This is pathetic. What's broke?

[magnethead]

It took 15 gallons to go 50 miles with no payload.

What'd I break? Haven't checked the plugs, but last time i pulled one it looked normal. We've never got a reading that low from that truck, even when towing a 5-6,000 pound trailer with the A/C on.

[ocbaud]

the odometer

or the gas gauge


????

[84 400]

I agree od or gas gauge working propery? Also cab or fule lineleak or tank leak but that would be quite noticable!

I am as puzzeled as you man

[magnethead]

no, the gas pump verified the 15 gallons, and the odometer runs off the same cable as the speedometer and both are still working properly, so......

[ocbaud]

i would probably plan on rebuilding your carb pretty soon than

[Rajas]

I had a similar problem with my old s-10, which was fuel injection though. This was prior to the tranny going out. Which was due to un noticable slippage on the highway.

[qksilver]

Need more info
Could be many things, plugged convertor, gas leak etc
Info like:
No power?
gas smell?
bucking?
RPM range?

[magnethead]

no smog

no gas leak/smell
normal highway RPM (give or take 100 or so RPM) (2800ish @ 55 MPH)
not having power/surge problems (although it does seem like just off idle isn't as strong as it once was)

[7387chevys]

thats like 3 1/3 mpg................did you get your gas at a different station?When we get gas at Gomart my truck has run on issues low octane crap
Shane

[72C203503ONTHETREE]

Using that much gas should make the exhaust turn black from being way too rich. My 73 with a crappy quadrajet blows black smoke out the exhaust under full throttle and it gets much better than 3.5 mpg. You have to have a leak somewhere. Take the dipstick out and smell the oil, if it smells like gas you are running rich or the pump is leaking internally, running fuel into the crankcase. Has to be a leak somewhere that doesnt drip when stopped. Try running the engine at higher idle while sitting in one spot and look under the truck carefully for wet spots along the frame or wherever the lines run to the carb. If youre not belching black smoke out the tail pipe at that much gas being used it has to be external loss.

[magnethead]

it only goes black above about 3400 RPM or so under no load..not somewhere i go often. It doesn't go black under load that I can tell.

I put 87 octane Exxon in it...

we just replaced the fuel pump about 2 tanks ago...last tank had normal mileage before this one.

I've been watching my gas needle (none to reliable, but still) and it looks like if I stay under 35 or 40 MPH the eyeballed needle mileage is normal...if I go to highway speed (2800 RPM) that needle starts moving. But if I run it to that RPM in first and second lock (shifting it like a manual), it doesn't appear to make the needle move as much (then again, i've only gone a mile with it in second lockout compared to half mile in first lockout and constantly in high gear).

[davidh]

I got 5mpg once out of 1/2 ton 85 pulling a short wheel base on a dolly. the motor was rebuilt but had not guts. found out the cam was going flat later..

[magnethead]

NORMALLY I get in around 8-10 MPG, regradless of loaded or not, give or take a few decimals.

I'm wondering on my cam, because it (the exhaust note) isn't very aggressive.

[Tom]

Its leaking gas somewhere while you drive is my guess.....

[Hoods69BadBowTie]

Its got to be leaking gas somewhere, how else is it possible to pull down like 3 miles to the gallon?

[magnethead]

I can't tell if my carb is out of tune or not, if it's a leak, or transmission slippage. Because now i'm noticing that it's at higher RPM's that it drinks..the needle (ign on, truck off, after sitting to let the tank settle) was right on the F when i left school and on the 7/8 mark when I got home..a whole 1 mile. Hasn't moved that much before on this tank or anything....

This is 100% erratic.

[Bender]

What about a hanging up wheel, such as stuck caliper, bad ebrake cable, etc.?

I guess it would be easy to tell if any of these were the problem if the truck doesn’t seem to “coast” normally.

I get about 6-8 in my truck, but I drive like an idiot and my carbs messed up

[magnethead]

it coasts fine, doesn't pull to one side or the other. The only conclusion i've come to (and my ears are backing me up on this) is that my engine is getting bogged down when it shifts to third gear. The second it shifts to third it tries to stumble and die unless it's fully warmed up..it's shifting to third at about 1500 or so RPM if not less, it shifts out of first at 1100-1200.

[72C203503ONTHETREE]

Sounds like the a lock up torque convertor is bad, locking up the convertor clutch too soon, but if your truck is an 80 and older thats not it. Does it short shift under heavy throttle or at normal light throttle? If it shirt shifts at heavy throttle the trans is screwed up, most likely the valve body is sticking, or the governor is screwed up. If it was the vacuum modulator it wouldnt shift until higher roms and shift hard. Id start looking at the trans.

[magnethead]

I can have it off choke and my foot off the gas, and if it's up to running temp already, it'll shift before the end of my (relatively long) driveway. Under hard acceleration it gets up near no-man's-land before shifting

Edit: It also has a B&M RV towing shift kit

[7387chevys]

Reckon that the neighborhood thief siphoned some? Seems the most logical explination I wouldnt think that gas milage would tank in 1 tank if its the engine
Shane

[magnethead]

that's what sucks about older trucks..can't use a locking gascap. I don't think so, but it's always possible. Thing is, i can see how much it's drinking as i'm driving, all i have to so is sit and let it settle.

We just replaced the fuel pump and filter, so the system shouldn't be clogged. I'm yet to pull a plug, i need to do that. Last time i pulled one it came out clean/normal.

[89 crew]

I really need more information. I can't make a guess with what is on here. My 89 crew cab got 4 mpg from a bad o2 sensor. Actually that is why I got that truck! the P,O, couldn't figure out the prob. it seemed to run ok didn't smoke except when cold. turns out the o2 sensor was hanging by the wire. now for yours, let's start off with year, engine, transmission, 2 or 4 wheel, is it running rough? if so when? anything else you might have noticed?

[magnethead]

Originally Posted by 89 crew

I really need more information. I can't make a guess with what is on here. My 89 crew cab got 4 mpg from a bad o2 sensor. Actually that is why I got that truck! the P,O, couldn't figure out the prob. it seemed to run ok didn't smoke except when cold. turns out the o2 sensor was hanging by the wire. now for yours, let's start off with year, engine, transmission, 2 or 4 wheel, is it running rough? if so when? anything else you might have noticed?

1977, 454, no emmisions equip, TH400 (non-lockup) with RV shift kit, 2wd, runs smooth, long tube large tube headers with 3" collector, H pipe, otherwise true duals, Dana 70HD, 4.10's, flipped air cleaner lid, 2.675"ish exhaust pipe

It seems to be RPM sensative..it doesn't appear to drink as much under 35 MPH as it does going down the highway (highway speed is 2800 RPM @ 60 MPH) or when warming up (2000 RPM on high idle). But we used to get consistently 10-12 MPG with a trailer on it going 70 down interstates.

[lurch115]

not knowing how many miles are on it...i would investigate more towards the cam....just had to put a new cam in a friend's truck....his gas mileage went to crap so he parked the truck for a couple of weeks....started it back up and it was misfiring out of one cylinder.....after pulling the valve covers we found a flat lobe on the cam....and after pulling the cam found several worn down lobes.....put a Lunati RV2 cam in and now that thing gets good milage and can pull like no other.....another reason why i say it could be cam related is from timing chain wear...valves not opening close enough to the right time will have a big effect on milage