PCM Tune = poor gas mileage

[DragonTX]

I own a '07 with an I6 with 72,000 mi, and it has been a number of months since I have been running a PCM tune, and sadly my mileage results have been very poor. In september, I got the PCM for less tune, installed the PCM along with a Volant CAI, Electric fan, and the big 3. Before all of this was done I was getting 16.8 Mpg average by my calculations, and around 16.2 on the DIC. Granted, I was very impressed with the performance enhancements and expected the mileage to drop for a while due to "lead foot disease"(in which I was having a lot of fun with my very happy foot and its association with the seat of my pants). It is a new, all around better performing truck now. I used to hate the lag off start, and the shifting was horrible, especially when I was towing my Triton 20' bass boat. No more! I got the stage 2 shift mod and what a difference! I recommend the stage 2 firmness for it is just right with good pull throughout the whole power range.
Impressions aside though, after some months my foot has been healed and I had started to do some real world efforts to see what mileage I was getting. In the last 3 months of calculations and comparing them with the DIC I am shocked (to say the least). I am now averaging 11.8 MPG!! (11.6 on the DIC) This is appauling since with all of the mods I was expecting to see a GAIN in mileage after the inititial phase of lead footing was over (and really I did not race the thing at all I take very good care of my TB and expect to have it a long time). For the last 2 months I have been driving as economical as possible (slow gradual acceleration, foot off gas and lengthy coast to stops); more economical than I have ever driven it when I was in the 16's mpg. I do not have to drive far and go through 20-30 gallons a month so even though I was shocked 4 months ago I felt the need to give it time to make sure my calculations remained constent; and they did.

So what I am hoping is that someone out there can help me out with some advice.


Note: I am kicking my a** right now because I did the core exchange so I do not have the stock PCM to try.
I did do the CASE relearn procedure and also did the security learn

 

[CaptainXL]

16.8 on the highway is in my estimation not good. I have and XL and get 19 mpg hwy.

I would definitely look into the leading causes of poor gas mileage. Namely the thermostat sticking open and restricted exhaust.

Also check to see if you are supposed to be using high octane for the tune. Timing will retard if there is knocking.

Wouldnt be a bad idea to check compression as well.

Also make sure you use the refill method to calculate mileage. The DIC has been known to be inaccurate at best.

 

[The_Roadie]

Also check for vacuum leaks, and loose intake manifold bolts. Get a scan tool on it and measure Long-term and short-term fuel trims. There's some discussion here and elsewhere about how excessive fuel trim adjustment can kill mileage due to not delivering the proper A/F ratio even if the PCM thinks it is. Fuel injector leaks may also be an issue, along with fuel leaks if you're losing it while driving along.

 

[HARDTRAILZ]

I got the PCM for less tune, installed the PCM along with a Volant CAI, Electric fan, and the big 3.

So you do not know if it was the PCM tune or not?

The CAI may actually be to blame.

 

[Wex]

This might also possibly be related to winter gas and the cooler temps. I always have a significant mpg drop during this time of year. Not to the extreme as yours but to lose an easy 2-3 mpg.

 

[DragonTX]

So you do not know if it was the PCM tune or not?

The CAI may actually be to blame.

It was only after the install of the PCM for less tune and CAI that the mileage dropped. I was on it for the 1st month or so and figured that was what caused the decrease, but when I started eco driving the mpg remains the same. Lately I just started to drive more heavy footed and that doesn't seem to change mpg much. I will know more accurate #'s on the next fill up but on the DIC for now it is a .1 difference.

How do you think the CAI would be to blame? I did tell them when I ordered the tune that I was installing an Efan and CAI.

 

[DragonTX]

This might also possibly be related to winter gas and the cooler temps. I always have a significant mpg drop during this time of year. Not to the extreme as yours but to lose an easy 2-3 mpg.

I understand this. ...My calculations started in sept. I live in NY and temps did not drop below 55 or so until late Nov-Dec and lately the DIC is down to 11.4 which matches my calculations

 

[HARDTRAILZ]

How do you think the CAI would be to blame? I did tell them when I ordered the tune that I was installing an Efan and CAI.

How do you think the tune could be to blame? I know of lots more stories of CAI creating lost fuel econmy than I do tunes causing it.

I understand this. ...My calculations started in sept. I live in NY and temps did not drop below 55 or so until late Nov-Dec

That has zero bearing on what fuel is delivered to the stations. They still stock winter blend gas even if it 90 degrees in the winter months.

 

[jrSS]

Is the pcm tuned for reg octane or higher octane?

 

[CaptainXL]

Is the pcm tuned for reg octane or higher octane?

Asked this a week ago and he doesn't know apparently. I would try using 93 octane for a while to find out or call pcm for less and find out.

 

[jrSS]

This^^^

 

[DragonTX]

Is the pcm tuned for reg octane or higher octane?

Yes it appears so, here is the data PCM for less had given me:


Broadcast Code (1993 & older only):
Engine: 4.2L
Transmission: Auto
Built Transmission?: stock
Rear End Gear Ratio: stock
Current Tire Size: stock
Stock Tire Size: stock
Shift Firmness: 2
Torque Converter: stock
Stock Fuel Injectors?: yes
Injector Size: stock
Injector Make: .
What PSI are injectors rated at?: .
Fuel Octane: 87
Delete 9th Injector?: No
Disable Displacement On Demand?: Not Applicable Speed Limiter: Remove Rev Limiter: tuner's choice (recommended)
Thermostat: stock
Skip Shift: stock
Desired Idle: tuner's choice (recommended) Trouble Codes:
Elevation: 275 FT
Do you tow?: yes
Electric Fan Patches: 02-07removecodes
Mass Air Flow Sensor: no
MAF:
Is the MAF ported?: No
Is the MAF descreened?: No
Throttle Body Size: stock
Cold Air Intake: airboxstock
Intake Manifold: stock
Exhaust Manifold: stock
Catalytic Converters: stock
Catback: Stock
Nitrous: no
Size of N20 Shot:
Are you using a Timing Retard Box?: no
Dry or Wet?: .
Aftermarket Cam?: no
Camshaft Part # or Name: .
Intake Duration @ .050": .
Exhaust Duration @ .050": .
Intake Lift at .050": .
Exhaust Lift @ .050": .
Cam Lobe Separation Angle: .
Cam Advance: .
Cubic Inches (if not stock): .
Compression: stock
Rocker Ratio: stock
Cylinder Head Information: stock

 

[CaptainXL]

Ok. It's time to start looking at other possibilities. Moreover make sure you are up to date on scheduled maintenance items.

For starters make sure you have replaced the coolant because according to GM it is due every 5 years. I also highly recommend you get a new thermostat housing installed when doing the coolant otherwise your just putting the old one back on which could completely fail at any time (if it already hasn't). On top of that a bad/stuck thermostat is one of the prime reasons for bad gas mileage. Also, check the spark plugs and see what condition they are in. Are they sooty/black? Perhaps you are running rich?

Do you have a scan tool and are you able to look at Short and Long Term fuel trims? Maybe look at the 02 sensor voltage and such? Also, if you are using an oil + cotton gauze air filter it may have gunked up the MAF and is causing some issues with the computer calculating air flow density. Have you checked compression which is also a leading cause of bad gas mileage? Etc...etc...etc. There is so much more that could be looked into. Just don't give up.

 

[MacMan]

.....I did tell them when I ordered the tune that I was installing an Efan and CAI.

From your PCM4less "build" sheet: Cold Air Intake: airboxstock

 

[DragonTX]

From your PCM4less "build" sheet: Cold Air Intake: airboxstock

I thought that strange myself so I emailed them and I recieved a mail back stating in short "How we have set up the air intake will work for any of the I6 intakes, so I don't think that's causing it"; It sounds like a copout to me. Also they suggested to clean the MAF and check O2 sensor readings. I do not have a scan tool available at this time (they suggested that I get an HpTuners scan done- nobody has that setup in my area that I know of), so I replaced the upstream O2 with factory replacement and cleaned the MAF with MAF spray cleaner. Neither of them looked at all dirty or bad, but I know that it doesn't mean that they aren't bad. I have to put a bit more miles on it before I see if it made any differance, so far the DIC is only up by 1mpg. I will make hard calculations soon, but the DIC is actually very close to past calculations; close enough to see a dramatic difference for sure.

Also I want to thank you all for your replies

 

[AV8ER]

This may not be anything to do with your problem but where and how I driving really changes my mpg. Two weeks ago on a trip my average mpg was 19.3 mpg (EXT w/3.73) but now going to school everyday I average 12-13 mpg. If your commuting is similar to before you can forget this

 

[Bartonmd]

How long is your commute?

I ask because you said in Sept, you did the tune/mods, but couldn't keep your foot out of it, and only started driving easy fairly recently? (like after it was already cold?). 11-12mpg isn't bad in the winter, if your commute is like 4-5 miles or less. What did it do on your commute last winter, stock? That's a much better indication than what it did in the summer or early fall vs. winter.

Also, part of the reason the throttle lag is in there, stock, is so it averages out your foot movement, and doesn't do the "accelerator pump effect" of squirting extra fuel on transient throttle every time you get back into the gas quickly. If you're an on/off/on/off driver with your foot, the tune getting rid of the lag will make your mileage a lot worse, but if you have a steady food, it'll usually make it better.

Mike

 

[DragonTX]

Well it has been a little while but I had to run a few gallons theough her to determine if the O2 and Maf clean had rewarded any results. I am glad to say it has improved even if slightly. I ran 10 gallons of 87 octane and came up with 13.4 mpg. But giving I used the remote start a few times I would guesstemate it to be 13.6 or so. It has been really cold up here so I was lucky to have a few days around 50f before a couple of days of bitter cold. I did most of the miles before this happened hence the .2 guess due to car starter.

Hardtrailz suggested it may be the CAI but the Volant I am running with is the preffered one by PCM tune; for it has yeilded the best results for others, but I still have the stock one but besides the improved element, it looks not much different than the stock one. And I just remembered that I had the CAI installed a few weeks before recieving the PCM and although it was not much MPG difference; It did yield a slight performance gain.

Bartonmd- I had sufficient time to get an accurate measure because the weather wasn't that cold until the end of October, and like AV8ER, I also have school to transport myself too along with the regular commuting. And to answer both of you on the use of my foot on the throttle; I am steady with it unlike before the tune when I practicly had to mash the throttle to get going (especially on the on-ramps). I try to use the Crosstown highway for commutes so my mileage is a mix of 60mph and 35-40 in town. My commutes have been the same all throughout my ownership of the vehicle. The only time I seen 20+ on the DIC is a 650mi trip to and from R.I./Conn. on a summer vacation; and that was before the tune. My other trip after the tune was around 500 mi up and around the adirondack region (upstate NY) to my camp and around the surrounding areas (mostly highway and 55mph routes). On that one I was averaging 14.7, so big differance.

After saying that; because I do not know wether it was the O2 or cleaning the MAF that yeilded a little gain, I am wondering if I should change the MAF also (I can get a BWD for 62$).I wish I had access to a real analyzer but I have searched online and the phone book and there just seems to be no one that owns one that can provide me with a printout of any useful data (unfortunatly all the good speed shops have closed due the the poor economy and lack of biz). Everyone I talked with has the Snap-on type scanners and charge 75$ an hour or so to hook it up. This is why I would just rather change the parts one by one if they are around 50 or so dollars like the O2 which eventually will need to be changed anyway around 100,000 mi or so; am I correct in assuming that?

Also I did check for vacuum leaks and suprisingly the I6 does not have many and those it does have look incredibly new and tight (my TB has had a lot of TLC before I owned her, and a lot more since

 

[Bartonmd]

OK, you answered one of my questions, but it was the least important of the 3. The other 2 are:

1. How long is your commute?

2. What mileage did you get last winter, when you were stock?

Mike

 

[DragonTX]

I just bought the truck in the spring of 2012 so this is the 1st winter. I commute maybe 20 mi 3 days a week with it, thats why it takes me so long to get back with mileage.

 

[HARDTRAILZ]

I get 3 mpg less from my dd on winter blend fuel. It does not matter the outdoor temp, can be-10 or 50 out...the same crappy gas is at the pumps. I drive it 750 miles a week. I could tell when they switched fuel...it was still warm out then.

 

[Bartonmd]

I just bought the truck in the spring of 2012 so this is the 1st winter. I commute maybe 20 mi 3 days a week with it, thats why it takes me so long to get back with mileage.

OK, so 20 miles/day, or 20 miles one way?

Your mileage is low for 20 miles/trip, but isn't too far off for winter temps, winter fuel, and 10 mile trips from a cold start every time, especially if you let it warm up at all before you start driving.

Mike

 

[marshall@pcm]

Warm up straight murders fuel mileage!

 

[DragonTX]

OK, so 20 miles/day, or 20 miles one way?

Your mileage is low for 20 miles/trip, but isn't too far off for winter temps, winter fuel, and 10 mile trips from a cold start every time, especially if you let it warm up at all before you start driving.

Mike

40 mi a day 3-4 days a week, I usually have 3-4 stops and shut the truck down. I understand about the cold weather and gas quality. We were just down into the single digits last week and the DIC is down to 12.5. used the car starter 5 times last week and give her 5 min. to warm up before leaving. But I still feel that I should be around 14mpg even in this weather (around 16.8 in the warm weather minus the 2-3mpg due to weather).

CaptainXL's comment on the coolant may be a thing to try, I checked the service sheets and the dealer serviced everthing for the 75,000 mark before I picked it up. BUT. ..I only see coolant service; no thermostat change, and if there was I am sure it would have been more labor than what was marked and a thermostat would have been listed in the parts section (all the brake parts, fluids, filters, are listed). Does anyone think that the Tstat would make that much difference?

Unfortunatly the garage is shut down until the weather becomes decent again (over 55 at least), so I cannot see anything major going on until then. 1st thing I will change are the plugs and Tstat. I wonder if the tune makes these parts more critical to be fresh? Anyone concur?

 

[Bartonmd]

20 miles one way shouldn't be too bad on the gas mileage, then... However, 5 minutes of warmup on a cold morning, from what I've seen on mine, will drop trip mileage by ~3mpg on my 40 mile highway commute. Actually, even starting it up and driving easy, my morning commute is ~1-2mpg worse than my evening commute, just because of the temperature difference.

Unless you just like getting into a warmed up vehicle, there's no real reason to warm up a vehicle, IF you can drive easy until it's reasonably warm. Starting it up, letting it idle for ~30 seconds, and driving easy not only doesn't waste as much fuel, but warms up the vehicle quicker, so there is less wear on the engine than sitting and letting it idle to warm up.

Mike

 

[HARDTRAILZ]

I do not concur at all.

You are getting appropriate mpg for what you are driving and how you are using the vehicle(warmup). Tune or no tune, it does not matter. General maintenance like plugs may help a bit, but everything you have posted points to the tune not causing any harm in your fuel economy.

 

[SBUBandit]

I get 3 mpg less from my dd on winter blend fuel. It does not matter the outdoor temp, can be-10 or 50 out...the same crappy gas is at the pumps. I drive it 750 miles a week. I could tell when they switched fuel...it was still warm out then.

I keep pretty ridiculous records on my TB, including a very detailed gas spreadsheet for every fillup since I bought it 4 years ago, and I can definitely see the mpg drop during winter months. Mine is not a daily driver, but it is what we use on the weekends and on long highway drives. I'm planning on sending mine in for tune this spring and it won't get any other changes at the same time, so I should get a pretty good idea of the overall effect.

 

[DragonTX]

I keep pretty ridiculous records on my TB, including a very detailed gas spreadsheet for every fillup since I bought it 4 years ago, and I can definitely see the mpg drop during winter months. Mine is not a daily driver, but it is what we use on the weekends and on long highway drives. I'm planning on sending mine in for tune this spring and it won't get any other changes at the same time, so I should get a pretty good idea of the overall effect.

I absolutly love the tune and highly recommend it, but I believe we are on a path to the term that 1 part just won't do. If you change one thing it does not necessarly mean that it is done and over with, Factoring in that the new O2 and MAF clean yeilded me some better mpg is telling me that there is more sensitivity to old components such as the new O2, and as suggested - plugs, thermostat, and maybe a new MAF is in order.
Admitadly I always felt that my TB sould get better mileage before the mods such as 17-19, so I was hoping these mods would yeild me that. I was checking out a few TB's and Envoys before settling on this TB. One of the TB's said 17.9 on the DIC (and you know it was suffle'd around the lot quite a few times), and an Envoy Denali the guy was driving daily still with a V8 that was getting 16.8. I know with an I6 I should get better. Being that buying any car or truck from a dealer their "full used car prep and service" does not mean that they serviced everything as they say. I am going to do a lot once it gets warmer out and hope that we can keep this thread going for others to benifit from it also. I learned that I will replace one part at a time in hopes to find the right Gremlin causing the mpg problem.
The responses to my inquiries to PCM for less tells me that they are very reputable and will back their tune 100% and extended the offer to switch out the tuned PCM with a core and they will run a few tests on it, all they want is the core charge; This time I will keep the core like I should have before it would have saved a few diagnostic headaches!

 

[HARDTRAILZ]

I find it funny that even when those of us that have owned these trucks for longer and have dealt with tons of them, tell you that the mpg is appropriate, you think you still need to waste money on changing things be you "feel" it should get better mpg.

 

[Shdwdrgn]

FWIW I was getting around 21mpg on the highway the first Summer I had my TB, but noticed it went down dramatically after I got my tune. I still have my original PCM but unfortunately never had a chance to try the old PCM again before upgrading my tires (the new ones are 1" larger diameter). When we get back on Summer gas, I may go ahead and and try my original PCM anyway, just to see if there is a difference.,

For comparison, I have had to refill my tank twice in the past week due to some extended highway driving (between 55-75mph) and I'm only getting 15.8mpg on those tanks. My usual driving which consists of roughly 75% in-town (30-35mph) gets me 15.2mpg. My t-stat was replaced a couple years ago when it failed, and my forward O2 sensor was replaced last year, and I clean my throttle body whenever I start feeling a hesitation off the line (about every 10k miles). I know I should be seeing a more dramatic difference between in-town and highway driving, but its just not there. My next step is pulling the plugs to see what condition they're in, but been too sick lately.

 

[DragonTX]

I find it funny that even when those of us that have owned these trucks for longer and have dealt with tons of them, tell you that the mpg is appropriate, you think you still need to waste money on changing things be you "feel" it should get better mpg.

Yes I understand, I am trying to find someone that will not charge an A&L for a scan and be able to print everything out so I can share the info. In the meantime I just do not understand why changing a PCM which should produce better mileage under same driving habits does not. I would have addressed this issue earlier in September but I was too busy with a new semester (5 Courses Advanced level neurosciences and STATISTICS OW!) + my PT job. I also know that since it is cold out I will lose some more, but an improvement (such as O2 and Maf clean) is an improvement. This points out that it is definatly a combination of things, If I rule out as much as I can (things that are maintenance anyway that the dealer probably did not do), what is left must be the culprit.

 

[WarGawd]

Just something I thought you may find of interest - This is my fuel economy chart for each month from last July until now. I changed my PCM for a PCM4 tune today. The chart represents a pretty consistent 3000km (1850 mi) per month average. The totals represented in the chart are ~1400 gallons of consumption and ~17000 miles driven.

My baseline is worse than most partly because of my 62/38 city hwy split (again very consistent) with the 3.42 rear end (def more of a hwy vehicle) and the winter mileage suffers more than most because I spend a LOT of time idling during winter astrophotography sessions. But the chart trend tells the clear picture of how the crappy winter gas blends and colder temps affect things:

View attachment 27323

FWIW.....

Edit - updated chart with March data complete, as well as first week of April, including todays highway road test of the mileage with new PCM - and it pains me to see I've now spent more on gas in 9 months than I paid for the truck

 

[Shdwdrgn]

Thanks for reminding me about this thread... I just wanted to clarify, for anyone who is wandering through, that the pcm4less tune was NOT related to my decreasing gas mileage. I recently purchased HPTuners, and have had a lot of fun changing my settings around and learning what it takes to fine-tune for a specific engine. In the process, I have taken log files while driving with both the stock tune and the pcm4less tune, and have found that their airflow changes were definitely a step in the right direction and certainly helped my gas mileage despite all the other changes I made around that time.

I am finding that with my custom CAI, I have to take my airflow tables even further than what pcm4less set. After driving around all weekend with some changes made, I'm anxious to take some logs tomorrow and see where I'm at. Their tune certainly got me a long ways down the road, and I still feel it was worth every penny.

 

[gmcman]

FWIW I was getting around 21mpg on the highway the first Summer I had my TB, but noticed it went down dramatically after I got my tune. I still have my original PCM but unfortunately never had a chance to try the old PCM again before upgrading my tires (the new ones are 1" larger diameter).

One set of tires I went with about 5 years ago were a plus size and were street tread. I went with a set of 265-60-17 Kumhos and I lost 3 MPG, 50 miles per tank. I sold them after 17K miles to lessen the blow.

 

[Shdwdrgn]

I really like the grip of these tires, but they are definitely not living up to their price. Not only the huge drop in mileage, but I've just passed 15k miles and the tires are well over half way worn (they're supposed to be 50k tires). Good thing I've got a good warranty on them. I want to stick with something that gives me a good balance between street mileage and traction in the snow, so I'll have to do some shopping before its time to replace these.

 

[GENO]

I really like the grip of these tires, but they are definitely not living up to their price. Not only the huge drop in mileage, but I've just passed 15k miles and the tires are well over half way worn (they're supposed to be 50k tires). Good thing I've got a good warranty on them. I want to stick with something that gives me a good balance between street mileage and traction in the snow, so I'll have to do some shopping before its time to replace these.

Just wanted to inject a little reality concerning the expectations for gas mileage. The Monroney label for a 2WD EXT is 15 city and 19 highway and we all know how accurate those figures are. I just bought an '04 last summer and I get around 16 with 50/50 city highway driving.

Good Luck trying to improve your gas mileage.

 

[Shdwdrgn]

Reality is for quitters! Next year I'm installing jetpacks so I can have a flying brick!

 

[SEMIJim]

This may not be anything to do with your problem but where and how I driving really changes my mpg.

I can certainly vouch for that. For years I've been driving a 1998 Honda CR-V. One day, 2-3 years ago, I decided to check my gas mileage. I was shocked to see I was only getting about 22 MPG (combined city/highway). Long story short: Got my foot out of the injectors and got my mileage up to over 24 MPG straight away. Engaged in a little casual hyper-miling (not anal about it), and got it right up to 26 MPG.

There's this one 4-lane (each way) divided highway, with traffic lights each mile, I travel every morning and most evenings. It never ceases to amaze me to watch the pack go blasting by me, while I accelerate gently and "putter" along at the speed limit, only to have me catch them at the next light, which they've had to stop at, and pass many of them. Every morning. Morning after morning

Warm up straight murders fuel mileage!

Particularly if the trips are short, of course.

I don't do warm-ups, unless I've got windows to clear. I just take it easy on the drive train for the first few minutes.

I find it funny that even when those of us that have owned these trucks for longer and have dealt with tons of them, tell you that the mpg is appropriate, you think you still need to waste money on changing things be you "feel" it should get better mpg.

*shrug* No harm in playing, right?

My wife's 2003 TrailBlazer LS, 4WD, I6, bought with 82k on it, was getting an average of 18 MPG, combined city/highway, when we first bought it. I haven't checked it lately. I'm hoping whatever TB I end up with will do at least the same.

I'm encouraged, tho. Of three gas mileage reporting sites I checked: The later TBs are generally reported to get marginally better mileage than the earlier ones. 2003 LS 4WD I6 average was about 17.4 MPG. The 2007 LS 4WD I6 average: 17.8.

Based on watching how my fellow drivers behave (see above): I expect to beat the averages

Jim

 

[Shdwdrgn]

There's this one 4-lane (each way) divided highway, with traffic lights each mile, I travel every morning and most evenings. It never ceases to amaze me to watch the pack go blasting by me, while I accelerate gently and "putter" along at the speed limit, only to have me catch them at the next light, which they've had to stop at, and pass many of them. Every morning. Morning after morning

See, I have exactly the opposite problem around here. When people actually do the speed limit, or when the road is clear and I can drive normally, I can drive from one end of town to the other without ever stopping for a stop light. Then you get the yahoos who think its their job to be traffic cops for the entire highway and ensure that nobody can even approach the speed limit, and the entire group of cars ends up stopping at every single light. They keep the lights pretty well timed around here to allow for smooth-flowing traffic in-town and between towns, so my mileage will go up by 1-2mpg when I'm mostly alone on the road.

 

[marshall@pcm]

Particularly if the trips are short, of course.

Short trips have nothing to do with warm ups. They are each bad all by themselves! lol