Things to check for low mpg

[swathdiver]

Mine gets 8 mpg when my wife drives it!

 

[sonic_the_hedgeh]

I just bought a 2003 Yukon Denali. Check engine light came on.

I checked the previous owners service records he gave me. Previously reported as a faulty map sensor. Which is about 400 dollar fix. I have now found out the shop that checked it failed to diagnosis the problem correctly. Instead it was a pcv valve that had not been changed out in years. Now the fuel milage is much better no rough shifts either. Hope this helps some.

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[sonic_the_hedgeh]

if you figure it out im in the same boat with this 8 mpg ********. please let me know.

2004 5.3 v8 no CEL codes.

got a tune up 18 months ago with all ac delco stuff.

changed out the 02 sensors on driver side. was getting rough idle and misfires. fixed it.

had the computer reprogrammed. diagnostics said this is the issue not fuel injectors as there is no code and the numbers were all off on the scan due to old programming. waiting to see a couple tank fulls see if it worked.

what are normal fuel trims?

Make sure you change the pcv valve. Mine is at the back near the firewall on a thick hose.
There is a YouTube video that is really good.


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[gpd4]

I guess I shouldn't ***** too much about the 15.5 I averaged yesterday headed home from fishing...

 

[Big Mama]

Ok after all of the suggestions make a list. Go through them one by one and add checking your tire pressure. Did the tuneup you had done include plugs and wires? Did you add aftermarket wheels? If the drop was sudden it’s most likely vacuum, plugs, wires, or O2 sensors. A bad injector would most likely throw a code the oxygen sensor should catch it. If not it may be both of them. Keep us posted. My 07 6.2 has dropped a little each year but is still 12-13 city.

 

[HACK BLOCK]

Hi Charles,

Is that +20% or -20% for your LTFT?

positive. I'm getting about 7mpg city and 11mpg hwy

 

[HACK BLOCK]

NYC stop and go driving will kill your mpg easy.

if thats a + ltft you likely got a vac leak somewhere.

if its a neg - number you could have a stuck open injector, check both banks, see if ones richer then the other, then you know its an injector if one side is higher neg #.

I've read that if the ltft is still high when keeping the rpm at 1500 and 2500 then its a fuel delivery problem and thats what im going though right now

 

[jmm121783]

I just had my buddy at the dealership do ALL of my FLUIDS...every single one. This took me from 10-11 to 12.9-14 with E85! Very happy, he said my transaxels and transfer case fluid was black, same as my ATF so I assume there was a lot of lost efficiency through the entire drive train, with a 20% loss on auto trannys already I can imagine it was more like 25% in my car with the old fluids.

 

[Big Mama]

That’s a great tip. Mine are due now. Anyone used Amsoil for the pumpkins and trx case? I use it in 350 in my old boat. It’s a bit expensive but let’s me change oil every 2 years.

 

[swathdiver]

I just had my buddy at the dealership do ALL of my FLUIDS...every single one. This took me from 10-11 to 12.9-14 with E85! Very happy, he said my transaxels and transfer case fluid was black, same as my ATF so I assume there was a lot of lost efficiency through the entire drive train, with a 20% loss on auto trannys already I can imagine it was more like 25% in my car with the old fluids.

Yeah when I first drove mine home it felt like it was trying to move in three directions at once. It settled down nicely once all the fluids were changed. Is that 14 highway on E85?