MPG issue

[99ABTJ]

My '99 TJ Sahara, 4.0L, is getting p-poor mileage. I just drove about 45mi (80km) on the first 1/4 tank. This is consistent. A couple of months ago I put on 31 BFG's (no lift) from 30's; recent tune up/oil change, K&N filter. It idles a little rough for a minute on a cold start, but other than that no other issues are noticed.

I had feedback previously regarding the O2 sensors, or maybe the fuel filter being an issue. It has 140k on it, on acceleration the engine has a chattering noise to it - been like that since I've owned it. Is that normal, or an indicator of a performance issue?? Thank's for any idea's...

 

[cheech66]

try the plugs first could be as easy as that

 

[yo momma yj]

chattering on acceleration sounds like low octane gas and pre-ignition.

if so, then those pings are aluminum popping off the top of the piston and will eventually erode a hole in the top of the piston.

do a compression check and look for numbers around 145-ish.

also look in your intake to see if there's carbon at the butterfly...if so then the breather is dumping carbon on the valve seats and low compression can act rough on idle

if so, then you gotta pull the head and do the valves

 

[TerryMason]

How many miles per gallon are you getting? You should track 2 or three tanks and let us know.

I'm a fan of fuelly.com for comparing real world gas mileage:

 

[SteelHeadz]

Moving up to a larger tire would have a bearing on fuel economy compounded by other possible contributing factors on this situation. Fuel filter, dirty t.body,worn sparkplugs, distributor cap and rotor would be my first step in diagnosing a high fuel consumption issue. What gearing ratio do you have will also be a factor not to be ignored for the engine pinging/ chatter on a load(Acceleration ) situation and with that, fuel grade also contributes to this.

 

[riden dirty]

i got the 98 4.0 when shes cold shes rough at first like until 3rd gear if u crank and go. i got the 31 bucks without a lift and typically 200 miles before refueling, so 45 on a quarter is pretty close. roughly 13 mpg

 

[Mooseknuckles]

13-15 is about where I'm at, depending on how much i'm driving on the highway or low traffic. Didn't really see a drop when I went from 30's to 31's.

 

[Ken72]

Definatly check your fluids for unusual discoloration also check the plugs. Other than that you just put a brick higher in the air creating more drag. 13 to 15 MPG is ok.

 

[99ABTJ]

Thank's for the replies. Good to know others have the same numbers. I know MPG shouldn't keep Jeep owners up at night, I've just been wondering if this change I noticed was an indication of a performance issue - maybe, as mentioned once, - it might be the winter gas or additives they put in the fuel here in arctic Edmonton AB.

 

[Bounty__Hunter]

Could be a combination of the increase in tire size and the error in your speedometer throwing off your mileage calculations. You need to correct your speedo after a tire height change for the engine to run its best and to correctly calculate mileage.

Google "Jeep speedometer gear" for a ton of info on correcting it. There's also electronic gadgets that allow you to easily correct a speedo on the fly, mine was $80 from Dakota Digital.