Low, LOW mpg

[Captkaos]

Actually the 4.3L/TH400 was common in the later years as the TH350 wasn't offered, and if you didn't want a overdrive trans you could get the TH400 with the 4.3 and the 5.0L.

[bryanedp9]

I try to be pretty easy on the pedal, and it does like I'd imagine it should city driving, but I put cruise on at 65 usually on the commute. That's when it starts to tank. Picked up a bike today so hopefully I won't be in a rush to get it all taken care of.

[bryanedp9]

Okay, at about 40mph in 2nd it was begging for another gear, so I went to D and third came right in. Can I discard trans problems now and go straight to fuel/exhaust or could it still be a gearing thing?

[bryanedp9]

Okay, I have a couple ideas as to low mpg so far, so chime in with input. I haven't yet had a look at the intake, but this is what I know:

1) What I think is the charcoal canister has rotted lines. I'd imagine that's setting me back. If so, what size hose do I need?

2) Tank selector valve is either burnt out or has a wiring problem. I've only been able to use the right side tank since taking ownership. New selector switch did nothing to help. This might relate to...

3) I think my fuel pump is going. After coasting downhill, it might sputter before the uphill climb and sometimes doesn't want to start at all unless I tap the framerail in that area.

So any advice you have pertaining to this any other causes of low mileage, please let me know. It's a family vehicle but the money for pumps and an exhaust and whatnot might put me into sell mode instead.

[87454westernhauler]

fom what u are saying and from my personal exsperiances with TBI systems, i would say that your biggest problem is not having a muffler for back pressure. My dads 87 454 was getting about almost 16-17 mpg and about a month ago the mufflers fell off and it is lucky if it get 10mpg. what i would do if u are trying to go cheep is stick a cheep muffler on it. how much of the exaust is left on it?

[bryanedp9]

fom what u are saying and from my personal exsperiances with TBI systems, i would say that your biggest problem is not having a muffler for back pressure. My dads 87 454 was getting about almost 16-17 mpg and about a month ago the mufflers fell off and it is lucky if it get 10mpg. what i would do if u are trying to go cheep is stick a cheep muffler on it. how much of the exaust is left on it?

It has one of those flexpipe deals from the headers. Someone (probably Pa) used bailing wire and a U-clamp to keep the muffler on so I lost it.

[Irish_Alley]

most states are ok if you have the exhaust run past the cab so put some straight pipe in then the muffler. unless you want to run the whole thing where you will see the most back pressure is a cat. but its illegal to use used cats from another vehicle so i think you would have to buy them new

[bryanedp9]

What about dual glasspacks behind the cab? Would that give reasonable backpressure?

[87454westernhauler]

it should, would be better than nothing. that is what i had on our 87 454 before they rusted out and fell off. turbo muffler are cheaper and quieter. 

[Irish_Alley]

glasspacks do give you something but not not a muffler will do. i love load trucks but wouldnt want to hear it everyday

[bryanedp9]

Alright, so backpressure is needed and a clean intake. What about the weak fuel pump?

[87454westernhauler]

sound like u have a loose ground if u have to wiggle or tap tank.

[zieg85]

Anything that is a problem to how the truck is supposed to run normally is going to affect it.  If you have to tap/hit/beat the side of the frame to wake up your fuel pump than it is time to replace it.  Could be a bad ground but more than likely it is in need of replacement.  I used to know of a guy who had a 4.3L with a 5 speed in a full sized 1990 extended cab C1500 and he was around 22mpg.  My $.02

[87454westernhauler]

yah. now that i think about it, it could be a bad fuel pump.

[bryanedp9]

Ugh, I need a job guys. Or better bike weather.

Should I drop the tank or lift the bed for a pump replacement?