Thread: in town fuel economy

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I have been searching this and other forums regarding the in town mileage on my 03 GMC 4WD 5.3. My first thought was pulling the engine and recamming, possibly a bump in compression, smaller valve and port heads if they exist and whatever else I could think of at the time to trap cylinder pressure in the lower rpm range. I haven't found anyone yet that has tried a strictly mileage build on one of these and it just occurred to me that maybe I am trying to use a shotgun to kill a fly. The highway mileage is around 17-19 depending on speed. In town mileage however is deplorable. I have owned this thing for a little over a year and I doubt that it has ever seen the high side of 9 regardless of how I drive. Is this normal? This is the first LS engine I have ever had and do not know what to really expect from it. If this mileage is not normal is it possibly an engine management problem? I have plugged it into a scan tool and there are no codes, so either everything is operating within parameters or the ECM thinks that they are. The truck runs fine, has good throttle response and gives no indication of any problems. It just sucks fuel the way I used to drink beer in my 20's. If this mileage is normal the engine is coming out. If not does anyone have any suggestions where to start/

It is the 5.3 V8 not the V6. As for the 4WD it is 4WD not all wheel drive like most GMC's I have seen which means that in 2WD the front axle is disconnected and not turning the front differential like my 99 suburban did. Even at that the difference in city fuel economy between my 4WD and a 2WD suburban was only 2 to 3 mpg even turning the front differential constantly. So the net result of the 4WD is only that it is carrying the extra 300 to 400 pounds of equipment around, transfer case front diff and driveshaft etc. which is like throwing a small load of dirt in the bed and driving around. I would not expect a small payload in the bed of the truck to dramatically reduce fuel economy. That would be like putting two or three friends in your car and having the gas mileage drop dramatically, just not going to happen. The advertised mileage on the truck is 14 city and 18 highway. The factory ratings are determined with a professional driver generally downhill with a tail wind, or so it would seem. interestingly enough this truck exceeds the factory ratings on the highway but falls 5 to 6 mpg short around town which is why I would like to hear from someone with a similar truck and get some real mileage figures.

Have you checked your air filter

Something is wrong here-I have owned (or own) at least 7 of these Units (in either Yukon/Tahoe or Sierra Pickup) and, most of them before the 6 speed (I'll talk about those in a second)-

The pre-2008 models were all 4 speed Autos, and the worst Gas mileage I've seen was in a Tahoe that had 4.11 Gears (it was a terror around Town-not so much on the Hiway)-the best was with a Sierra (lighter, equipped with 3.42 Gears), and it got 22 mpg on the Hiway, and about 14-15 in Town (by the way, the only full-size GMC's that are AWD are Denali's)-

As far as the newer 6 speed models, they suck (as far as Gas!), especially if you live where I do (western US with lots of Mountains, Hills, and undulating terrain)-best mileage I have seen with the 6 Speed was 11 in Town and 16 Hiway (the eight speeds aren't any better)-

First off, do you have 3.42/3.73 or 4.11's in it? This made the single biggest change that I've seen (I've owned them all), and where/what kind of driving do you do?

I would agree that you have some kind of problem, as even with my '99-(first one-3.73 Gears), got about 20-21 Hiway, 12-13 in Town-do you tow a lot of loads? Are you getting this info from the Trip Computer or are you doing the math? Lots of variables here-

I never had a single Injector problem, as a matter of fact one of my friends bought my '06 (it has over 350,000 on it now) and no Injector problems-

I also noticed that Tires make a difference, and after trial and error I now only run Michelins (they can make up to a 2 mpg difference)-are you running the Stock size Tires (is it lifted)? I've put up to a 6" leveling/2" rear lift without issue-

You need to find a driveability guy who knows these Trucks/SUV's and have him give it the once over-has it had a Tune-up?

Alignment issues with quadrasteer-----------even if its aligned perfectly ( good road milage) any time you turn it any direction except straight ahed, you will hav terrific road drag because of weird geometry in that system to accomplish those turn radius---- and in town, you are never running very straight for very much of the time------

tire types -------best at Micheln LTx----------

Driving tip--watch all the dummies/crazys around you and DON'T drive like them-------

Originally Posted by old guy 44

does anyone have any suggestions where to start?

Trade trucks.

.

Have you replaced the oxygen sensor on it lately? I've got some friends with these in hot rods and they started getting horrid mileage and another guy mentioned that dirty oxygen sensors will cause this. He said they still read but not accurately and it'll cause the computer to compensate and run it rich but not overly rich to flag for codes. They changed theirs and got back to good mileage again. Just a thought.

Originally Posted by jerry clayton

Alignment issues with quadrasteer-----------even if its aligned perfectly ( good road milage) any time you turn it any direction except straight ahed, you will hav terrific road drag because of weird geometry in that system to accomplish those turn radius---- and in town, you are never running very straight for very much of the time------

tire types -------best at Micheln LTx----------

Driving tip--watch all the dummies/crazys around you and DON'T drive like them-------

Aren't these designed to lock out the rear steering function above 25mph so at high way speeds and as long as the alignment is correct, this shouldn't be an issue. I'm failing to see how the road drag will eat the tires here? It's a Dana 60 designed to be in the rear. The fronts sure don't have this issue. Any of these I've been around didn't wear tires any worse than a regular truck. Unless it had worn components like anything else.

I have submitted a detailed response twice now and it will not post. I am trying this just to see if it posts.

OK that posted so we will try again.

Gears, tires, modifications. As near as I can tell the truck is bone stock except for aftermarket wheels with 265/75/16 tires. I crawled under it and could not find a tag on either differential but the engine is not spinning up enough to have a gear set in the 4's.

Driveability, I grew up in my dads repair shop and wrenched professionally for over 20 years and the truck has good throttle response, the engine is crisp and not laboring like something is wrong. I have not kept a detailed log of fuel usage however I used to have a second home in the mountains. It was a 230 mile round trip and if I went up and did not do much driving up there I would get home with slightly less than half a tank. If I put 30 to 40 miles on the truck up there I would need to stop for gas on the way home. My normal usage is Los Angeles traffic which if you know it no explanation is necessary if you don't no explanation is adequate. In that environment it will suck the tank dry and set the low fuel light in less than 200 miles, sometimes a lot less.

Tune, It had new plugs and filters in it when I bought it, I am assuming that they are the correct ones because I found it at a dealer and in 20,000 miles of driving the economy has not changed. On the subject of tune what about different plugs like E3's they make a lot of claims but what is reality? Also what about one of the aftermarket tuners? If the thing had a carburetor and distributor I would be playing with re-jetting and re-curving the distributor but it is not so easy with electronics. I never thought about the O2 sensor, cheap enough to throw one at it and see what happens.

Quadrasteer, I normally drive in 2WS because of an undiagnosed problem with the rear steering dropping out of 4WS into 2WS and not centering the rear steer. That is another thread.

Denali, my truck is the only non denali quadrasteer I have seen. Do not know if it was ordered that way or not but it seems to be a one of or close to it.

Originally Posted by old guy 44

I have submitted a detailed response twice now and it will not post. I am trying this just to see if it posts.

I learned a long time ago to copy my post before I hit send. That way, it isn't so much hassle to re-post. It's really sad, but this forum has turned into a second class operation.

.

Well gas gauges aren't very acturet----------You need to fill it up, note odometer milage, drive it right around 100 mile fill up again and do calculation-repeat several times--- even go further-but keep a running record of miles driven and gallons used-I always only fill to auto shut off and don't top out ---------make it one of those graphie things and add notes to if long or short trips-------

You may have as many as 4 O2 sensors--also chech coolant temp sensor and Inlet Air sensor

Other things to check are fuel pressure and injector size. Parts may have been changed to make the truck run for sale. No way to know what it has in it. Even the pcm may have been changed at some point.

I'll add to what the others have said. Since you don't know the truck's history(and it's done this since you have had it), you may have to go over every part of the EFI system and powertrain just to verify each part is what it should be and is working correctly.

Once you know that everything is mechanically and electrically correct, the only other option (unless you trade it like Tech said) is to take to a tuner and have the pcm program altered. There are dyno's that can do road load tests, not just full throttle pulls. Find someone that has one of these and can use it and you can see and alter the tune for increased mileage. This would be cheaper than a new engine swap. If you alter the internal engine parts very far from stock, you are most likely going to have to tune it anyhow.

OK---I just looked up Oak Park Ca on Goole maps---------you ain't going to get any milage in town-----not only are you dealing with Cali gas, but hills every where-----------



If you have any specific questions about dyno- I'll ask my son Corey as he probably has done more dyno tuning that anyone

When I get the time and someone to help me I am going to have someone drive it while I have a scan tool connected and see exactly what is going on. I have checked it in the driveway and everything is within normal parameters however having the engine under load definitely changes things. Sometime today I plan to pull the plugs and have a read. I have not been out of town with it in a couple of months and the exhaust pipe is now pretty sooty which makes me wonder. If the plug read shows it to be running fat I will screw in a new O2 sensor. What I can see from the top there is a sensor in the RH head pipe and none in the LH one. Seems a bit strange but I can see the whole pipe upstream from the primary cat. When I put it in the garage to check the plugs I will jack it up to have a better look.