MPG Stinks after front dif and transfer case replacement

[Jolly Roger]

Hello everybody, loving this forum and all the people who make it great! Thank you!

Short version:
I have terrible MPG now after a front differential and transfer-case replacement.
Went from 16mpg in city to 11.1.


Long version:
Last winter I took my 2007 Tahoe into the shop to have them look at the four-wheel drive. It was making a gear noise that would slow down or speed up with acceleration. At first I thought it was the transmission because the sound seemed to be coming from under the passenger seats, but the noise crept up closer to the front of the truck the longer I drove it.
The streets were drier the day I took it in so I took it out of 4x4. Noise gone. I told the shop owner this and now suspected a transfer-case instead of the transmission.
He called me hours later to inform me that it was fixed. $2000 bucks for a new front differential. I drove it home and put it back in 4x4. Same noise. I called him back, brought it back to them and met the mechanic who preformed the work. (I wouldn't trust this guy to air tires, a true idiot) We take it for a test drive where he informs me that they didn't. They put the truck up on the lift with a stethoscope and they all agree that its the transfer-case now. I have a talk with the shop owner expressing my disappointment that they didnt even test drive it after they "fixed" it. He told me he would work with me on the price of a new t-case $2400. I told him pound sand and took it to another suggested shop with good reviews..

So, now this NEW place takes my truck, disassembles it and confirms it is indeed the transfercase.
What they DIDN'T tell me before hand is that the guy who does the work wont be back for a week.
With a few choice words I told them to put it back together. They still charged me $100 service fee.

3rd times a charm right?
took it to yet another shop. The owner expressed his anger at what Id been through and replaced the t-case with a new one and charged me $2200. (I also had them check the front diff) This guy has many awards with the BBB, I should have gone there to begin with.

I've been driving it ever sense then but my MPG absolutely stinks. Went from 16mpg city to 11.
I dont know what is to blame, the front Dif work or the new transfer-case.

I have tested the 4x4 and it works great without any noise.
It did have a VERY loud clang when I put it into 4X4 low but I read that this is somewhat normal.
I've also noticed a squeal in the front when I turn corners, cant tell if its the belt or diff.
I've thought that maybe they put the wrong diff oil in?

Any and all help would be greatly appreciated. I drive long distances and gas is killing me!
Thinking about taking it back to the last shop I used and beg for help, but cant afford to with the amount Im spending on gas!

 

[swathdiver]

Sorry to hear about this fiasco Brandon. I'm wondering if something is too tight, causing the engine to work harder and burn more fuel. With a Tech-2 or other bi-directional scan tool and manybe some of the Bluetooth adapters, you can see how much load is on the engine as a percentage and how many foot pounds of torque the engine is producing.


Tire squeal is normal on hard surfaces while turning when the front differential is engaged. Don't make a habit of that.

If it were mine, I would get the front wheels off the ground and spin them and see if there is any unusual resistance.

Keep this post going so more guys stop by and have a look.

 

[Jolly Roger]

Thanks for the reply swathdiver, I've been looking to get a good scanner. Most of them I've been looking at really can't tell me much more than what the DIC already does.
What scanner would you recommend?
My differential is not engaged but squealing is still there are dry pavement. I don't think its the belt, almost sounds like the tire itself is making the noise...
I've read that the wrong dif fluid can cause noise...

 

[thompsoj22]

Thanks for the reply swathdiver, I've been looking to get a good scanner. Most of them I've been looking at really can't tell me much more than what the DIC already does.
What scanner would you recommend?
My differential is not engaged but squealing is still there are dry pavement. I don't think its the belt, almost sounds like the tire itself is making the noise...
I've read that the wrong dif fluid can cause noise...

If it were me i would jack it up with all four wheels off the ground, mark a line on the top center of the wheelwell on both driver side wheelwells, mark a line on both drivers side tires that lines up with the lines you put on the wheelwells, put it in neutral, mark a line on both front and rear driveshafts and use a tape measure to extend an accurate pointer up to the driveshaft line and lock it, have a helper rotate the rear wheel and count the revolutions of the driveshaft to complete one revolution of the tire , repeat for the front. the actual ratio will be evident in where the line stops on the driveshaft in relation to the tape measure/pointer. it should be an identical distance for front and rear. Can anyone else explain this in more simple terms?. It is possible front ratio was fubar on the install, it will destroy your new transfercase if this is the problem. Now im just guessing, it could be engine related in regard to mpg but you should not hear tire squealing, is the transfer case engaged electrically? check to make sure that is functioning correctly and your not in 4wd constantly.

 

[ivin74]

I have a feeling the diff is to tight, did the first shop rebuild it themselfs or replace it with a rebuild unit from a manufacturer? How many miles has it been since diff was rebuilt? Drain the front diff and see if you see metal shavings on the oil, if there is go back and have them fix it right this time.

 

[swathdiver]

If it were me i would jack it up with all four wheels off the ground, mark a line on the top center of the wheelwell on both driver side wheelwells, mark a line on both drivers side tires that lines up with the lines you put on the wheelwells, put it in neutral, mark a line on both front and rear driveshafts and use a tape measure to extend an accurate pointer up to the driveshaft line and lock it, have a helper rotate the rear wheel and count the revolutions of the driveshaft to complete one revolution of the tire , repeat for the front. the actual ratio will be evident in where the line stops on the driveshaft in relation to the tape measure/pointer. it should be an identical distance for front and rear. Can anyone else explain this in more simple terms?. It is possible front ratio was fubar on the install, it will destroy your new transfercase if this is the problem. Now im just guessing, it could be engine related in regard to mpg but you should not hear tire squealing, is the transfer case engaged electrically? check to make sure that is functioning correctly and your not in 4wd constantly.

Good call, didn't even think they would be dumb enough to put the wrong gear ratio in, but remembering how he described them, yeah, it could certainly happen!

However, I think if it's not engaged, and it's working right, it shouldn't be making noises or killing his mpgs.

 

[Jolly Roger]

If it were me i would jack it up with all four wheels off the ground, mark a line on the top center of the wheelwell on both driver side wheelwells, mark a line on both drivers side tires that lines up with the lines you put on the wheelwells, put it in neutral, mark a line on both front and rear driveshafts and use a tape measure to extend an accurate pointer up to the driveshaft line and lock it, have a helper rotate the rear wheel and count the revolutions of the driveshaft to complete one revolution of the tire , repeat for the front. the actual ratio will be evident in where the line stops on the driveshaft in relation to the tape measure/pointer. it should be an identical distance for front and rear. Can anyone else explain this in more simple terms?. It is possible front ratio was fubar on the install, it will destroy your new transfercase if this is the problem. Now im just guessing, it could be engine related in regard to mpg but you should not hear tire squealing, is the transfer case engaged electrically? check to make sure that is functioning correctly and your not in 4wd constantly.

Holy cow.. I dont know why this never occurred to me,....the possibility of the first shops "mental midgets" installing the wrong differential altogether! (wrong gear ratio) Is this even possible? Would it even drive? The last shop supposedly checked it... my gear ratio is the 3.73 ....I've sense then driven it in 4X4 on the highway with no obvious problems.

I was under the impression that the T-case is only engaged when the electronic differential is locked. Besides testing the 4-wheel drive after the new transfer case was installed, I've only been driving it in 2-wheel HI. The 4-wheel HI and LOW has that stereotypical feel to it of a locked front dif, lurching when turning a corner, and overall tank-like drivability. So I know Im in 2-wheel drive right now and have been all year.
I dont have the ability to raise my truck and do the test you have suggested, I no longer have a garage or even a level safe place for jack-stands. I have no other noises while driving except for the occasional chirping when turning.

I have a feeling the diff is to tight, did the first shop rebuild it themselfs or replace it with a rebuild unit from a manufacturer? How many miles has it been since diff was rebuilt? Drain the front diff and see if you see metal shavings on the oil, if there is go back and have them fix it right this time.

The first shop replaced the diff with a brand new unit, told me it was fixed and I drove it home. When I turned into my neighborhood I shifted it into 4-HI and the gear noise was still there. (the whole reason I took it in) I drove it back and they decided that it was a T-case failure instead of the diff that they had just changed.

The third shop put in a new T-case a week after the front diff was changed by the first shop.
I've driven it less than 1500 miles sense both were done and have watched my MPG go from 16 to 11.
I just dont know which of the two repairs are to blame.


Thanks for your replies and help gentlemen! The truck has less than 70,000 miles on it and shes my pride and joy. Would love to get this nailed down.

 

[swathdiver]

Thanks for the reply swathdiver, I've been looking to get a good scanner. Most of them I've been looking at really can't tell me much more than what the DIC already does.
What scanner would you recommend?
My differential is not engaged but squealing is still there are dry pavement. I don't think its the belt, almost sounds like the tire itself is making the noise...
I've read that the wrong dif fluid can cause noise...

Bought a Tech-2 when I bought my truck. No longer was I going to work on cars blind, these scan tools are a must to know what the computers are seeing and they are full of information that works hand in hand with the shop manual and troubleshooting guide. This is the tool, the Tech2, that the dealerships use/used and works for the GMT400s, GMT800s and GMT900s.

Not only can it see all codes in all modules, it can perform resets and is used for programming. It can also graph and record and replay snapshots of trouble code information and a hundred other functions.

 

[Rocket Man]

Following. This has me interested.

 

[Jolly Roger]

Bought a Tech-2 when I bought my truck. No longer was I going to work on cars blind, these scan tools are a must to know what the computers are seeing and they are full of information that works hand in hand with the shop manual and troubleshooting guide. This is the tool, the Tech2, that the dealerships use/used and works for the GMT400s, GMT800s and GMT900s.

Not only can it see all codes in all modules, it can perform resets and is used for programming. It can also graph and record and replay snapshots of trouble code information and a hundred other functions.

Where is the best place to get it? Been looking online but results are far and wide. Can it disable my AFM? You wouldn't happen to have a part number or link would you?
Thanks again James!

 

[swathdiver]

Where is the best place to get it? Been looking online but results are far and wide. Can it disable my AFM? You wouldn't happen to have a part number or link would you?
Thanks again James!

No, it cannot disable AFM but it can test it thoroughly. Bought mine from AliExpress two years ago. Fellas on here have bought them off ebay and Chinacardiags. You want the latest software, 33.004 and a seller with good reviews and or one that has a sold a bunch of them.

 

[Jolly Roger]

double post...

 

[thompsoj22]

No, it cannot disable AFM but it can test it thoroughly. Bought mine from AliExpress two years ago. Fellas on here have bought them off ebay and Chinacardiags. You want the latest software, 33.004 and a seller with good reviews and or one that has a sold a bunch of them.

also got mine from aliexpress $275 to my door fedex.

 

[Jolly Roger]

Ok, had my son place his cell phone under the truck while I drove slowly over it...
My front drive shaft going to the transfer case is turning.
I'm not in four wheel drive. Is this correct operation?

 

[thompsoj22]

Ok, had my son place his cell phone under the truck while I drove slowly over it...
My front drive shaft going to the transfer case is turning.
I'm not in four wheel drive. Is this correct operation?[/QUO

yes that is normal, the shaft allways spins because you dont have the style hubs that allow axle disengagement. ask your son to go for a ride with you and find a smooth quiet culdesac/parking lot and with the windows down do a smooth 360 degree turn in both directions, there should be no chirping/lurching in 2wd. you have to rule out the drivetrain and than i think your mpg may reveal itself to be engine related in some way? hey does your sons phone still work after "i drove over it slowly" id like to do the same thing to my daughters phone! lol

 

[Jolly Roger]

I'm on it John! And thank you for the suggestion!
I'm slammed most of the day today but I surely can find time for this test and get back to you with the results asap.

I'm 100% positive that its not in 4-wheel drive but I'm pretty sure that even in 2-wheel drive the frt diff binds a bit and makes a whining noise when turning sharply. Ill pay close attention today and report back!

And yes, when I pulled up behind Home depot yesterday and told my son to get out and put his phone under my truck, he had those dear in the headlight eye's.

 

[Jolly Roger]

Test complete.
Took it to a parking lot and drove in the tightest circle possible in both directions. Nothing! No squeaks, no binding no lurching notta!

But something did come to mind:
When replacing the transfer case they said the the O2 sensor wire was wrapped around a drive shaft. They repaired it. I cant see any new codes on my dash because the CIL stays on all the time due to a gas tank purge valve error.
Could a faulty o2 sensor/wire cause such a drastic mpg loss?

 

[Rocket Man]

Test complete.
Took it to a parking lot and drove in the tightest circle possible in both directions. Nothing! No squeaks, no binding no lurching notta!

But something did come to mind:
When replacing the transfer case they said the the O2 sensor wire was wrapped around a drive shaft. They repaired it. I cant see any new codes on my dash because the CIL stays on all the time due to a gas tank purge valve error.
Could a faulty o2 sensor/wire cause such a drastic mpg loss?

Oh yeah. The ECU uses O2 data to determine fuel mixture.

 

[Jolly Roger]

I dont currently have a scanner but maybe one of the auto parts places will plug up for me real quick....but with my luck I can just about guarantee that no o2 code will return. That would be way too easy. lol

 

[Justinj360]

also got mine from aliexpress $275 to my door fedex.

Wait... Why was I under the impression that Tech 2's cost thousands of dollars..? Am I missing something?