Help!! 10 mpg on premium!!

[big_c_snow]

I have a major problem, and I hope someone can help me out here. I have a 1986 chevy 1/2 ton, 305, TH700R4, 3.73, and 29" tires. I am getting 10 miles per gallon, which is a combination of city and highway driving. It probably wouldn't be that bad, but I have to put 92+ octane in it, and it still pings under anything but very low throttle. I also live 30 miles from anywhere, and have one 18 gallon tank. 30 bucks every three days sucks when you work fast food. Anyway, I hope someone can help me with my fuel mileage problem. I drive it kind of like a 17 year old kid would, but i try to go easy on it and still get 10 mpg. I also lost to my friend in his Chrysler New Yorker, which was embarassing. Thanks for the help!

[MylilBowTie]

Check the timing and start looking at the distributor if the timing is right. Weights could be sticking or some parts are just worn out. Have you done a good tune up lately? Could be a lot of carbon built up in the engine. What do the spark plugs look like?

You should have a 16 (short bed) or 20 (long bed) gallon tank. 10 mpg isn't all that bad but its not good for it to be acting like it is either so you might get a little bit better mileage but not much. That is about what the majority (not all) of these trucks run is about 9-14 mpg. Unless your me and get 4 mpg!

Let us know what happens.

[TC70C10]

You might want to see if you Electronic Spark Control (ESC) module, or your knock sensor is bad. I've got an 81 K5 that I dropped a new 305 in, and didn't put in the knock sensor. I've had the same trouble but last week I bypassed the knock sensor circuitry by cutting 2 wires going to the distributor and tying them together. I unplugged the module under the dash and got the timing set around 8-10 BTW.

I got about 16 mg this weekend between Alabama and N.C.

Around town I get 13-14.

There's a post on this message board about the ESC. Just do a search and you'll find the link.

Good luck

[rockman20]

Not sure if the 86 still has it or if they have made everything electronic now, but I would check the vacuum advance as well. I know nothing about the knock sensor, so I would look at timing issues first. Initial and total timing and mechanical and vacuum advance. If everything looked good, I would then research into the knock sensor.

[rockman20]

PS 10MPG isn't too bad.

I am getting average of about 10-12 around town and just took it out on the highway yesterday. Made a 390 mile trip at about 14.5 MPG and not sure what it got on the 390 miles back, but it was better because I used less gas. (Even though I was now hauling a complete 350 Pontiac engine with a complete TH350 attached to it.

[Slonaker]

My 2wd '86 with a 305 and TH350 and 3.08 gears gets a little over 11 mpg with combined city/highway driving.

Slonaker

[apstguy]

Welcome to our nightmare! I have a 85 305 3.08 TH700R4 and get around (9.8) 10 mpg. These old 305's get 10 miles per gallon when they get old. Really want better fuel economy? Upgrade to a 350!

[big_c_snow]

The distributor "should be" okay, it was installed shortly before I got the truck. It is the Pertronix flamethrower dist. I say should be because I have had to replace the ignition module and coil -separate times- to get it to start. I have tried to do the timing once before, but we couldnt see the mark and were getting impatient. The plugs better be good, as they are the third set in the last three months. Both times, it required new plugs to start. I finally figured out why, and got the carb rebuilt, and it is not tuned right. Oh another Q- does anyone know what tool to use to adjust the mixture screws? I tried the other day, but couldnt tell the right tool to use. And you say 10 mpg is normal for a 2x4 with overdrive trans? I hope not, I can't afford that. Thanks!

[gchemist]

I get ~15 MPG in the city!! 158000+ miles on the 305 too. ~18 MPG on the freeway with OD and 26" tires. 700R4 with 3.08 rearend.

Is the torque converter locking up? Are your brakes dragging at all? How is the drive shaft working? Is the original cat still there?

Ditch the stock air cleaner and go to an open element. Use a good carb cleaner on the carb. Oh, is the speedo corrected for the tires? Oh drop the tail gate or add a bed cover!

[rockman20]

There is a tool that I picked up from CarQuest many years back that has a screwdriver handle, a flex cable, and then an end where you can install different bits. Looks basically like one of those magnetic screwdrivers but with a flexible shaft. And instead of phillips bits, it has those odd looking bits that they use on carbs.

I don't recall price, name, or anything, but I know it came from CarQuest.

Clean up the harmonic balancer really well and look for the knick in it for the TDC mark. Clean it up and then take a white crayon and rub it over the mark a couple of times to make it stand out better and then do your timing.

If you are going through plugs and such at that rate, something is definitely wrong with the ignition. My guess would be the timing is WAY off which is killing your fuel economy, causing the ping, and destroying your plugs.

[big_c_snow]

Well, I wish I could say that the speedo was calibrated, but I have no idea. It has this problem with bouncing as I hit around 60 mph, it then goes straight to around 70. None of my other factory gauges work right either, so thats a problem. There are no cats on the truck, just manifolds into 2.25 inch pipes to 2 flowmaster mufflers out the back. Would an across the bed toolbox hurt fuel mileage? It came with the truck and I have nowhere else to keep all my crap at this time. Thanks!

[gchemist]

To calibrate your speedo correctly go here http://www.tciauto.com/tech_info/speedo_gears.htm

Slap on a 26" tire and the New Yorker is history.

[big_c_snow]

Well, I did the timing and it was about 15 degress btdc at idle. I fixed that small problem, and it stopped pinging, but now has even less power. Just thought I would post an update. Thanks!