NEED HELP Poor Gas Mileage -

T

tippy

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Joined
Aug 31, 2025
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Canada
Hi all,

I'm getting 5.1 KM/L - I think this is poor but not 100% sure though - would be nice to know what the best mileage one can get from this 2002 Envoy Vortec 4.2L I6 engine - and / or perhaps what ways one can improve the gas mileage (gas is getting way too expensive nowadays) Thanks
 
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If you are doing a lot of stop and go city driving, you are going to be hard pressed to get any better ... maybe as much as 6 ... my experience on suburb driving.

Certainly, if your driving is more highway, then that number is bad. Place to start would be to look at fuel trims during idle and constant throttle to see if things look ok... then go from there depending on what's happening.
 
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tippy said:
Hi all,

I'm getting 5.1 KM/L - I think this is poor but not 100% sure though - would be nice to know what the best mileage one can get from this 2002 Envoy Vortec 4.2L I6 engine - and / or perhaps what ways one can improve the gas mileage (gas is getting way too expensive nowadays) Thanks
Click to expand...
That's roughly 13 MPG for those south of you, mine gets combined 15 (mostly street, not much highway driving) so it wouldn't hurt to do at least a little diagnostic per @budwich but probably doesn't warrant extensive/expensive repairs.
 
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Rather than buy a dedicated scanner I would buy a ELM327 Bluetooth or WiFI OBD dongle and then use it in conjunction with an app like Torque Pro or CarScanner on your phone.
 
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I like to start with basics. Age and miles on the existing plugs? Plus, like @budwich and @dhanger alluded to, please answer about the kind of driving you do. Live data scanners are not that expensive.
Also, age of the fuel filter and when was the last time the throttle body was cleaned and the MAP sensor and Intake Air Temp sensor cleaned?

Basic maintenance goes a long ways toward getting the best miles per gallon you can. Nothing wrong with looking at fuel trims but maintenance pays to do.
 
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And I'll take that idea one more step down that "Fundamentals Step Ladder" and start with the "Big Three of ALL Four Stroke Engines"

(1) Fuel
(2) Air
(3) Spark (Compression)

By now, ANY GMT360 Platform with the GM LL8 4,2L Engines having 24 Years in the Rear View Mirror will be suffering with an accumulation of in-cylinder Black Carbon Mung congesting the Valve Seats, Piston Tops and most importantly, Gas-Gum Lacquer Trapping the Two "Low Tension" Thin Compression Rings at the tops of all six Pistons that Reduces Compression, Raises Quench, Invites Misfires and ruins Gas Mileage as a result.

Visit THESE Threads to get a better idea of how you can TREAT this condition, without performing exotic, expensive or difficult ways to alter these aging conditions using Chemistry to Dissolve and Remove the Carbon Build Up Contamination. Please note that in lieu of using the ACDelco TEC (Top Engine Cleaner) which is no longer available... Choose the alternate Berryman Product instead:

1780863362391.png


C

Thread 'I'm going to use the A.C. Delco T.E.C. Liquid or Aerosol?'

Hi all. I've decided to do the top end soak with A.C. Delco product. They make it in an aerosol also. Any suggestions on which might be better? After doing some add'l research...It appears the aerosol is made to use while the engine is running by spraying it through the intake.

It looks like I'll stay with the liquid and soak the cylinders as mentioned in previous threads.

C

Thread 'A.C. Delco Foaming Top End Cleaner....'

Good Evening All,
I've read about this procedure here on the forum. I'm wondering if it would be OK to do, regardless of if I have carbon build up or oil consumption.

I have noticed a small amount of oil accumulated in the intake plenum. I do have a little piston "slap" that goes away upon heating up to operating temp. I tend to use about a quart of oil in 3000-4000 miles. Engine currently at 190 k miles.

Any drawbacks to doing the top end cleaning process? Or best to leave things alone?

ZD3SF

Thread 'Carbon cleaning the 4.2L LL8. Experiment'

I have recently performed carbon cleaning on the pistons and valves using a method recommended by @MRRSM and I figured to document the experience for those who would like to pursue this in the future. The method involves pouring AcDelco Top engine cleaner in the spark plug holes. This is NOT a guide, nor do I pretend that the performed method is the flawless.

Required tools/materials:
  1. 10 mm socket, 13mm socket ,5/8' spark plug socket, ratchet, and suitable extensions.
  2. AcDelco Top Engine cleaner (PN: 10-3015 ), 2 x 32 oZ cans
  3. Shop towels
  4. Optional: seafoam, new spark...
  • Like
 
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mrrsm said:
And I'll take that idea one more step down that "Fundamentals Step Ladder" and start with the "Big Three of ALL Four Stroke Engines"

(1) Fuel
(2) Air
(3) Spark (Compression)

By now, ANY GMT360 Platform with the GM LL8 4,2L Engines having 24 Years in the Rear View Mirror will be suffering with an accumulation of in-cylinder Black Carbon Mung congesting the Valve Seats, Piston Tops and most importantly, Gas-Gum Lacquer Trapping the Two "Low Tension" Thin Compression Rings at the tops of all six Pistons that Reduces Compression, Raises Quench, Invites Misfires and ruins Gas Mileage as a result.

Visit THESE Threads to get a better idea of how you can TREAT this condition, without performing exotic, expensive or difficult ways to alter these aging conditions using Chemistry to Dissolve and Remove the Carbon Build Up Contamination. Please note that in lieu of using the ACDelco TEC (Top Engine Cleaner) which is no longer available... Choose the alternate Berryman Product instead:

View attachment 119688


C

Thread 'I'm going to use the A.C. Delco T.E.C. Liquid or Aerosol?'

Hi all. I've decided to do the top end soak with A.C. Delco product. They make it in an aerosol also. Any suggestions on which might be better? After doing some add'l research...It appears the aerosol is made to use while the engine is running by spraying it through the intake.

It looks like I'll stay with the liquid and soak the cylinders as mentioned in previous threads.

C

Thread 'A.C. Delco Foaming Top End Cleaner....'

Good Evening All,
I've read about this procedure here on the forum. I'm wondering if it would be OK to do, regardless of if I have carbon build up or oil consumption.

I have noticed a small amount of oil accumulated in the intake plenum. I do have a little piston "slap" that goes away upon heating up to operating temp. I tend to use about a quart of oil in 3000-4000 miles. Engine currently at 190 k miles.

Any drawbacks to doing the top end cleaning process? Or best to leave things alone?

ZD3SF

Thread 'Carbon cleaning the 4.2L LL8. Experiment'

I have recently performed carbon cleaning on the pistons and valves using a method recommended by @MRRSM and I figured to document the experience for those who would like to pursue this in the future. The method involves pouring AcDelco Top engine cleaner in the spark plug holes. This is NOT a guide, nor do I pretend that the performed method is the flawless.

Required tools/materials:
  1. 10 mm socket, 13mm socket ,5/8' spark plug socket, ratchet, and suitable extensions.
  2. AcDelco Top Engine cleaner (PN: 10-3015 ), 2 x 32 oZ cans
  3. Shop towels
  4. Optional: seafoam, new spark...
  • Like
Click to expand...
I have this exact Berryman kit on top of my tool box. I am waiting for Linda to get back from Texas and she wants to be here when I do the service. Her engine is 24 years old but only 78k miles. Still gonna do the cleaning.
 
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