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post #1 of (permalink) Old 10-25-2002, 08:27 AM Thread Starter
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Bad Gas Mileage? Help me out here.

I think I'm getting terrible gas mileage, but I can't really say because I haven't doen a full tank average yet. I drove 50 miles this morning, just a few stop signs, and I burned up 1/4 of a tank, and I don't know what everyone else gets, but it seems horrible. I have the usual crack in the 4.0L Header before the O2 Sensor, which I think may be contributing, but would it really affect it a lot? Any suggestions would help, cause the extra gas money is really starting to cut into my weekend beer funds.
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post #2 of (permalink) Old 10-25-2002, 08:48 AM
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Re: Bad Gas Mileage? Help me out here.

15 gal tank x 1/4 = 3.75 gal
50 miles / 3.75 gal =13.3333 mpg

that is if you have a 15 gal tank.... you get the picture.
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post #3 of (permalink) Old 10-25-2002, 10:44 AM
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Re: Bad Gas Mileage? Help me out here.

13.5mpg isn't all that bad. I get less than that with my 4cyl. And yes a bad O2 and a busted header will not help the cause
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post #4 of (permalink) Old 10-25-2002, 11:23 AM
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Re: Bad Gas Mileage? Help me out here.

I don't know what kind of Jeep you're talking about (assume a TJ), but I just did a complete write up for an exhaust manifold change on a 4.0L ZJ for another board. Most all of it should apply. Gimme your email if you want me to send it to you.
post #5 of (permalink) Old 10-25-2002, 11:31 AM
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Re: Bad Gas Mileage? Help me out here.

I normally get about 230 miles per fill-up. You're in the ballpark with what I'm getting. I'm not crazy about it either, but with lift and oversize tires I didn't help my mileage any. Don't know about the header causing a problem.
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post #6 of (permalink) Old 10-25-2002, 04:54 PM
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Re: Bad Gas Mileage? Help me out here.

A common problem that causes poor gas mileage is the coolant temperature sensor. The injection system uses the coolant temperature for many computations including emulating a choke. If the coolant temp reading is too low it will continue running rich and will not go closed loop. The O2 sensor is ignored in open loop.

Hey, they're cheap. If you are in fact running rich, change it. If you are going to do much work on injection systems, you really need a real time scanner, not just a code scanner. The coolant temp sensor is what caused me trouble. It gave no codes and Precision Tune couldn't find the problem. I bought an Auto-Xray scanner and the first time I hooked it up wondered why it was giving a -38 reading for the coolant temp when ambient was 66. Changed the sensor, which corrected the problem, then demanded a refund from Precision Tune. Got more than half my money back.
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