in town fuel economy

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-------

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Originally Posted by old guy 44

does anyone have any suggestions where to start?

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Trade trucks.

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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.

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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-------

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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.

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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.

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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.

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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.

Jerry,
Interestingly enough now that you mention it the economy does not seem to change appreciably between winter blend and summer blend and even factoring in foothills and traffic when you are putting in 20+ gallons of gas every 175 miles, give or take, there is a problem somewhere. I have been driving these roads since 92 with a variety of vehicles and this is the first one that is this thirsty. There is fuel economy in these engines somewhere, if I am not mistaken the hot rod Corvette gets good enough mileage that it is not subject to the gas guzzler tax. Granted there is little comparison between the Corvette and my truck but it is still a gen 3 small block.

Cleaning the injectors can't hurt. I supervised an LAPD repair shop in the 90's, I rarely did any work on the cars but as I recall it is just a matter of attaching a device to the fuel line and screwing a can of cleaner to it. Any idea if any of the chain parts stores rent the equipment?

As for dyno tuning ask Corey about the E3 plugs, do they really live up to the hype or is it just advertising smoke? Also what about an aftermarket tuner, can I tweak and peak the timing and mixture with one of these. While I am not totally opposed to pulling the engine and playing around with camming and heads it is not at the top of my to do list. If there is something that I can do externally to get the economy into the acceptable range it would be the preferable solution. Thanks for the offer.

The more I think about this the more I am leaning toward a mechanical problem not electronic. My scan tool shows nothing and the truck recently passed smog. I know for a fact that a vehicle will not pass smog if anything is even the slightest bit out of factory parameters. I even had a vehicle fail because it had a hard code set for the knock sensors that would not cancel. It was an Infiniti and I needed to take it out and drive it on the freeway for about 50 miles to get the code to cancel and then go back for a retest.

Soooooo I just bit the bullet and ordered a new set of injectors, not reman not aftermarket, and the upstream O2 sensors. As someone in this thread observed they can coke up and send false information. The downstream sensors are only checking cat efficiency and have little or nothing to do with engine management. Parts should be here next week. I will get them in and report back after a tank or two.

Thx for the O2 sensor tutorial. Educational.

I do know that the sensors are working because winter has finally arrived in So Cal. When I leave the house in 50 degree weather the truck runs like it is about 10 degrees late in timing until they warm up and it goes into closed loop. How well they are working I hope to answer later in the week.

OK so I finally got the injectors and O2 sensors in. I started and when I looked under the hood I realized that it probably would be a good idea to have the o rings for the crossover tube and return line. I tried to get them from the local dealer and they needed to order them so I just found some on Amazon for less than half the price and ordered them myself. But I digress. We have been experiencing a very strange phenomenon here in southern Cal, someone told me it is called rain. Haven't seen it in so long I forgot what it was. Anyway I was having a problem finding dry weather and the time to do it happening at the same time but did prevail. With the new parts in, the truck does indeed run a lot smoother. I have only driven it a couple of times but I was in a parking lot with the engine running , trans in park and was making some notes. I thought that the engine had died until I looked at the tach and saw that it was idling at 500. It was running so smooth and quiet that I could not feel it or hear it.

When I get a hundred miles on it I will stop and fill it back up and see what happens. Here's hoping.

Also I found out why it runs so sluggish in open loop. The timing does not advance until the system warms up and goes into closed loop and then it will advance properly. Is this normal or do I have a computer glitch which could explain a lot.

It does have heated sensors and goes into closed loop within about a half a mile but it seems odd that it would kill the advance in open loop. I have never noticed that on any other vehicle but admittedly I have never looked for it either.

I understand everything you are saying. All that I am saying is that it is going into closed loop, but before it does the ignition advance is not what it should be and I found that strange. But it does make sense that as most of the system is "off line" that the pre-sets are rather conservative to prevent possible damage. Driving it with the scan tool on the O2 sensors are not active initially and all the read outs are pretty stable. As soon as the O2 sensors come alive all the readouts change and move around as they should.

I do have a new problem though. When I put in the new injectors I noticed that the vacuum tube to the fuel pressure regulator had a tear in the upper connector. I assumed that I had damaged it when I removed it but if it was torn before I started the fuel pressure regulator was not getting a vacuum signal which is probably adequate explanation for poor low speed economy. When I drove it I was taking care of a couple of chores. Made a stop and when I came back about a half hour later the engine was reluctant to light. it cranked for about ten seconds and finally reluctantly picked up all cylinders and then ran fine. Made a second stop same scenario. IF the vacuum line was damaged before I started working on it that would mean that the fuel pressure regulator has not been off of its seat for a very long time. I seem to recall that the fuel rail is supposed to maintain pressure, so what is the possibility of the regulator now being reluctant to close, thus leaving the fuel rail with no pressure when stoped. It would take the fuel pump some time to pressurize the rail again with an unseated pressure regulator possibly explaining the slow start.

If I pull the regulator off and connect it to a hand vacuum pump are the mechanics visible enough to see if they are moving properly?

Now you are trying to make me computer literate, it is a lost cause. At my age it took me a couple of days just to figure out how to turn the d.... thing on.

I find it a whole lot easier just to call my computer guy, have him log on to my computer and make it happy.

With regards to the truck. Been driving it around for a week or so and put 98 miles on it. Stopped for gas, same pump at the same station, and put in 8.4 gallons. That is a dramatic improvement over what I had. I was filling up 20+ gallons at 175 to 180 miles. It may still improve marginally as I have noticed that the throttle response is improving as the computer learns. I believe that the vacuum line to the fuel pressure regulator was damaged before I worked on it causing full pressure in the fuel rail which would be reason enough for the poor economy. The new injectors and O2 sensors probably did not hurt but bottom line is that it is dramatically improved over what it was.

I still have the starting problem but it only happens if I stop when the engine is at operating temperature and the truck sits for 45 minutes to an hour. It will take around 5 seconds of cranking for the engine to light. If the truck cools off completely first or second piston up will light and the engine runs fine. I haven't had time to wrap my fingers around a fuel pressure gauge to check for bleed down but I still wonder if it isn't the pressure regulator sticking from long time lack of movement. That will be my next task when time permits. It is not critical as the truck is running fine and economy is up to an acceptable level for now.

It sounds reminiscent of heat soak, when the fuel boils out of the carb.

Bleed down sounds likely.

Got my fingers wrapped around a fuel pressure gauge and checked on a hot engine. KOEO 49 pounds. Running 44 pounds, I pulled the vacuum hose off and it jumped back up to 49 pounds. Shut the engine off and in a couple of minutes it crept up to 55 pounds. I am guessing that it was the fuel heating up in the fuel rails. Within 45 minutes it had bled down to 0. Not sure what spec is but as I recall the police chevy's with TBI ran on 35 pounds and the ones with port injection ran on 50-55 pounds. Ordered a new regulator should be here early next week.

OK. I hate it when I follow a thread all the way to the end and there is no conclusion so here goes. At the beginning of this thread I was fueling up every 150 to 200 miles 22 to 24 gallons. The last tank I went 293 miles on 23.6 gallons, 12.42 miles per gallon. I believe that I can state that this is the first time since I have owned this truck that it went any where close to 300 miles on a tank around town.

The largest change was installing new upstream O2 sensors and a new set of injectors. They went in at the same time so I do not know which of the two made the change or if it was a combination. I also found a tear in the vacuum tube to the fuel pressure regulator which I replaced at the same time. The fuel pressure regulator had no vacuum signal for an unknown period of time and when I put vacuum to it the diaphragm ruptured bleeding a small amount of gas into the plenum. This created a hard hot engine start as it was in effect flooding the engine by filling the plenum with raw gas fumes. The running fuel pressure was low also. It was running at 43 psi and with the vacuum signal disconnected it would jump up to 50. I ordered a new regulator and put it in. I rechecked the fuel pressure and it is now running at 51 psi and with the signal tube disconnected it jumps up to 60 which as near as I can tell is in the range it should be in.

The tank of gas mentioned at the beginning was after installing the O2 sensors and the new injectors but before the new pressure regulator. The pressure regulator went in somewhere in the top half of that tank and for the most part cured the hot start problem. Every change I have made has resulted in the engine running differently until I got enough miles on it for the computer to start re-learning. Right now the truck runs fine but it still occasionally is a little slow starting if I shut it off with the engine hot and leave it sit for about an hour. When it does happen it is the exception not the rule so for the time being I am tired of looking at an open hood and will let it go to see what happens.

I will continue monitoring the in town mileage for a couple of tanks and if it changes after the computer completely re-learns I will update the miles per gallon.

I still haven't ruled out a cam change if I can find someone that will grind what I want.