Poor fuel economy

[jtmarten]

Hi
I recently purchased a '98 K1500 w/5.7 Vortec and am getting horrible mileage, 12.5-13.5 . About 90% of my driving is country roads and interstate. The best I've gotten is 14.5, all interstate with cruise control set at 68.
So far I've replaced:
Plugs - Autolite platinums, gapped @ 0.060"
Plug wires - Taylor ThunderVolts (50 ohms per foot resistance)
Cap
Rotor
Both upstream O2 sensors (Bosch sensors)
Fuel filter
Thermostat (old one was a 180, new one a 195)
Radiator (all aluminum, 2 row)

Also added a cold air intake, cleaned MAF with MAF cleaner.
Has 3.73 diffs
Tires are new Uniroyal Liberators @38psi
Had full trans fluid exchange, serviced both diffs.
Nothing has made any significant improvement.

Any ideas where to look? I'll be receiving a new EOBDII scanner later this week so I can check live data to hopefully track down the problem.

Thanks much!!
Jeff

[J-Ri]

Is this a decrease from what you got when you first bought it? That doesn't seem too low for a Suburban. Maybe an owner of one can offer a comparative number, but I'd be overjoyed if my truck did that good

When you get that scanner, look at the MAP sensor data. If it's showing too low of a vacuum, the computer will dump in more fuel than is needed.

Also look at the IAC counts (20-40) and the injector PW (under 3mS) once at normal operating temperature and at idle. If either are higher (or even in the upper part of that range), you may want to consider an induction flush, or at the very least remove the throttle body and give it a good cleaning. Be sure to clean the IAC plunger and seat.

[jtmarten]

Thanks for the info!

Same mpg since I bought it in October. I wouldn't mind the mileage if it were a 454. Several 'burb owners I've talked to in my area have 454 K2500's that are getting the same mileage. A friend had an older (non-vortec) K2500 with a 350 that was getting 17mpg highway with 4.10 gears.
I'll have to wait until I get the new scanner and see what the data shows.
Thanks again!
Jeff

[J-Ri]

You're very welcome.

The biggest MPG factors are wind resistance and weight. The 454 is only slightly heavier and the wind resistance is the same. The big block is capable of burning more gas, but only when needed for extra power. My old 350 will pull well over the max trailer weight at 45 MPH with no problem (almost 18,000 lbs)... but a 454 would be fun

It is my understanding that the S-10 pickups actually get better MPG with the 4.3 than the 2.2. That's probably because with the 2.2 you have to put the pedal to the floor to get it moving.

[Mark Hubley]

I have a '97 K1500 with the 5.7. The best mileage I've ever noticed was 15 MPG, and that was on a road trip to North Carolina, mostly highway. I don't even want to know what my mileage is when I'm hauling a horse trailer

[J-Ri]

I'm pretty sure I was getting gallons per mile when I pulled that 18,000 lb trailer. It was up and down steep hills, I had one pedal or the other almost on the floor the whole way. Only about 10 miles, but felt much further.

[jtmarten]

OK, got the scanner today. Here's the data I got with her idling after a 25 mile drive.

ST FTRM1 0 to -3.1%
LT FTRM1 1.5 to 3.8%
ST FTRM2 -3.1%
LT FTRM2 -4.6%
MAP 6.8 Hg"
RPM 627
IGN ADV 22.5
IAT degF 79
MAF lb/min 0.000
ABSLT TPS % 0
O2S11 0 to 0.715V 0 to 0.7%
O2S12 0.725V na
O2S21 0.065 to 0.770V 0 to 1.5%
O2S22 0.715V na
OBD2 STAT OBD2-CALIF

There were no freeze frame codes, no trouble codes, all the fuel, emission, EGR, etc systems showed OK.

Anything in here that would indicate poor mileage?
Thanks!
Jeff

[Mark Hubley]

Jeff,

Unfortunately, I can't tell you anything about the scanner codes. However, the mileage you reported in your first post (12.5 to 14.5 MPG) is what you can expect for your vehicle. I really don't think you're going to get any improvements by replacing any more parts. As J-Ri mentioned, the laws of physics are against you: aerodynamics of the Suburban are very poor, and the vehicle is also very heavy.

My Suburban stays in the garage unless I am hauling something that I cannot haul with my car (e.g., horses). The thing is a pig to drive, it eats way too much gas, and that's the way it is. I do not drive it to get groceries, or to take my son to basketball practice, or to take the wife and kid to church. I use an '08 Mitsubishi Lancer for those tasks, and I get 30 MPG.

If you are serious about getting better gas mileage in the Suburban, here are some suggestions:

1. Put a Plymouth Road Runner style nose on the front end to reduce drag.

2. Remove all the rear seats, HVAC system, carpeting, sound insulation, 4-wheel drive system, mother-in-law, and anything else you can think of in order to reduce weight. You might be able to break 15 MPG that way, and this will also improve cornering and braking.

3. Do everything you can in terms of driving habits to keep the fuel consumption down: go light on the throttle, keep speeds within the speed limits, draft close behind 18-wheelers, and drive downhill whenever possible.

Seriously, your truck is a heavy-ass, flat-fronted, pig of a Chevrolet Suburban, born of the American belief system that we are entitled to drive huge trucks and that we have a God-given right (indeed a duty) to consume gasoline in massive quantities. Enjoy it!

[Happy Meal]

Originally Posted by jtmarten

Hi
The best I've gotten is 14.5, all interstate with cruise control set at 68.

Your vehicles original sticker listed 14 city 18 highway.
Under the new MPG formula that sticker would list 12 city 17 highway, average of 14.

Seems like you are right where you should be.

[J-Ri]

MAP should be high at idle. Verify with a vacuum guage and check sensor. Lower MAP indicates higher load, using more fuel. I was playing around with my car today, and noticed that the scan tool I was using reads the pressure inverted. It showed about 6" at idle, and 27" WOT. Maybe that's a GM thing, or maybe mine's screwed up and yours is bad... just check that out.

The MAF reading is off. All the scan tools I have ever used show it in grams per second. 5-7 grams per second is what I would expect. Converting to lbs/min, that would be 6.6138 lbs/min for 5 g/sec. Double check that number... www.convertit.com.go/convertit My math has been getting progressively worse since graduating.

However, if the reading is lower than actual flow, the computer will think there's less air coming in and use less fuel. So, that may or may not be worth looking into. It seems odd that there is no MAF code set.

The LTFT may show something is off... but not by much. having one positive and one near zero means one bank is runing richer than the other (also means one is running leaner). +/- 5% is considered good... but there is a difference side to side.

Mark Hubley is right, a smaller daily driver would be benneficial to everyone. Even after the extra insurance and registration, I save hundreds of dollars a year by not driving my truck every day (not to mention saving tons of gasoline, thereby keeping the demand lower, which keeps the price lower for everyone... so you can thank me for gas prices being .000000001 cents lower ). I only drive it for offroading, trailer pulling, or if the snow is so deep I can't get my car out of the driveway.

[Fastball]

Hi,

I have also just purchased a "new to me" 99 K1500 Sub with 5.7 and 3.73 gears and 265X75R16 Michelins (45 lbs Psi) with 225,000 K's. This is my 3rd 4 wheel drive suburban and I have been driving them since 1985. The 84 with the old 350 Carb'd would run 18.5 to 19 mpg (Cdn) at Hwy speeds and 3:42 gears.

My 1994 with 425,000K's on it when I sold it in Sept of 2007 would still turn high 17's to low 18's with 3:42 gears. When I bought it, it had 273,000K's and would get just over 19 if I held my tongue right.

The 99 is another story altogether. I, am also getting what I consider to be unacceptable mileage and have changed the front of cat 02 sensors, new AC Platiinum plugs, cap and rotor. The thing runs just great and pulls real hard with the 3:73 gears. I pull a double sled trailer at 105-108K's and get right on 14 mpg and this past week-end I pulled my 8 X 20 sled traielr with 3 sleds and averaged right on 12 mpg. These are very respectible numbers for towing anything. It handles the double in the overdrive almost anywhere and the big trailer the same way as long as I do not run the cruise. The funny thing is, after unhooking the trailers and running empty, NOTHING CHANGES AND IT SITS AT 14 MPG. That being said, it is winter here but if I had 14 pulling the trailers and 16 to 17 running empty I would be satisfied as it would do 18 to 19 in the summer. This truck is just so much better to drive than my 94 I just want to improve the mileage and you will have one very happy guy.

I have spoken to a few other people running the 350 vortec in SUBS and pick ups and for the most part they run in the high teens when running empty. I have no reason to believe that this truck show do any different.

I have just changed the thermostat for the second time as I can't seem to get the needle to stay in one place as it bounces back and forth. after changing it for a factory issue thermostat tonite it does the same thing and after driving it for 10 miles and soem at Hwy speeds, I came back to teh garage and found the rad cap to be cold and the antifreeze inside the rad to be cold as well. The top of the thermostat housing was hot so I have circulation issues and perhaps these issues may be related to the fuel economy as a cold engine asks for more fuel.

That is next on my list of "to do's" and then a fuel system purge and hopefully we find something. I will be monitoring this thread to determine if the questions regarding the Maf sensor show something.

I do feel that although these are 5000 lb vehicles, they are only slightly heavier than the thousands of 4 door and extended cab trucks running around that do better on fuel. I guess the worse thing is to fiond out if all of these trucks were built on the same Friday afternoon but I doubt it.

Frustrated in Saskatchewan!

[Fastball]

Originally Posted by Fastball

Hi,

I have also just purchased a "new to me" 99 K1500 Sub with 5.7 and 3.73 gears and 265X75R16 Michelins (45 lbs Psi) with 225,000 K's. This is my 3rd 4 wheel drive suburban and I have been driving them since 1985. The 84 with the old 350 Carb'd would run 18.5 to 19 mpg (Cdn) at Hwy speeds and 3:42 gears.

My 1994 with 425,000K's on it when I sold it in Sept of 2007 would still turn high 17's to low 18's with 3:42 gears. When I bought it, it had 273,000K's and would get just over 19 if I held my tongue right.

The 99 is another story altogether. I, am also getting what I consider to be unacceptable mileage and have changed the front of cat 02 sensors, new AC Platiinum plugs, cap and rotor. The thing runs just great and pulls real hard with the 3:73 gears. I pull a double sled trailer at 105-108K's and get right on 14 mpg and this past week-end I pulled my 8 X 20 sled traielr with 3 sleds and averaged right on 12 mpg. These are very respectible numbers for towing anything. It handles the double in the overdrive almost anywhere and the big trailer the same way as long as I do not run the cruise. The funny thing is, after unhooking the trailers and running empty, NOTHING CHANGES AND IT SITS AT 14 MPG. That being said, it is winter here but if I had 14 pulling the trailers and 16 to 17 running empty I would be satisfied as it would do 18 to 19 in the summer. This truck is just so much better to drive than my 94 I just want to improve the mileage and you will have one very happy guy.

I have spoken to a few other people running the 350 vortec in SUBS and pick ups and for the most part they run in the high teens when running empty. I have no reason to believe that this truck show do any different.

I have just changed the thermostat for the second time as I can't seem to get the needle to stay in one place as it bounces back and forth. after changing it for a factory issue thermostat tonite it does the same thing and after driving it for 10 miles and soem at Hwy speeds, I came back to teh garage and found the rad cap to be cold and the antifreeze inside the rad to be cold as well. The top of the thermostat housing was hot so I have circulation issues and perhaps these issues may be related to the fuel economy as a cold engine asks for more fuel.

That is next on my list of "to do's" and then a fuel system purge and hopefully we find something. I will be monitoring this thread to determine if the questions regarding the Maf sensor show something.

I do feel that although these are 5000 lb vehicles, they are only slightly heavier than the thousands of 4 door and extended cab trucks running around that do better on fuel. I guess the worse thing is to fiond out if all of these trucks were built on the same Friday afternoon but I doubt it.

Frustrated in Saskatchewan!

I have since taken my vehicle to a local rad shop where they proceeded to change the 2 day old OEM thermostat and put in one of their aftermarket ones. No improvement exceopt that I am $85 out of pocket. They claim that the circulation is good and can't figure out why the needled bounces back and forth everytime the thermostat opens and the water cycyles.

I then took to a garage that I deal with periodically and we did a leak down test to make sure the head gaskets we not leaking. Everything came back fine. I then changed both sensors, the one on the thermostat housing that feeds info to the computer and the heat guage sending unit loacted on the side of the head (driver's side). No change.

Next up was the local tuen up shop. There I encountered a formed GM mechanic with 13 years experience. He advised that the cycling thing is common and with the massive amounts of coolant flow that the rads have, the change in temperature when the thermostat opens is normal. He went on to say that the guages that GM use are very sensitive and that is why they fluctuate so much. He said that at Hwy speeds and under loads they woudl still bounce but not as much as in the winter.

I have a infrared heat detector and have been monitoring the cab tempo when the thermostat opens and although it drops off it will run to over 180 at the top and not below 150 at the bottom so I guess I have been chasing my tail and will have to get used to it.

I am also going to change out the original 02 sensors (upstream) as I have already changed the other two due to coding and go from there.

That being said, they also told me about a change up kit for the injectors worth about $500 cdn and 3 hours to change out the old pop-it style for the new version. Anyone know anything about the kit and done the install yourself? You have to remove the top 1/2 of the plenum to change the electronics so it looks like plug and play.

[mountainchef]

I;m getting just over 17 mpg. 98% of my driving is in the southern rockies in northern new mexico. This is the first truck I have ever owned and had no idea what to expect for gas mileage.

[sub006]

Setting your cruise control at 60 will give you at least 1 mpg improvement. Suburbans have brick-like aerodynamics!

Gap of .060 is pushing it unless you have a really exotic ignition. Try .040 max with stock system.

K & N air filter, 0-30W oil plus low-restriction muffler might add almost 1 mpg.

[ricnor]

I asked a similar question with regards my 454 vortec suburban as I was only getting 10mpg on a run. The concensus was that it's a big vehicle and my driving habits are not good (too fast, too quick).
The answer would be to change my gear ratios but that would be expensive and the return would take many years to recoup my out goings. I have slowed down a bit and now on a run with cruise control set to 70mph I can just about get 11mpg.

Some of the replies to my question were from people with 350 engines and they were getting low teens and thought that 10 for my engine wasn't too bad.

I use my vehicle for everyday use but have just recently bought a Blazer with a 4.3l engine and when sorted will be selling my Suburban as fuel costs here are now equivalent to $9.62 a gallon!!!