Picked up a 2006 gmc envoy Xl gas mileage...not so good

mrrsm

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Performing an Engine Un-Sludging Procedure could NOT hurt the OP's Motor either. Here are the "Even Dozen Steps for a Proper Anti-Sludge Treatment":

(1) Drain the Old Motor Oil and remove the Old Oil Filter.
(2) Install a "Cheap" Oil Filter and 6 Quarts of "Cheap" Organic Motor Oil.
(3) Pour in 1 Quart of "Cheap" Automatic Transmission Fluid.
(4) Start the Engine and allow it to Warm Up and Idle for 10-15 Minutes.
(5) DO NOT RACE THE ENGINE OR PERFORM "SNAP THROTTLE" ACTIONS.
(6) Shut Down the Engine and Immediately Drain the Trashed Oil and Trashed Filter.
(7) Install a HQ Mobil1 or K&N Oil Filter. ALWAYS PRE-FILL !!!
(8) Fill the Crankcase with 7 Quarts of Mobil1 5W-30 FULL Synthetic Motor Oil.
(9) Start the Engine at Idle for 5 Minutes & Check the Oil Level & look for any Leaks.
(10) "Grease Pencil" your POST-OIL Change Mileage somewhere convenient.
(11) Enjoy the Ride...and REPEAT this procedure in 3,000 Miles.
(12) Return to Regular Mobil1 Oil and Oil Filter Changes after the 2nd Treatment.
 
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budwich

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Jun 16, 2013
2,027
kanata
Correct me if I'm wrong, but I didn't see a vacuum reading on the OP's screenshots, maybe you were looking at mine?

Vacuum should be higher at idle and drop under load. You can almost correlate the vacuum to fuel economy in most cases, higher load/throttle opening will result in lower vacuum.



Not knowing the history of the vehicle, and the fact you have a stock LWB and getting 14 mpg at a steady 60 MPH cruise, if that were mine and I was in your position, I would start with cleaning the MAF and pull a few plugs to check their condition and try to get a peek inside the cylinder and look at the piston crown for any carbon build up.

I would also run a can of BG44k through it, it's not snake oil, works very well. If you have some carbon build up it will lnock it down quick. The fact you had better MPG's with higher octane fuel makes me highly recommend this trearment.

I don't like to throw $$ at things just because, but since you want to figure this out and seem like you want to keep it, I know I would change the plugs and the upstream O2 sensor.

With respect to your screenshots, the ignition advance and coolant temp should be a numerical display/dial and not a graph.

If your operating temp is below 190 deg I would also look at replacing the thermostat and coolant temp sensor, both at the same time.

Two things to consider is:

1) The history is unknown and would not be a bad decision to replace the plugs, upstream O2 sensor, clean the MAF, replace thermostat, coolant temp sensor to ensure you have brought those areas up to date.

2) When driving, if those above areas are ok, you are applying higher load for a given condition, so that makes me curious to your cylinder compression given the mileage. That's not saying it should be slightly low, but a possibility given your MPG's.

Not saying you need a compression test, but always good to know what you have and it's easy to do once a plug is removed.

Also, have you checked for dragging brakes? The wheels should have some resistance as the pads run at zero-clearance, but shouldn't feel as the brakes are being applied.

Take it for a spin in the morning for a few miles, can you feel excessive heat radiating from one of the wheels? Not likely but something to check.
my comment about vacuum wasn't about any measurements that were posted or otherwise, it was about the fact that at idle, vacuum is large which IF there are any air leaks in the intake air areas, then these will throw off the mixtures and thus you will see an issue at idle BUT not at speed. The posted graphs that deer... posted appear to show differences at idle versus speed and hence the suggestion to look at the MAP (not MAF) and for possible air leaks.
 
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gmcman

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Dec 12, 2011
4,656
comment about vacuum wasn't about any measurements that were posted or otherwise, it was about the fact that at idle, vacuum is large which IF there are any air leaks in the intake air areas, then these will throw off the mixtures

Gotcha, my mistake, I misread your comment.
 

deerslayer1985

Original poster
Member
Feb 26, 2015
148
I mentioned awhile back I thought the plugs looked good. I had pulled one out and just looked at it. Not even paying attention to the brand of plug. SO today I went looking for the stuff you mentioned. I couldnt find it. So the guy told me lucas fuel system cleaner and fuel conditioner worked really well too. So I got a thing of it, and put it in the tank. I bought some maf cleaner, and I bought some seafoam combustion cleaner. It comes with a straw you tuck in between the throttle body and intake hose. So I followed the directions, and sucked in the whole can of seafoam. Then shut off the engine for 10 minutes for what they call a hot soak. While this was going on. I removed the maf sensor, and cleaned it, and went ahead and cleaned out the intake duct too. Got it all dried, and reassembled. Then I went for a "spirited" drive as the can said. To blow out the hydrocarbons. The average mpg was on 12.6...I was flooring it, and running it pretty hard, and my mpg calculator was going up on the 10 minute drive. I wasnt easy on the gas, but it went up to 13.8 mpg in the rough trip. I let it cool down, and pulled out a spark plug to have a second look. I'm honestly not sure about them....but they arent acdelcos. They are bosch double iridiums. Here is a picture of the tip.
 

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mrrsm

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Looks like you're on the Right Track with your "Carbon-Busting" Tactics....

From zooming in your Image... It appears that one of the two mini-tips has lost its ultra-thin 'cap' of Platinum-Palladium.

In support of your efforts with using Sea-Foam Products... THIS Guy took your Tactics one step further when he:

(1) Changed His Oil and Oil Filter.
(2) Poured in a substantial amount of Sea-Foam Liquid to the Crank-Case Oil Reservoir. (...From 00:2:02 into the Video)
(3) Allowed the Engine to High Idle and Warm Up.
(4) Later Changed Out the HOT, BLACK OIL that carried out all of the Carbon Contaminants that were in the Oil Pan and circulating through the Engine.
(5) Exhibited remarkable Engine Performance afterwards.

 
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Chickenhawk

Member
Dec 6, 2011
779
We are getting somewhere. I agree with everyone else to stay with your plan to replace the plugs and the upstream O2 sensor. Plugs MUST be AC Delco 41-103. Trust us on this one. No other plugs work well, and many can cause misfires that will plug the cat if gone on too long.

Your upstream O2 sensor is definitely a bit lazy. Most of us routinely replace the O2 sensor with every plug change. Even if nothing shows on the graph, it helps prevent a sensor getting so rusted into the manifold that it is almost impossible to remove.

Next step is to do an exhaust backpressure test to see if your low coolant temps or misfiring plugs may have caused some blockage in the past. Readings should be 0 at idle, and no more than 1 PSI at 2500 RPM. Replace plugs, O2 sensor and cat if needed, and you will be up into the 18s, plus your acceleration on the highway will be back.

As for the advice you got on Lucas for being "as good" as other fuel system cleaners, ignore the advice. The ONLY fuel system cleaner that works are ones with the chemical PEA. Everything else is snake oil. You will find PEA in only a very few. Here are some:
- Redline SL1
- Gumout with Regane
- Chevron Techron/GM Fuel System Treatment
- Royal Purple Max Atomizer
- BG44K
 
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gmcman

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Dec 12, 2011
4,656
FWIW, and not the end-all value for all our exhaust systems, but the last couple times I checked the backpressure on my stock exhaust, I was at 1/4 PSI at 2500 RPM.

At just over 1/2 PSI with my short-lived Walker system I was losing MPG's on the highway.
 
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mrrsm

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One other oft overlooked issue with a "Sluggish Response... Right out of the Gate..." has to do with having "Excessively Low Tire Pressure". You'll be surprised at just how 'positively peppy' a Large SUV or Truck will behave once the Tires are no longer shy of Proper Air Pressure... by as much as 10 PSI.

If nothing else, your Gas Mileage WILL also Increase when so much Power is not being wasted while trying to overcome so much unnecessary additional friction from having Much Wider Tire Foot Prints meeting the surface of the road. :>)
 
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deerslayer1985

Original poster
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Feb 26, 2015
148
Ok guys. I do appreciate all the help. I will try to rent the exhaust pressure test thing, and do that hopefully over the weekend. The oil still looks new in this. The oil life monitor shows 23% life remaining, but it is mobil 1 full synthetic in it. The guy I bought it from said thats what has always been used in it. So I dont think the oil is sludged up. My tires are at 42 psi. I've checked them frequently. I just put air in a couple of them the other day. They had dropped to 39. I try to check them every now and then. I will definitely be putting acdelco plugs in, and an upstream o2 sensor....hopefully I dont have to do a catalytic converter. I'll keep my fingers crossed, but I've been driving the envoy all evening, and the on dash mpg calculator is going up slowly just from cleaning the maf, and cleaning the top end. Im at 14.3 mpg now. Started earlier today at 12.6. Id be happy if I can get it to do 16 ish in the city, and 20 ish highway.
 

Chickenhawk

Member
Dec 6, 2011
779
Here's another tip. Many muffler shops will do the exhaust backpressure test for you. Bring the new O2 sensor with you as they will have to take out the old one to get the gauge into the manifold.

If you are doing your own test with a rented gauge, soak the sensor threads in PB Blaster for a few days beforehand. Some members report it is easier to change if slightly warmed up. If you are renting a gauge, be sure it is not a regular vacuum gauge. Exhaust backpressure gauges go from 0 to 6 PSI. Vacuum gauges can go up to 200 PSI, and it is difficult to see the difference between 0 and 1 PSI on those gauges, and yet 1 PSI makes a difference, especially as my friend gmcman points out above.
 
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JayArr

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Sep 24, 2018
504
Mission BC Canada
I did notice it does seem to not have the get up and go going up a hill from a stop after I have driven for awhile. It goes, but not like it should I feel.

Mine was like that, 2005 XL, terrible mileage and no guts. My cat was almost 100% clogged. Replaced it with a generic Walker version (Part#93253) and got both my mileage and power back.

I know the cat is expensive and so you're looking for any answer that keeps you from having to buy a new cat but it sounds like that's what's wrong, the sooner you face the elephant in the room the sooner you'll know for sure. Check the backpressure.

From AxelAddict.com

  • Set the transmission to park (automatic) or neutral (manual), and apply the parking brake.
  • Disconnect the vacuum hose at the brake power booster and connect the vacuum gauge to the hose or to another direct intake manifold port.
  • Start the engine and let it idle for about 15 to 20 minutes so that it reaches operating temperature.
  • At idle, vacuum reading should be between 18 and 22 in-Hg (inches of Mercury).
  • Increase and hold ending speed to about 3000rpm. Vacuum reading should drop but should go back to the previous level in a few seconds; otherwise, it's likely the exhaust system is blocked. Continue to the next step.
  • Repeat the test, but this time, snap the throttle valve open about four times, rising engine speed to about 2500rpm. Take note of the vacuum reading with each snap of the throttle.
  • If you see that gauge needle dropping steadily to near zero, most likely the catalytic converter or muffler is restricted.
 

TobyU

Member
Oct 31, 2019
34
SW Ohio
We just did a 350 mile round trip in wife's 04 TB ext and it got 18.3. It more than likely needs some plugs and has 237K on it but it runs well and no codes.
It has always felt anemic even on slight inclines but that is coming from an expedition with a 5.4 V8 in it.
It will go if you give it gas but we are used to just barely pushing pedal and having more power or acceleration. This one has always felt like you have to push more than other vehicles.

I don't see how you got 22-23 in your other one.... Lol
 

deerslayer1985

Original poster
Member
Feb 26, 2015
148
No updates yet, been very busy. My other one wasnt the extended version. So it was quite a bit lighter. It had low miles, and that was highway miles per gallon.
 

deerslayer1985

Original poster
Member
Feb 26, 2015
148
My local autozone didnt have the exhaust pressure tester available at the moment to rent...So I cant do that today, but I watched some youtube videos. They say you can check the temp at the inlet and outlet of the cat and tell if it is bad, or clogged up. I have a laser thermometer. Has anyone done this on theirs? It says the outlet of the cat should be 30-100 degrees warmer than the inlet. If its less than that it is more than likely clogged. Im going to try it. It says to warm the car up first. Up to operating temp. So I have to go to town. I will check it as soon as I get back.
 

TollKeeper

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Dec 3, 2011
8,047
Brighton, CO
... Id be happy if I can get it to do 16 ish in the city, and 20 ish highway.
Good luck on that one.. On my XUV with a v-8 and 3.42 I can get 22mpg at 72mph, and 18mpg at 75mph 100% hiway. My general fuel mileage for combined 70% hiway and 30 city was 16.8 with the 10% blended gas, and 17.1 straight summer gas. With the same metrics above, 16.2 blended and 16.3 straight on winter fuels.

I would also like to know what kind of unicorn you have for 22-23 on your TB. Those are just unheard of mileages.

When I moved to Denver, I got 32.2 mpg, but cruise was set at 53mph and was drafting the moving truck that my wife was following (I was driving the moving truck). That made for a VERY LONG DRIVE going from St Louis to Denver in a truck that would not go above 53.
 

deerslayer1985

Original poster
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Feb 26, 2015
148
My trailblazer was an 02 short wheel base. It had 3.42 gears. I had a flowmaster muffler in place of the giant muffler, and ditched the big trash can muffler at the back and didnt have the spare tire mounted. It was nothing to average 22-23 on highway with it....but to continue with the envoy xl. I just checked the cat temperature. I had been driving for about 20 minutes. So it was up to temp. I shot the laser thermometer on the front of the cat just at the inlet. The temp was fluctuating between 370-376. Then I shot the back of the cat in the same area as I did the front. It was reading 430-438. So based on that alone it should be good shouldnt it?
 

gmcman

Member
Dec 12, 2011
4,656
It shows that the cat is lighting off but may not tell the whole story. Could have a partial clog and still react with the unburned fuel, resulting in the temp difference.
 

deerslayer1985

Original poster
Member
Feb 26, 2015
148
Ok, I'm hoping to have some free time this weekend. Hopefully autozone has the exhaust pressure gauge available then. So what pressure am I looking for? I've watched different videos and there is a lot of conflicting amounts of allowable pressure.
 

budwich

Member
Jun 16, 2013
2,027
kanata
you have two vehicles, one you like the way it runs and one that dislike. You might consider doing some odb data comparison between the two.... things like you have done, idle fuel trims, constant throttle (light / medium), o2 sensors at idle and at constant throttle.
 

Chickenhawk

Member
Dec 6, 2011
779
As far as exhaust backpressure, our factory exhaust systems are VERY efficient. This means that at idle, you should not get any backpressure. (0 PSI). When they run it to 2500RPM, pressures should still be near zero. Some say 3 to 4 PSI, but on the Trailblazer, that is way too high. I would suggest 1 PSI should be max.
 
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coolasice

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Oct 27, 2013
1,019
Northern Maine
As far as exhaust backpressure, our factory exhaust systems are VERY efficient. This means that at idle, you should not get any backpressure. (0 PSI). When they run it to 2500RPM, pressures should still be near zero. Some say 3 to 4 PSI, but on the Trailblazer, that is way too high. I would suggest 1 PSI should be max.
Service manual says maximums 1.25psi @ idle, 3psi @ 2000RPM
 
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WarGawd

Member
Sep 2, 2012
468
Just picked up a 2006 GMC Envoy XL for a great price. It is in really good condition. I knew the guy who owned it. It has 185,000 miles on it, but has been babied, garage kept, and maintenance performed regularly on it. Just the usual brakes, oil change, air filter. Never had any issues from what he has told me. I have found out it doesnt do nearly as good on gas as my 2002 trailblazer did. I constantly averaged 22-23 mpg in my trailblazer. I have ran a couple tanks through the envoy, and Im only averaging around 14 mpg. The only differences between the envoy xl, and trailblazer I have beside the obvious size difference is....my trailblazer didnt have the spare tire underneath. It didnt have the big can muffler on the back I removed it. I cant imagine those 2 things doing much if anything at all for gas mileage. Do the Envoy XL's really suck that bad on gas, or is there anything I can do to increase it. Even if only slightly?

My '05 XL had the 3.73 rear end gears and it was DEFINITELY more of a highway cruiser. Gas mileage really suffered in town doing a lot of stop and go driving. Check my signature for my gas consumption records (until I got 2 years behind, gave up and then started working backwards to enter them LOL)
 

pell

Member
Jun 5, 2017
88
Pelham, NH
Late up-date. Finally got to pulling my plugs. Wrong ones were installed. It had Acdelco 41-981 in there. Pulled heat shield off manifold has three bolts broken two clean-off at head. Truck runs great with new plugs no hesitation when throttle applied. Can wait to pull my camping trailer next weekend. Going to order new manifold, power steering lines and transmission lines and probably barter with my mechanic to do the work.
 

gmcman

Member
Dec 12, 2011
4,656
Hopefully the plugs were the root cause, I don't know what you have for heat protective sheathing on the trans lines, but I'll find what I used and link it later today.
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Late up-date. Finally got to pulling my plugs. Wrong ones were installed. It had Acdelco 41-981 in there. Pulled heat shield off manifold has three bolts broken two clean-off at head. Truck runs great with new plugs no hesitation when throttle applied. Can wait to pull my camping trailer next weekend. Going to order new manifold, power steering lines and transmission lines and probably barter with my mechanic to do the work.
41-981 were not the wrong plugs... just the original ones... they were superseded by the 41-103
 

pell

Member
Jun 5, 2017
88
Pelham, NH
Don't remember seeing that number anywhere, assumed if it wasn't the 41-103 you where going to blow the engine up. Learn something new everyday. Well if they are the originals they had 238,000 miles on them so they might have needed to be changed. They gapped out at .058.

Here is a pic of a pair
 

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Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
Well if they are the originals they had 238,000 miles on them so they might have needed to be changed.

Yeah, I think they are supposed to be changed every 100K miles, so they were twice overdue.
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Don't remember seeing that number anywhere, assumed if it wasn't the 41-103 you where going to blow the engine up. Learn something new everyday. Well if they are the originals they had 238,000 miles on them so they might have needed to be changed. They gapped out at .058.

Here is a pic of a pair
Page 6-14 of the owners manual....
 

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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
They are available from GM's site, well at least in Canada.
 

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