Gas Mileage Part Duex

[OUMike]

ok...so I've got a 94' 4.3. I got this truck with about 112,000 on the odometer. I was getting anywhere between 350-400 miles out of a gas tank (Regular Unleaded). Slowly but surely I noticed a slight decline in gas mileage, and then before I knew it I was getting about 9 to the gallon. I changed the PCV Valve, all 6 plugs, all 6 wires, air filter, and went on and did an oil job too. HUGE difference, but still couldn't quite get what I was getting before. When I got the truck my daily commute was a little further (more HWY driving, but not much more). I've since moved a little closer, so my stop and go traffic has picked up a little. I still wasn't convinced that would be the cause of the drop in mileage. My truck runs fine, no trouble with the motor. Then yesterday (7/8) my battery light came on, and gauge was reading about 12 (usually at about 14.5). I only had a two year battery (purchased about 3 yrs ago), so I decided to change that out....didn't fix it. I replaced the alternator and problem solved.

This past weekend I went camping in Jasper, TX (about 150 miles from where I live). I filled the truck full of gas before I left and drove home. When I got home (this is before alternator/battery swap) I had about 154 miles on the tank. I have now drivin' 318 miles on this tank, and I think I got about another 30 or so.

My theory is...changing the alternator and batter will improve my gas mileage. I'm not sure how I can support this. Does the alternator effect spark in any way? Maybe I'm gettin' better spark. Any thoughts??

[woyster_00z71]

If your alternator was going bad, it may have been dragging causing more power from the engine to be able to turn the pulley. Just a thought. I'm no expert.

[Tucson_02Silverado]

Check the vacuumn lines. There shouldn't be too many of them, maybe just replace them because of age. When is the last time you changed the cap and rotor? Maybe time for a new coil as well. If you charge at 14.2+ when you start the truck and you never see the guage drop below 13 it's probably no need for a new alternator unless you see the guage bouncing a little (regulator going bad) or you hear a little squeal from the pulley (bearings bad).

[OUMike]

It sounds smooth, and no flux in the gauges. Never changed cap and rotor, and vaccum lines I believe are also original. Think I should change em just cause?

[varmint2506]

If your alternator was going bad, it may have been dragging causing more power from the engine to be able to turn the pulley.  Just a thought.  I'm no expert.

If the alternator had a bad bearing, it could very well add a lot of drag.

It sounds smooth, and no flux in the gauges.  Never changed cap and rotor, and vaccum lines I believe are also original.  Think I should change em just cause?

I would defiantly change the cap and rotor especially if they have some age on them.

[Tucson_02Silverado]

Also for vacuum check, you should have what looks like a black plastic ball with a couple vacuum lines going to it. It's a vacuum reservoir for vacuum actuate things like HVAC doors. Check that for cracks/leaks as well. The stock vacuum lines would most likely be hard nylon with rubber ends. This amount of time the rubber ends are probably cracked/brittle/hard and be the source of any leaks.

[Black02Silverado]

One more thing that a lot of people forget is the tires. A good front end alignment and keeping the proper pressure in the tires can make a difference in gas mileage.

 

Changing out the cap and rotor is a good idea.

 

Anything worn can put a strain on the engine. Like stated above, warn bearings in the alternator. Power steering pump with fluid that never been changed, (yea this is stretching it a little ) Could put a slight drag since the pump isn't pumping fresh lubricating fluid as it once was. Is the AC up to snuff?

 

Yea I'm looking at things that probably don't effect it one bit but if you lose mileage over time it could be some of the cause. The other is the fact that the engine is just worn out. Not saying yours it but lower compression in the cylinders means power loss which = lower gas mileage.

 

Now that you have gotten a good laugh reading this, it is just what came to mind.

[stebob]

May want to slap a new fuel filter in there, too!

[Black02Silverado]

May want to slap a new fuel filter in there, too!

Duhhhh How did I miss that one?

[100R1]

02 sensor

 

with 112,000 miles if original could be a problem.

 

I had a jeep cheroke that I started getting extremly good gas millage (28 mpg) on the road well above normal. I thought, it gets better with age!

 

oops, next thing I know it's the s*its, this gas millage sucks.

 

not to much longer I start getting check engine light some times on and sometimes off

This being a Jeep i could cycle the key on and off and get trouble codes.

 

yep it was the 02 sensor, replaced and gas millage went back to the original millage of 15 city and 21 highway

 

This started about 110,000 miles, I now have 248,000 miles on it with the second 02 sensor in palce and no problems. It's for sale since I have my Yukon.

[OUMike]

I changed fuel filter at about 145k miles. O2 sensor though...probably original. Where is it located on the 4.3L??

[bigtruck21]

On the exhaust after the manifold and somewhere near the cat. You might have 2 sensors one before and one after the cat.