New Motor = Terrible Fuel mileage

[RedfishBluegrass]

I have a 1993 pickup, 31x10.50 BFG muds, new motor (everything but the block). Fuel Filter, plugs, wires, cap, rotor, air filter, radiator, alternator, clutch, muffler, and bearings all new. The motor has approximately 300 miles since the rebuild and I am getting about 15.5mpg. Pulled some codes; knock sensor and intake air temp on the AFM. Checked resistance on the AFM and found the temp part to be off but not by much. Engine idles at 850 - 900 but has a definite miss at idle. Timing is right on at 5 deg BTDC. Checked the ground on the plenum and tried to search as much as possible. Any ideas or things to try? Thanks.

 

[missouriman]

well my big time rebuild. the only one that I have ever done, the power and mileage didn't come in untill 4k miles.

but mine never missed. it ran good, but low power untill it broke in..

I can give you my advice on break in if you want. but it is only advice, nothing more. if it dosn't work or breaks I am not resposible.
just worked very well for me.
if I were to rebuild again I would break it in alot different then I did. I think you would have power and milage much sooner.

maybe I will start a new post and take the flameing that I am sure will follow. watch for it..

 

[GRM]

What milage were you getting before? I'm getting 16-17.5 out of my 22re/5 speed truck, and thought it was pretty common with 31's and 4.10's.

 

[RedfishBluegrass]

missouriman, advice on the break in would be great. I feel like I actually have plenty of power, more than my 3.0 that I had previously. Maybe it will just take some time to get worked out. The truck was not mine while it was running before, i bought it broken down. The PO said he was getting around 20 mpg, with the same tires. Searching on here it seems it should be a bit higher, but maybe that is what it should be.

 

[Jukelemon]

No, it should not be higher than 20 with 31's. I get 20-22 avg on stock rubber. When I ran 31's, it sunk into the high teens.

As well, it depends on how you drive.

It is well researched that newer engines do not get the same fuel economy-look at long term vehicle testing reports. Should increase 1-2 mpg's at maybe 5k-7k miles.

Then you would be right at avg I think for 31's.

 

[RedfishBluegrass]

No, it should not be higher than 20 with 31's. I get 20-22 avg on stock rubber. When I ran 31's, it sunk into the high teens.

As well, it depends on how you drive.

It is well researched that newer engines do not get the same fuel economy-look at long term vehicle testing reports. Should increase 1-2 mpg's at maybe 5k-7k miles.

Then you would be right at avg I think for 31's.

Thanks for the advice, might be switching up to stock tires!

 

[WarDamnEagle]

missouriman, advice on the break in would be great. I feel like I actually have plenty of power, more than my 3.0 that I had previously. Maybe it will just take some time to get worked out. The truck was not mine while it was running before, i bought it broken down. The PO said he was getting around 20 mpg, with the same tires. Searching on here it seems it should be a bit higher, but maybe that is what it should be.

Are you adjusting the odometer mileage to account for the larger tires before you do your mpg calculation? With larger tires your odometer will show less miles than you actually traveled (assuming you haven't regeared your diffs). The best way to check the mileage would be to use the mileage driven as shown by a GPS.

 

[RedfishBluegrass]

I did not adjust for the tires! How much could that actually help?

 

[WarDamnEagle]

I did not adjust for the tires! How much could that actually help?

5% to 10% depending on exact tire sizes. It's just the ratio of the diameters but I don't know what tire size came on your truck when it was stock. Again, the best way to check your mileage is to use a GPS.