Poor Fuel economy

This is probably the MAF sensor becoming dirty or needed replacement, though the sensor is starting to go bad the computer will not set a code for this here is a guide to help walk you through the repair.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please run down this guide and report back.

Bad gas mileage is usually tied to an oxygen sensor or a leak near it. The sensor sees a lean condition and adds gas that is not needed. Other possibilities are blockages in the air intake path or exhaust path.

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:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

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.

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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

Removal Procedure

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.

Picture 3

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.

Picture 4

6. If equipped with vacuum assisted brakes, remove the vacuum brake booster hose from the rear of the intake manifold.

Picture 5

7. Remove the Positive Crankcase Ventilation (PCV) hose with valve.

Picture 6

8. Remove the MAP sensor (1) from the intake manifold.

Picture 7

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).

Picture 8

10. Remove the EVAP purge solenoid bolt (2), solenoid (3), and isolator (1) from the intake manifold.

Picture 9

11. Loosen the intake manifold bolts.
12. Remove the intake manifold.

Picture 10

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.

Installation Procedure

Picture 11

1. Install NEW intake manifold gaskets (1) to the intake manifold.

Picture 12

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
pic 13

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.).

Picture 14

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.).

Picture 15

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.

Picture 16

7. Install the MAP sensor (1) to the intake manifold.

Picture 17

8. Install the PCV hose with valve.

Picture 18

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.

Picture 19

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.

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Let me know what you find.

Joe

Welcome back. Thanks for letting me know.

Take care,
Joe

Welcome back:

It sounds like it is all related to the fuel level float in the tank. If it is failing, it may be sending false signals to the gauges. What is the actual MPG you get with the vehicle?

Welcome back:

You are welcome to come back as many times as you want. What's why we are here. I'm very patient. I have to be honest, the MPG you are getting is about typical for this vehicle.

I may have asked you this already, but do you have a live data scanner you can use? If so, can you tell me what the fuel trims are, both short and long term?

Welcome back:

I will be able to tell by the fuel trims. And yes, I honestly feel that MPG isn't out of the ordinary for this type of vehicle. If the fuel trims are within spec, then it's running normal.
Here is what I need.

Short term fuel trims STFT

Long term fuel trims LTFT

Let me know.

Joe

Welcome back:

Sounds like a plan. Let me know.

Joe

Hi Doug:

The fuel trims indicate the engine is running too rich, as you expected. The negative 25 means the computer is trying to limit the amount of time the injectors are open to lean the fuel getting to the engine. With that, I can't remember if you checked fuel pressure and the regulator or have done anything specific to the injectors. The trims should be close to 0.