Gas Mileage DROP

[79D604SPD]

I have a 1991 GMC Sierra C1500 pickup with 4.3L V6 TBI engine with 170,000 miles. Since fall ended, I have noticed a good drop in the mileage I can travel between gas fill-ups. When I bought the truck in August I was getting about 450 miles until I filled the truck up from 1/4 tank(was about 19mpg then). It has the 34 gallon tank. Since winter began it has dropped to about 350 miles until I need to fill up from 1/4 tank. I haven't calculated the actual gas mileage now. I know cold weather may cause some drop in gas mileage, and I don't expect wonderful gas mileage from a high-mileage vehicle, BUT a 100 mile drop between fill-ups?

I have just replaced plugs, wires, O2 sensor, patched a small hole in the exhaust and changed fuel filter. I don't like to throw parts at a problem to fix it, but with high miles on the truck, replacing that stuff won't hurt and was not expensive. Seems like gas mileage has not improved much.

One thing I noticed is the thermostatic air cleaner damper door in the air cleaner housing snorkel doesn't seem to work all the time. Yesterday(outside temp was about 55 degrees) after a 10 mile drive I checked the door in the air cleaner while the engine was running and it was about half-way open. Today(outside temp about 45 degrees) after 10 miles with running engine and the door was closed. My haynes manual says the door should begin to open and pull air from the outside, instead of the exhaust manifold, when the engine warms up. Is this right?, and if the door is malfunctioning, would it cause a drop in gas mileage? Thanks for any ideas or help.:headwall

[76chevyman]

i have a 2001 s10 blazer with a 4.3 and the only time i get over 300 is if im traveling on the highway. but going to work and home and it ideling a little i get like 160 miles a tank. thats like 13 miles a gallon. So a 4.3 in that truck getting that milage seems great to me. but it has droped as you say.
Maybe its the injectors are dirty im sure its TBI look at the spray pattern se if its a nice stream or if its dripping and doesnt look consistent.

Rich:burnout

[79D604SPD]

I bought off-the-shelf injector cleaner(called TL-3, recommended by parts store manager) and I will see what happens, I've never seemed to have good luck with them. I did price a TBI rebuild kit, minus injectors for 40$, I might eventually do that. Really surprised me too, that this truck got about 19mpg then. It has 3.42 gears and 700R4/4L60. Haven't really changed my driving habits since fall. My girlfriends 92 GMC C1500 350 TBI gets about 13mpg average. Thanks for your help.

[wyldman]

That is about normal for that truck.The cold weather has a huge effect on fuel injected engines,and they will use a lot more fuel.

[79D604SPD]

Have to hope for warm weather soon, then. thanks for your help:cool:

[76chevyman]

Try the Lucas injecter cleaner thats the only one ive found that works.

Hey Wyldman is that normal for a 2001 blazer with a 4.3 i get 13 a gallon city

Rich:burnout

[ZR2DK]

Ive got a 2000 S-10 (pickup) ZR2 with 31" mud tires. In the winter I get anywhere from 15-17mpg combination of highway/around town and in the summer I can get upwards of 18-20mpg combination highway/around town (23 is my best on a trip from Ohio to Maryland.)

I have a K&N FIPK cold air intake, a flowmaster muffler, & bosch platinum +4 spark plugs & bosch wires (they worked for 1-2mpg).

When I first got the truck, I was getting around 16-17 with bald BFG All-Terrains in the summer. Now Im getting that in the winter with mud tires and even better than that in the summer.

I feel that my mods worked well. The original plugs/wires only had 30k on them when I got the truck and replaced them w/ the bosch. I would recommend them.

An E-fan is in the plans for this summer.

Also, give a product called "seafoam" a try. Run some into the engine via the brake booster vacuum line while the truck is running and also send some through the injectors via the gas tank. Re-connect the hoses, shut off the truck and let it sit for at least an hour (more if you can). Start it up (it will be hard to start, but eventually it will turn over) and then drive it around town and get the temps up and then give it hell and watch the white smoke and *gunk* blow out of your exhaust. I use it about 4 times a year and it works nicely for a good engine clean-up.

[76chevyman]

Im going to add a K&N on it in the spring and maybe a performance chip as well. as for wires i use MSD wires great wires.

Rich:burnout