1998 5.7Vortec fuel mileage

fizure · October 25, 2006

I have a 98 k1500 with 5.7 Vortec, been running fuel from the same pump for over year, when outside temp drops below 70 F, my fuel mileage drops by 5-8 mpg.

I have 135,000 miles, new plugs, wires, fuel filter, air filter, changed oil and filter. If i remember correctly I had this same problem last year. I have done zero mods to the air intake on this truck. any help ?

Drive 80 mile round trip 5-6 days a week for work fuel mileage usually averages 16-19 now is 11-13mpg. this driving is all highway.

Mr. Smith · October 25, 2006

I have a 98 k1500 with 5.7 Vortec, been running fuel from the same pump for over year, when outside temp drops below 70 F, my fuel mileage drops by 5-8 mpg.
I have 135,000 miles, new plugs, wires, fuel filter, air filter, changed oil and filter. If i remember correctly I had this same problem last year. I have done zero mods to the air intake on this truck. any help ?

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I would look at the coolant temp sensor for the computer. It is the one by the thermostat. It may be out of spec causing the engine to run rich due to the sensor thinking the engine is cold.

1999 Tahoe 4x4 · October 28, 2006

Are you sure your gasmilage is that bad? 5 miles to the gallon seems quite unbelivable!

As a reference at Southern California temperatures I frequently get 9-10 miles to a gallon on my stock truck. HOWEVER that is only if I drive the truck just e.g. two short trips a day for all week long. This way the engine is always driven "cold".

So if you are getting 8 miles per gallon with short trips like me I would not be that much alarmed.

BTW: The best I ever got was 17mpg on a trip from LA to Houston. Most of the time on this trip I got 15mpg.

5mpg is the more unbelivable because the air fuel ratio is set as a constant 14.7:1 on a warm engine. That means if the ratio is so radically different you actually should get a check engine light and some codes with a warm engine. I would look into this matter serious because a rich fuel ratio will sooner or later damage your catalitic converter.

Also if any temperature sensor is faulty a check engine light should come on too: as far as OBDII engines, the computer performs a sensor check at cold start condition. Coolant temperature, cylinder head temperature and intake air temperature sensor readings must be the same. If not the same check engine light will come on.

Check careful for external fuel leaks too.

fizure · October 28, 2006

Are you sure your gasmilage is that bad? 5 miles to the gallon seems quite unbelivable!

As a reference at Southern California temperatures I frequently get 9-10 miles to a gallon on my stock truck. HOWEVER that is only if I drive the truck just e.g. two short trips a day for all week long. This way the engine is always driven "cold".

So if you are getting 8 miles per gallon with short trips like me I would not be that much alarmed.

BTW: The best I ever got was 17mpg on a trip from LA to Houston. Most of the time on this trip I got 15mpg.

5mpg is the more unbelivable because the air fuel ratio is set as a constant 14.7:1 on a warm engine. That means if the ratio is so radically different you actually should get a check engine light and some codes with a warm engine. I would look into this matter serious because a rich fuel ratio will sooner or later damage your catalitic converter.

Also if any temperature sensor is faulty a check engine light should come on too: as far as OBDII engines, the computer performs a sensor check at cold start condition. Coolant temperature, cylinder head temperature and intake air temperature sensor readings must be the same. If not the same check engine light will come on.

Check careful for external fuel leaks too.

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