Fuel Economy and Crate 5.7 350

JStorry · October 17, 2013

So I had a Crate 5.7 installed earlier this month. I know there is a break in period, but I am wondering how long that takes. I'm currently getting less than 11MPG

I thought I had done everything right... I replaced quite a bit, but wonder if I should have replaced the Intake Manifold. Or maybe poor fuel economy is normal after a crate?

Any ideas?

I replaced the rad, oil cooler lines, water pump, spark plugs and wires, distributer (came with cap and rotor),block heater, and even did the engine mounts while it was out. I don't think any of that relates to fuel economy though?

I do have a fairly new exhaust. I put a gibson on it in July, removed a cat and put an O2 simulator in its place so I don't think it's related to the exhaust.

Thanks in advance

LongrangeLauber · October 17, 2013

My buddy had a 350 in his 99 Sierra as well, and I thought he was messin with me when he said the best he got even on highway was 8mpg. I thought no way so we hand calculated it. Sure enough 7-8 mpg's. Your lucking out at 10-11!!! Fuel economy wasnt an issue until the NBS trucks (like mine) and maybe even later in the 2000's

Chris · October 17, 2013

11 is good for a 350

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KMGZ400 · October 17, 2013

sounds about spot on for MPG...look like in your picture your truck is lifted with bigger tires... thats not helping either

JStorry · October 17, 2013

Wow. Before the swap I was complaining about 15MPG, and that's with the lift and bigger tires. Before the lift and bigger tires it was getting 18-19

Thank you

MikeNH · October 17, 2013

Wow. Before the swap I was complaining about 15MPG, and that's with the lift and bigger tires. Before the lift and bigger tires it was getting 18-19

Thank you

What was under the hood before?

JStorry · October 17, 2013

Stock 350, had over 340,000 KM on it.

kstruckcountry · October 18, 2013

On my old 350, I worried more about the oil mileage than the fuel mileage.

I'd expect it to get a little better over time.

6.2max · October 18, 2013

Wow. Before the swap I was complaining about 15MPG, and that's with the lift and bigger tires. Before the lift and bigger tires it was getting 18-19

Thank you

Check your front 02 sensors. Maybe need replacement.

&CDFiXER% · October 19, 2013

Maybe time to think about a tune? Perhaps the stock ECM isn't liking something about the new setup? I bet if BB hasn't had a go at it, there is room for improvement.

The O2 simulator wouldn't be a factor would it?

LongrangeLauber · October 19, 2013

Maybe time to think about a tune? Perhaps the stock ECM isn't liking something about the new setup? I bet if BB hasn't had a go at it, there is room for improvement.

The O2 simulator wouldn't be a factor would it?

They can be a big factor. Tells the ECM if the motor is running too rich or too lean. Not sure how big of an mpg difference but sure does play a role

Badass69 · October 19, 2013

If its a front one sure. But its a rear which realistically do absolutely nothing useful. Just monitors the efficiency of the cats. Break in on a roller cammed engine is quick and dirty.... so your mpg now will be what it is. Check like maf, map sensor,fuel pressure etc. Stock spider injectors are junk too... Lots of variables

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LongrangeLauber · October 20, 2013

Its an old style 350. Even if you put a new manifold and injectors I doubt you will see a noticeable difference in mileage. It is what it is

Badass69 · October 20, 2013

Its an old style 350. Even if you put a new manifold and injectors I doubt you will see a noticeable difference in mileage. It is what it is

Poppet style injectors on those are notoriously problematic. Theymade a new spider assembly with actual injectors to fix it. If he dropped in a new stock crate and it gets 4 mpg less or worse than the original something is amiss. I doubt the injectors are the issue honestly but a pressure regulator certainly could be....

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