low gas mileage

[lionheart21]

I have a 1995 Chevrolet K1500 with a 350 in it. It is the original engine with 150,000 miles on it. For the past few months I have been getting about 11-12 mpg with it, and when I bought it 2 years ago I got around 17 mpg. I was wondering if anyone would know what the problem would be as to why I am getting such bad gas mileage.

[VortecWes]

My '94 Sierra (TBI) gets 10-12 while my '97 Silverado (Vortec MPI) gets 16-18. Your mileage seems right to me...

[daman]

Has it seen a compleat..... tune up lately???

[arod]

I'm having the same problem. I have a 93 C1500 ext. cab with a 350/auto, 130K miles on it. I bought the vehicle used in 1996 and ALWAYS got 17-18 mpg, city or highway driving.

I started taking the truck off the road in the winter (Nov-April in upper NYS) 3 years ago. When I did this, the mileage dropped the next spring to 11-12 mpg. Nothing was done to the vehicle in the interim, and it started right up in the spring. Sounds like what you are experiencing.

I also notice a definite lack of power, like the timing is retarded in an older vehicle. I know this is all controlled by the ECU, so I assume there is a bad sensor somewhere. The engine does labor when I pull a trailer, and the power output of the engine is much lower, so much it's noticeable on slight hills. The engine just feels sluggish.

I went through the ECU tests, reset it, checked what I could electrically. No dice. I gave it a complete tune-up (plugs, wires, cap, rotor, all filters), the oxygen sensor (six years old at that point) and checked the timing. It was correct with the timing plug under the hood disconnected. I changed the MAP sensor thinking it was the most obvious culprit based on what appears to be the vacuum/timing issue, but that did not work.

I took it to a local dealer after no change in mileage, and they hooked up their diagnostics to it. Nothing showed up, nor could they offer any advice. All I was told is that these vehicles "have to be driven. They can't sit for months at a time". Whatever that means.

Last spring I took it to Annapolis MD to pick up some engines and transmissions for another vehicle I am rebuilding. First tank heading south I got 11 mpg. Filled up just south of Williamsport PA, drove to Annapolis, back to York PA with about 1300 lbs of pig iron and aluminum in the bed. Got 18 mpg. Why? I don't know!

Got 18 mpg back home on the next tank of gas as well. The next week it was right back to 11 mpg. Something happened on the long drive that "kicked" something into working again. I'm thinking the continual heat generated by a 10 hour drive could have dried something out, but I would think a sensor shorted by water would show as a ECU sensor fault.

I went through all the hoses and other external devices I could. I checked all the pin connectors, hose connections and whatever else could crack, leak or get wet. I think there is one or a couple of other ECU sensors on the engine that may have failed (as in not changing values when the engine is on) but are not reading a fault code at the ECU. I hate to be turned into a parts changer, but that's what it's coming to.

Any more info you may have may lead me to figuring out what's going on.

[arod]

I also checked tha cat converter for plugging when I had the O2 sensor out- none. It has a Dynomax cat back exhaust system on it since 1996, so that does not seem to be the issue either.

[mjdaye]

Hey guys, some things to think about that almost everyone misses are these:

Altitude change, severly effects gas mileage when moving from one altitude to another without a retune on the computer.

Gas: Not all gas is the same, watch the gas pumps your using, some states push that 10% ethanol crap. When i moved from Georgia to Florida I learned florida loves that gas with 10% ethanol, and yes my gas mileage has dropped because of it.

Not sure if it helps but nobody ever thinks of those

[troubles789]

9-13 here and only 13 going down hill on a trip with the wind to my back.

[tblake]

This thread dates back to March of 2007. We never did hear the outcome, so I doubt the OP is still checking it.

[fmx_guy]

While i was searching through the posts i came across this one and i thought i would share my experience. when i got my 94 z71(k1500) 5.7l i did a full tune-up. napa didnt have the platnum plugs? or whatever is supposed to go in the motor, anyways i SUBSTITUTED WITH MUCH CHEAPER PLUGS. ran fine for 4-6 months then fuel mileage started getting bad. it was the plugs. i bought the correct 8.00 and change apiece plugs then my fuel mileage went back up to 17-20. the point i am making is that even though the truck felt like it was driving fine. it made a huge difference at the pump. Thats what i get for being a cheap a$$. hope this helps someone. dual exhaust,k&n,msd blaster coil,asp underdrive pulley,3.42 ratio.,change driving habits.(stay under or at 2000rpm) in high gear.

[davidx2000]

FMX, what plugs did you end up putting in? I need new plugs and am a new (used) 97 K1500 5.7 owner.

Thanks!

[fmx_guy]

My plugs may be different than yours, the napa guy said that my stock plugs were platnum? I took his word and paid the man. So i am using stock AC Delco plugs for a 94 chevy silverado 5.7l . Hope this helps you.

[davidx2000]

Thanks FMX - brand is what I was after so I'll pick up some AC's.