolt Torque Sequence
4. Tighten the intake manifold bolts.
4.1. Tighten the bolts a first pass in sequence to 5 Nm (44 inch lbs.).
4.2. Tighten the bolts a final pass in sequence to 10 Nm (89 inch lbs.).
5. Install the EVAP purge solenoid (3), isolator (1), and bolt (2) to the intake manifold.
Tighten the bolt to 10 Nm (89 inch lbs.).
6. Install the EVAP purge solenoid vent tube to the solenoid (1) and vapor pipe (2).
Important:Lightly coat the MAP sensor seal with clean engine oil before installing.
7. Install the MAP sensor (1) to the intake manifold.
8. Install the PCV hose with valve.
9. If equipped with vacuum assisted brakes, install the vacuum brake booster hose to the rear of the intake manifold.
10. Route the electrical harness into position over the engine.
11. Connect the knock sensor harness electrical connector to the intake manifold.
12. Connect the following electrical connectors.
- The MAP sensor (1)
- The knock sensor (2)
13. Install the fuel injectors.
14. Install the throttle body.
Let me know what you find.
Images (Click to enlarge)
Wednesday, March 20th, 2019 AT 8:26 PM