Poor gas mileage

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Jacob

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i have owned my 95 3/4ton 4x4 suburban for about a month now and finally figured up the gas mileage and got a whole whopping 5.5 miles to the gallon. I do let it idle to warm up some most days but not much more than five or ten minutes. I drive through my smallish and never have to sit at a light long. I only drive about 6 or 7 miles with longer trips on the weekends and most of that is at 55. The thing has 4.10 gears with 265/75r16 tires. The plugs and the cap and rotor didn't look very old and I gapped the plugs back to .35 and that seamed to help the got up and go a little. It doesn't have a cat but has the factory y pipe and a new muffler and tail pipe. The the drivers side exhaust manifold is cracked. The evap system is plugged off because the previous owner didn't hook the hose back up to the vent when he replaced the fuel pump. The fuel filter looks recently replaced. It only has 140k miles on it. For compairison my 2011 1/2 ton crew cab 4x4 Silverado got 11ish with e85 doing the same route.

I have a new O2 sensor for it as that looks like the factory one. Has anyone had one get that bad that it drops mileage? Is there something else I am missing? At this rate I should have gotten one with a 454 instead of a 350.

The guy that sold this to me boned me good. It was dark and I didn't have much time to work on it. He covered a lot of stuff up. Good thing I only gave 2200 for it.
 

Ironhead

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Winter temperatures, short trip usage, warming up for up to 10 minutes, lots of stuff not stock (cat, evap system disabled, cracked manifold, etc). Maybe you are getting about right for those conditions, miserable as it is.

I'd try cutting down the warm up time, bringing as much back to stock as possible, driving longer trips, and see what the result is.
 

Jacob

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I wouldn't think the evap plugged off and the cat being gone would hurt it that bad. The only thing I was thinking about the cat and the cracked exhaust manifold is that maybe the O2 sensor isn't getting up to temp in time. I have considered putting in a heated O2 sensor.


I did figure out today that my odometer is losing a mile over 20 miles. So that is some of it. The speedo is spot on. I did do a 150 mile trip today so I will see what that got for mileage but I can already tell it is better.
 

Stevea100m

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I actually had a similar issue. I swapped the cat for a high flow cat and this increased throttle response but still had bad mileage. I took the TBI off to do the TBI mods and would have never guessed it but someone put a crap gasket on and it was sucking air in in between the TBI and the intake. Now it runs smooth and has way more power
 

21Actual

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Thats a pretty short trip (6 miles). PCM will be in open loop mode for around half the drive (depending on idle time, which equates to 0 MPG)
 

Jacob

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I did a 200 mile trip and it got 12.5mpg. I gotta do a 250 mile trip Friday but now have the o2 and manifold replaced so we will see what that gets. I can tell though that running 70 eats the gas though.
 
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