Gas Mileage Way Down

97whitez71 · November 12, 2012

Truck has about 194,000 miles on it. No check engine light and still runs great. Over the last 4-6 months, the gas mileage has gone from around 18 down to about 13 mpg. The plugs and wires have about 70,000 miles on them. At about 188,000, I put two new O2 sensors upstream. Other than the mileage going down, you'd never know anything was wrong with the truck. Any ideas of what else I could look at for the reason in mpg drop? Thanks in advance!

00Silverado4x4 · November 12, 2012

Id get codes read first, may have old codes with no light. Also fuel pump failing, catalytic converters getting plugged up, crappy fuel, injectors getting gummed up, carbon build up internally or combination of all. You have a lot of miles and possibilities. My personal opinion is to take it to a shop/dealer to have it gone through.

kstruckcountry · November 12, 2012

My guess would be cats.

Bad Bowtie2 · November 12, 2012

Did you go back with AC Delco parts??

I would change the fuel filter & drop a can of Seafoam brand fuel treatment in the gas tank.

Then buy a 2nd can of Seafoam to suck into the intake manifold (while running).

ujtruck · November 12, 2012

spark plugs and wires ?

atleast check them, make sure they are not loose, I found 1 in mine now MPG improved 18+

SILVERADOSC540 · November 13, 2012

+1 Mark, Danny and Jon- fuel filter/cats. Joe is right on the easy check of plug wires.

hillbilly4u · November 14, 2012

Is there any way to have the cats checked for flow? My mileage is bad too, but I've been burnin a lot of oil.

kstruckcountry · November 14, 2012

You can take them off and put on new ones, or if you live somewhere not required replace them with tubing. Lots of people just wallow them out too. I've known lots of people who had the insides coming apart and plugging the exhaust. Pretty high mileage for that to happen.

SILVERADOSC540 · November 14, 2012

Had it happen on a Saturn. Biscuit burned down in size and turned sideways against the back of the cat. Mileage and power took a dump. New cat and my 85 screamin' horsepower were layin' waste to anyone who dared challenge her.

97whitez71 · November 14, 2012

Yeh, all AC Delco parts went back in. Plugs and wires look fine and tight. No miss or rough idle what-so-ever. Will a failing fuel pump cause this without causing hard starting, bad running engine, etc.? Same question as hillbilly, can the cats be checked? It may not matter though, if there are no check engine lights and truck runs great otherwise, I would think it would have to be the cats. I just can't imagine what else would cause this kind of drop. Thanks!

SILVERADOSC540 · November 15, 2012

My Saturn cat was an easy find because I could barely feel any exhaust coming out when I had the wife rev it for me. Also wound up much slower when she wound up the throttle in neutral. Does your truck have an inline fuel filter on the frame rail? Not familiar with your truck, but if it does, it's an easy swap and not very expensive. May not fix it but for the money it's not a bad investment anyway. Good luck.

97whitez71 · November 15, 2012

I'll see what it feels like coming out of the tail pipe. Yeh filter is on the frame rail and was replaced around 10,000 - 15,000 miles ago. Thanks.

hillbilly4u · November 16, 2012

The NBS Silvy's have no filter on the rail, its in the tank. Another bad idea I think.

Gurrzt · November 16, 2012

Did you go back with AC Delco parts??

I would change the fuel filter & drop a can of Seafoam brand fuel treatment in the gas tank.

Then buy a 2nd can of Seafoam to suck into the intake manifold (while running).

Isn't the fuel filter part of the fuel pump on these vehicles?

Bad Bowtie2 · November 16, 2012

Did you go back with AC Delco parts??

I would change the fuel filter & drop a can of Seafoam brand fuel treatment in the gas tank.

Then buy a 2nd can of Seafoam to suck into the intake manifold (while running).

Isn't the fuel filter part of the fuel pump on these vehicles?

Well, I saw that the OP had a '97 K1500 & a '01 Burban in his "sig". Both of these vehicles have driver's side frame mounted serviceable fuel filters.