1994 Gmc Sierra MPG ISSUE

[Marsal420]

Alright we'll here is the skoop, i have done many things that should have improved my MPG on my truck here is the list, *New Plugs/Wires,Cap/Rotor,Coil,FuelFilter,Changed oil with synthetic as usuall,replaced the PCV Valve, Sprayed SeaFoam into the Cylenders, am going to do the full SeaFoam treatment in about 1500miles or so.

On top of that i have, a Cold Air Intake, TBI Spacer, and TruDual Exhaust, i think the exhaust might drop it alittle since i dont think they put my 02censor back on. But the CAI and the TBIS should actually improve it.

So if anyone has idea what could be another way to go please post up, because im getting about 15hw and 10city and i know i should be getting alot better then that. I have the 5.7L V8 with an Auto Trans btw, and if i can't get this figured out by snow fall which should be in about a month, im putting it up for sale , no use keeping somthing i can't afford to drive.


Marsal

[Z15]

There are many things that can affect MPG. Those add-ons like cold air intake, TBI spacer and exhaust are not going to do much. People tend to drive harder with those items on the vehicle than they normally would.

Do you have oversize tires??

If you changed the tire to larger than stock, that will drop you mileage. First it puts more load on the engine and trans because now they are operating outside of their designed torque curve, trans is shifting before is supposed to and engine then has to work harder. And oversize tires will change your speedometer and you will be driving faster that what it says and you calculations will give inaccurate mph readings, on the low side.

But the CAI and the TBIS should actually improve it.

As you can see, all the hype about better mileage with these devices is just that, hype. There is no proof they do what they claim excpet get you to buy. Ever notice all the claims for more mpg, more HP are from dyno testing at maximum RPM (redline)? Who drives like that?

I know many who have made similar changes and gotten worse becuase they now drive faster and harder than before. As one friend told me, he was sorry he put on the dual exhaust because he loves to hear the exhaust rumble and that he is getting worse milage now that before because he often runs it up to hear the noise. Or he is down shifting to hear the rumble, all burning more fuel.

[Marsal420]

Originally Posted by Z15

There are many things that can affect MPG. Those add-ons like cold air intake, TBI spacer and exhaust are not going to do much. People tend to drive harder with those items on the vehicle than they normally would.

Do you have oversize tires??

If you changed the tire to larger than stock, that will drop you mileage. First it puts more load on the engine and trans because now they are operating outside of their designed torque curve, trans is shifting before is supposed to and engine then has to work harder. And oversize tires will change your speedometer and you will be driving faster that what it says and you calculations will give inaccurate mph readings, on the low side.


As you can see, all the hype about better mileage with these devices is just that, hype. There is no proof they do what they claim excpet get you to buy. Ever notice all the claims for more mpg, more HP are from dyno testing at maximum RPM (redline)? Who drives like that?

I know many who have made similar changes and gotten worse becuase they now drive faster and harder than before. As one friend told me, he was sorry he put on the dual exhaust because he loves to hear the exhaust rumble and that he is getting worse milage now that before because he often runs it up to hear the noise. Or he is down shifting to hear the rumble, all burning more fuel.

We'll i can say i dont drive hard even with my mods, didn't drive that way prior to them ither. Im pretty conservative with how i drive usually 55-60 dont go WOT to get up to speed or anything, smooth transition allow the engien to get up to 55 at normal time, rather then make it rush there which wastes more gas. That is why idk why im getting this type of mpg....


Marsal

[Z15]

You didn't say if you are running oversize tires?

Are you carrying around extra weight in the bed?

I had a 92 and a 96 ext cab 4x4 Z71's, best milege I would get was in the 16 mpg range and I was always travelling the highways, not much around town miles. You are not to far out of the normal range for a 4x4 truck. If anyone is telling you they get more, they are either full of BS or dont' know how to calculate mpg. My 2008 GMC for instance, that I just got from PSG 3 months back only gets in the 18.3 range around the area. A 550 miles round trip over the Duluth a few weeks ago only got me 19.7 mpg cruising at no more than 60 mph. Thats with a 5.3L with the active fuel management (4 cyl's de-activate).

[sector95]

First off, we're dealing with a 350 cu. in. V-8 shoving a 1-plus ton hunk of steel. What kind of driving do you do (highway vs. city/local)? What kind of gas mileage are you getting? My '98 (w/305 and 4L60E tranny) gets 17-18mpg in a mix of highway and local driving (very little stop and go stuff). What kind of gas mileage are you expecting? What kind of final axle ratio are you running?

Z15 sorta hit the nail on the perverbial head: most of that stuff you can put on your ride to "improve" just about everything but your sex life, in a word, doesn't (okay that's really two words...just seeing if you're paying attention). There may actually *be* an improvement in your mileage or HP but you'd have to measure it with a micrometer, at redline and under laboratory condidtions; in the real world, not so much if any at all. The perverbial "Butt Dyno" is usually way out of calibration and bears no relation to the real world.

A TBI spacer typically has nothing to do with improving mileage; what it will do, supposedly, is slightly shift your torque peak higer or lower in the RPM range.

A cold air intake.....well I have a '98 and it came from the factory with a "cold air intake" as probably did yours; intake air is drawn from outside the engine compartment through the fender area. Under normal driving conditions the factory intake system, with a clean air filter, is more than adequate in flow requirements...only when you get into the upper RPM range will a low restriction intake show its stuff...providing you have a low restriction exhaust and a high flow throttle body to match.

Your dual exhaust.....you need to find out if the installer put your O2 sensor back...and if you have true duals, then that may have an effect on the sensed fuel mixture by the single O2 sensor. When you start screwing with engine management computer inputs you better know what you're doing. That little device probably has the greatest effect on your mileage since it tells the engine management computer how rich or lean the fuel mixture appears.....the correction, lean or rich, will be applied by the computer to get the ideal fuel mixture. Like all things computer, if you put garbage in you get garbage out; if the O2 sensor is giving erroneous inputs to the computer then your getting an erroneous fuel mixture.

Modern computer controlled engines are about as fuel efficient as they can be. You might be able to tweak some more MPG's but you'd probably need an engineering degree to do it. With the brick-on-wheels styling of the older style full-size GM trucks, aerodynamics (or the lack thereof) also has a lot to do with your mileage; wind resistance is the biggest sapper of gs mileage in these vehicles at higher speeds.

Keeping your engine in tune, running factory-sized tires and driving sensibly are probably the best things you can do to maintain the mileage you're getting. Putting in a higher (lower numerically) final drive might get you a little improvement in MPG numbers and installing a tonneau cover on the bed can help reduce wind drag and maybe help in mileage.

Mike
'98Sierra

[Z15]

All those devices that supposedly save you fuel (and thats very questionable), cost more then the end result will net you. You spend $500 to save $50. I have owned trucks dating back to my 1st 4x4, a new 1975 Chevy Blazer. So I know a little something about the mpg these units get. When ever I hear someone with a 4x4 saying they are getting 20 mpg highway, I discount that right away becuase after owning over 10 of them and drivng them in my work as a surveyor both pu's and suburbans, I know that not the be true. I recently had a friendly arguement with a local neighor who claims to get 21 mpg with his '06 F150 4x4 V8. He said he would prove it to me. So when he went from Houghton to Green Bay a week or so back, he checked his mileage, sure enought he got 20.6 mpg. He had is written down on paper to prove it. But that was the only time he checked it, on his way down. He admits he was taking it easy but on the way back he was driving fast to get home early on a Sunday, so he did not check it that time. Yet by his logic, he still gets 20-21 mpg. See the problem with this? He is cherry picking the results as do most who claim great mileage with 4x4's.

To get a true representation you have to log all fuel for thousands of miles and then average, that will give you a more true reading than cherry picking on the best and ignoring all the other times you let it idle, drove 80 mph etc.

My 2004 GMC that I traded in, I had logged every mile and gallon of fuel becuase I have to have that for my business. In 35,000 miles, it averaged 17.3 mpg. Total miles divided by the total gallons used in that time. For the average to be that, it had to get in the 18's, 19's or maybe even the 20's at some points in time BUT it also was down in the 14's I am sure, thanks to the UP winters.

[Marsal420]

Yeah these Winters are Harsh , My truck has a tonn cover and i do have true duals, im unsure as to if there is 1-2 02sensors or if there is one? I havn't checked i will check tommorow morning, and now if there isn't one could i have 2 installed? i was actually thinking of just buy a single outlet exhaust if i do keep this truck but a performance exhaust since this was a custom build up by my mechanic for me. I have the same size tires that where on the truck when we purchased it 265,75,R16.

All i know is i need to figure somthing out, at a line under half a tank im alittle over 210miles , but i towed a friend from Rapid River with his Toyota so that could have killed my MPG alittle bit. I do understand what your saying about how these trucks where made, there W/T's regardless of the package and they weren't ment to get good mpg.. Although i really wish i did i dont want to have to sell my truck but i might have to if i dont see any improvments.

Any other ideas that could help my MPG, that i might not have tryed already?

Thanks for the help thus far everyone

Marsal

[Z15]

You say is has 265 tires but what does the tire pressure lable on the door say the tire size is? If it does not say the same, thats part of your problem. OS tires fool the sensors that tell what rpm the trans to shift at, when the torque converter is to lock up and give you inaccuate odometer and speedometer reading. Meaning, you are going faster and driving more miles than are recording. I think you need to determine if 265 are the correct tire size before going any farther.

No O2 sensors maybe part of your problem. They are plumbed into the exhuast pipe before the converter. I am not sure how many would be on your truck. I do recall my 92 305 engine had only one.



From a dealer servive bulletin

Before any diagnosis is performed for these types of concerns, inspect the tires/wheels for the proper tire and wheel combinations.

If a truck is found to have other than the above listed tire sizes, no further diagnosis should be undertaken until the original size tires and wheels are reinstalled on the truck.

The Engine Control Unit (ECM) and Transmission Control Module (TCM) use (RPM) and (VSS) inputs as operational parameters for engine torque, transmission shifting, cruise control operation and speedometer indicated vehicle speed.

Calibrations for different tire/wheel combinations are not available.

Therefore changing of tire and/or wheel size should not be undertaken by dealerships or truck owners.

Warranty Information
Reinstalling the proper wheels and tires on the vehicle, prior to performing any type of diagnosis of the truck, would be considered truck owner responsibility and not a warranty repair.

[Marsal420]

Same size as i have on there, the tires that i have on there are the ones that where on the truck when it left the lot. " I'm not the original Owner " but whoever bought it later switched to some Uniroyals and i later found a brand new GoodYear Td under the track as my spare, and since i needed tires bought 3 new ones .

So now that we have that settled..... What is there to figure out now? the whole thing with the 02 sensor is sorta confusing me because i dont know if i need one or not?


Marsal

[Z15]

Well, the tires can't be the problem then, that would have been to simple.

I think you are stuck with what you got. Everything is going to cost you more and more money and there is no guarantee you will get the results you want even then. Remember, its weighs over 2 tons.

O2 sensors can be expensive, you would have to drill and tap into the header pipe at the correct location. I don't know if a 350 has one or two O2 sensors. The computer does not know how to regulate fuel without the O2 so it probably is in a default mode. You may see have more problems in colder weather? Can't really say.

What does the mechanic who did this work for you have to say about your mileage situation?

You can put more air in the tires which would help increase mileage BUT it will ride harder, wear the tires more and possible affect handling in some highway dirving situations.

My Advice.
Cut your loss's, sell it (if you can) or trade it on a 4 cyl 4x2 Canyon (Colorado) and don't mess with the new truck, keep it stock.

[Z15]

I was talking with a GM tech who has fooled with a lot of these older trucks and he mentioned a few things to ask you.

Did you eliminate the cat. converters?

Does your check engine light come on when driving, go out or stay on all the time?

Does is come on when you start the truck?


If you put put the O2 sensor back in, you will have a add an H-pipe (connector between the 2 exhaust pipes) and tap the O2 sensor in there so that it gets a reading from both banks of the engine. If there are cats and no O2 sensors, they will probably get plugged up from the excess fuel going through them. In open loop the computer can not adjust the mixture and run as lean because it is not monitoring the O2 sensor reading.

Can you put the oem exhaust back on, maybe you would see better milage. Dual exhaust is not help on these computer controlled trucks for mileage.

[Marsal420]

There is no cat , hence "True Dual Exhuast"
The Check Engine Light did come on but i removed it, because the engine had been replaced and they put the computer on the truck and couldn't find anyting wrong with it, so i just took the light out, but it did stay on when i started the truck and even when i drove it.


Marsal

[Z15]

Was the previous engine also a 350?

I will pass it on but he did mention that without the O2 the computer is in open loop and that will account for your bad mileage. I don't know how you are going to get around that. Don't take it anywhere where you would have to pass an emission test, it will fail big time.

[Marsal420]

Originally Posted by Z15

Was the previous engine also a 350?

I will pass it on but he did mention that without the O2 the computer is in open loop and that will account for your bad mileage. I don't know how you are going to get around that. Don't take it anywhere where you would have to pass an emission test, it will fail big time.

Yeah , i know that but if that where to be the case and i where to move id have everything brought back to code. Yes the only engine in this truck has been a 5.7L 350, so maybe your mechanic friend might know how many 02 sensors there was? If he does let me know and when i bring it in to have the right Cv shaft replaced i'll have my mechanic install a new one , if it's a good price. But i might just say forget it with the 02 if it isn't there because im planning on selling the truck , i just dont know what i should get? Need a truck but unsure as to what would be a good choice? Funds are at there highest so im not looking to spend anymore then $5,000 maybe someone on here would have an idea as to what would be a good 4x4 or awd choice?


Marsal

[Z15]

You did not mention headers so I’m assuming it has stock manifolds on it?


My 1992 only had one O2 sensor in the drivers side exhaust manifold and he is sure a 94 350 is the same. Take a look and see if it has a sensor or a plug in the sensor hole. If it has a sensor, it would most likely be bad and plugged with deposits.

Install an O2 sensor.

If it has headers he suggests a heated O2 sensor. This would require a little wiring. Have to wing that one on your own.

He said to not install an H-pipe to put the O2 sensor into. It would not run hot enough in the H-pipe in our climate and you would have poor sampling. The stock setups that are on an exhaust manifold sampling from one bank of will work.

Reinstall the check engine light and hope for the best.

If you have headers, quality ones should have come with an O2 provision.

He said if you were to come into thier shop with this setup, he would not work on it because of the removed cat's. Licensed business's are breaking the law by defeating emission control devices.


Let us know what happens..