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I have 98K on my truck and I took it into NTB for the following

Spark plug replacement (I didn't change the cables)
Transmission flush (they overfilled the tranny and it leaked out the tranny vent for 3 days)
Fuel injector treatment, there the put treatment in the oil, fuel tank and I guess the injectors

Them I performed the following myself, diff lube change and coolant drain and replacement

Well I typically would get 19 -19.5 mpg fuel economy, I ran one tank and got ~17.3 and the a 1/3 tank and got 17.6 WTF????

I replaced my air filter, and checked tire pressure after the first tank and don't know what could have caused this. Any ideas? Was it because they overfilled the transmission? I'm stumped.
 

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Did you change your fuel filter? Maybe its about time, should change those roughly every 10,000 miles. They get nasty fairly quick.
 

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you only ran 1/3 tank. Rather than a full tank.
 

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fuel filter and maybe have your O2 sensors checked, and cats checked...I personally am not a big fan of the pour in fuel inj. cleaners I just run super in it .
 

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Proper diff. oil? too heavy wt. will bog you down; I'd run a whole tank+ of gas to get all the "treatment" outta the system for a final MPG. read.
 

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I'd be a little worried about the tranny now. Maybe enough to start doin' a drain and fill every 10K, w/Amsoil synthetic. I'd be wondering about the condition of my spark plug wires, also. Put some high test in for the next couple of tanks and see where you're at. Check to see if you're setting any codes.
 

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The service engine light came on now

P1448 vent control valve
P0328 knock sensor
P0455 large evap leak
Sounds like you have a vaccuum leak on it, they could have spli one of the lines while doing the plugs, or also a possibility, they might not have seated one of your plug boots might not be seated properly, causing a weak spark.
 
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