Decreasing Gas Mileage

clomax · February 7, 2011

My 2001 Yukon's MPG has dropped by nearly 1/3 in the past few months, going from roughly 330-350 miles a tank to barely over 200 now. This started after running a can of sea-foam through the gas... It's not throwing any codes, but the exhaust seems pretty rich from the soot on the tailpipe. Prior to the sea-foam I had changed the plugs and wires. I haven't pulled a plug to see how they look. Thoughts, suggestions, questions, comments??!

Stevens03 · February 7, 2011

When was the last time you've cleaned the MAF sensor and/or throttle body? Have you changed location of where you normally get gas? Has anything else changes like tires or even tire pressure? I'd start with those and see if that changes anything.

clomax · February 7, 2011

Haven't cleaned the MAF. I've been getting gas at a Sam's prior to the decrease in mileage. I did notice that they do have the contains 10% ethanol sticker, almost impossible to avoid stations that blend. Tires were changed but stayed with the size prior to.

When was the last time you've cleaned the MAF sensor and/or throttle body? Have you changed location of where you normally get gas? Has anything else changes like tires or even tire pressure? I'd start with those and see if that changes anything.

thebandnerd03 · February 7, 2011

i love sea foam but there is one downside to it. if for some reason you have sediment or any type of build up in your tank, filter, or lines it can cause this to dislodge and begin building up carbon in the engine. the main issue on your year of vehicle is on the control plate in the throttle valve assembly. this can get a lot of debris on it and cause issues with MPG. go have your intake cleaned at your local shop (usually not more then $50) and you should see some differences

Hieneken49 · February 7, 2011

My 2001 Yukon's MPG has dropped by nearly 1/3 in the past few months, going from roughly 330-350 miles a tank to barely over 200 now. This started after running a can of sea-foam through the gas... It's not throwing any codes, but the exhaust seems pretty rich from the soot on the tailpipe. Prior to the sea-foam I had changed the plugs and wires. I haven't pulled a plug to see how they look. Thoughts, suggestions, questions, comments??!

if it make su feel better my 09 sierra went from 16 in the warmer months to 14 the last 2 months in the cold.

clomax · February 7, 2011

Just pulled the MAF. Haven't located a tech article on properly cleaning this... Bought a can of electrical cleaner specified mass air. I'm assuming just spray the same direction as air flow?

When was the last time you've cleaned the MAF sensor and/or throttle body? Have you changed location of where you normally get gas? Has anything else changes like tires or even tire pressure? I'd start with those and see if that changes anything.

HawkScot · February 7, 2011

Future reference, if you want to use SeaFoam, change the plugs AFTER you use SeaFoam. Also, change brands of gas and run some Techron cleaner through it. I have had good luck with SeaFoam.

My dad had his mileage drop and drop on his 2008 Chev 2500 with 6.0 and he had been getting gas at Costco. He changed to Shell and it went from 12.6 to 16mpg.

Also check your air cleaner and make sure it is clean.

clomax · February 9, 2011

Wasn't immediately after changing plugs but definitely won't be using it again in the future. Cleaned the MAF and reinspected the air filter to make sure it was still clean. Almost through another tank and still no difference...

Had read about the fuel pressure regulator as a potential culprit. Anyone changed one out on the 5.3? Is it under the plenum or on the fuel rail?

Future reference, if you want to use SeaFoam, change the plugs AFTER you use SeaFoam. Also, change brands of gas and run some Techron cleaner through it. I have had good luck with SeaFoam.

My dad had his mileage drop and drop on his 2008 Chev 2500 with 6.0 and he had been getting gas at Costco. He changed to Shell and it went from 12.6 to 16mpg.

Also check your air cleaner and make sure it is clean.

Spurshot · February 11, 2011

If you've convinced yourself that the intake system is in good working order, you might look into ignition timing as a culprit. That kind of drop in mileage might be linked to a timing issue. Inadequate timing advance at cruise/light throttle can reduce mileage. On these engines, there could be a number of reasons the timing isn't getting to where it should. The nock sensor might be something to look at. You might do well to look into a custom tune where they hook up your truck and do the tune and verify everything while driving/dyno testing. The tuner will likely spot the problem right away.

clomax · February 11, 2011

I'm not convinced of anything yet. Regarding scanning the vehicle, while unrelated to tuning, I am still not seeing any trouble codes or pending codes. I have a Landrover that requires a proprietary platform (testbook) from the dealer that enables them to scan more than an OBDII scanner. Are GM vehicles similar in this regard?

If you've convinced yourself that the intake system is in good working order, you might look into ignition timing as a culprit. That kind of drop in mileage might be linked to a timing issue. Inadequate timing advance at cruise/light throttle can reduce mileage. On these engines, there could be a number of reasons the timing isn't getting to where it should. The nock sensor might be something to look at. You might do well to look into a custom tune where they hook up your truck and do the tune and verify everything while driving/dyno testing. The tuner will likely spot the problem right away.

Spurshot · February 11, 2011

Chris,

I'm not all that knowledgeable about the diagnosis software, but, I do recall seeing some of the software capabilities of EFI Live sofware when my Blackbear tune was done. It's far and away more than a simple code reader can do. You can see all sorts of engine/trans parameters, including fuel and ignition. That's why I suggested the custom tune route.