Mechanical reasons for low MPG's? Serious answers only please.

[chill]

In the past I have done everything in the world possible to effect MPG's with my Jeep. I have had both very good and very bad experiences.

At this point in time I am having a bad experience. My average MPG is about 13. and that would be on a good stretch of normal "county" driving. In town it is not crazy to see 9-10, with best highway at 18.

In the past the combination of intake throttle spacer and computer tweaking has had the overall result of a bit more hp, a bit less mpg. Weight in a larger tire and lifting has added weights well to further decrease mpg's which negate the increase I gained.

All this being said; I have the number 6 plug that IMO does not seem to seat correctly by lack of distance it goes into the motor. Has been this way since purchase, i think someone cross threaded it. Can this effect MPG's a lot? or is this a negligible loss? And how much would lift and tire size increase actually decrease MPG's? These are the only thing that have been changed, really, and what else could be an invisible drag and decreas on MPG's?

I have done it all, back and forth, and twice, to get ANY results, so I don't want a stupid lecture on "look it up", or "jeeps... gas mileage... not why I bought it...", or "The computer resets and there's nothing you can do" I have been a member long enough to have realized this already. I'm looking at what is mechanically wrong potentially that I can address.

 

[cplchris]

it could be perfectly fine but we need more info to really analyze the situation. what do you mean when you say normal "county" driving/ things such as average speed, distance between stops, how you drive (lead foot or granny), the more specifics you can give the better. i have a 2006 sport with about 3.5 or 4" of lift, 245/75/16 at3's on soft 8's and for my daily commute i average 12-13.5 to the gallon on 89 octane. my average speeds are between 35 and 45 with a lot of sitting in traffic, the trip is about 5 miles one way to work, i run synthetic motor oil, keep my tires at the proper pressure, just did new plugs (champion coppers), and drive somewhat conservatively (1/2 to 3/4 throttle max), i keep up on every bit of maintenance i can (transmission, t-case, diff fluids, brake system, intake air filter). so like i said there are a ton of factors, and tire size is definitely a big one for 3 reasons 1) bigger tire=more weight (rotational mass) 2) wider tire=more surface drag on pavement 3) tread pattern (mud tires and most all terrains are not great for daily driving.

edit: i just saw in your sig you have 17" wheels, you could save some weight dropping down to a 16 (but the cost/benefit would make it pointless unless you got a killer deal on them)

 

[dude1116]

Intake and exhaust could be causing issues. On the highway you're not losing all that much MPG (18 isn't bad. I can usually only max about 20 if i'm MOSTLY highway, and 21/2 if I'm legitimately ALL highway).

What kind of plugs are you using. I hope coppers?

 

[CactusJacked]

All this being said; I have the number 6 plug that IMO does not seem to seat correctly by lack of distance it goes into the motor. Has been this way since purchase, i think someone cross threaded it. Can this effect MPG's a lot?

With the spark plug being further up the hole (shrouding the spark) in the head, that cylinder isn't going to perform as efficiently, performance and mpg wise, as the other 5. Can't say with numbers how much of an effect that will have. Plus, with the plug not in all the way, it isn't fully seated (sealed) and you would have cylinder pressure blowing past the threads, again negatively affecting cylinder performance. You really want to have that messed up plug hole taken care of.

 

[tommudd]

So much comes into play here
no mention of what lift or size if any
No mention of what wheels, tire size or how many ply and tread pattern
Type of plugs and brand
The spark plug has already been addressed so that needs taken care of

Also style of driving of course, mine bangs off the rev limiter as often as possible but still get great mileage

WAIT JUST READ THE VERY SMALL HARD TO READ PRINT SO KNOW TIRE SIZE NOW
SO THAT TIRE SIZE IS GOING TO KNOCK YOU BY 3-4 mpg POSSIBLY

 

[chill]

I did remove the intake at the beginning of the warm weather and returned it to stock box. I did realize a 1ish mpg uptick by doing this, so good thinking. As far as county driving, I was trying to convey a type of driving that is some in sub-urban town driving, very little city at all (stop and go), and more often trips around locally at 40-50 mph. This is how driving is in my neck of the woods locally.
Now the plugs are copper core NGK. So gotcha on that one no worries there. My style of driving is careful, say mom driving not granny, because of the mpgs.

You know its funny because to others, say like my wife with a VW TDI @ 48mps. a couple up or down is nothing. For me that's the difference between 18 and 15. Certainly a more tangible spread.

How about gear ratio? Mine are stock because, I couldn't justify a reason to switch them out.

 

[tjkj2002]

How about gear ratio? Mine are stock because, I couldn't justify a reason to switch them out.

4.10's will help out a lot.Cleaning the throttle body and intake would be wise,DO NOT USE SEAFOAM,have it professionally done at a shop.




"Taking it easy" is actually worse for mpg's down the road as carbon build up is more severe.Need to blow the cobwebs out every few weeks,that means 30mins+ of WOT driving,seems backwards but in the long run is better.

 

[cplchris]

4.10's will help out a lot.Cleaning the throttle body and intake would be wise,DO NOT USE SEAFOAM,have it professionally done at a shop.




"Taking it easy" is actually worse for mpg's down the road as carbon build up is more severe.Need to blow the cobwebs out every few weeks,that means 30mins+ of WOT driving,seems backwards but in the long run is better.

or you can just add something like bg cf5 to the tank every month or so...not sure i would say 30 minutes of wot is good for the motor. but i do agree that too much time sitting and idling and giving almost no throttle will cause excessive carbon build up.

 

[tjkj2002]

or you can just add something like bg cf5 to the tank every month or so...not sure i would say 30 minutes of wot is good for the motor. but i do agree that too much time sitting and idling and giving almost no throttle will cause excessive carbon build up.

Only adding a cleaner to the tank does not clean the throttle body or the intake,only cleans the injectors and tops of intake valves.


Don't sit there and hold WOT in park or N,gfo out and drive it with plenty of WOT accelerations while the engine is under a load.

 

[cplchris]

Only adding a cleaner to the tank does not clean the throttle body or the intake,only cleans the injectors and tops of intake valves.


Don't sit there and hold WOT in park or N,gfo out and drive it with plenty of WOT accelerations while the engine is under a load.

cf5 isn't an injector cleaner...it is for preventing carbon build up and breaking up carbon that is already there. i don't see the need for WOT to prevent carbon deposits, 1/2 to 3/4 is more than sufficient acceleration without going over 5,000rpm or bouncing off of the rev limiter.

 

[tjkj2002]

cf5 isn't an injector cleaner...it is for preventing carbon build up and breaking up carbon that is already there. i don't see the need for WOT to prevent carbon deposits, 1/2 to 3/4 is more than sufficient acceleration without going over 5,000rpm or bouncing off of the rev limiter.

Anything put in the tank is just a injector cleaner,and cleans the top of the intake valves.No fuel moves through the intake so nothing added just to the gas tank can help clean the intake or TB.


Oh and nothing wrong with going to or over 5000rpm's under load,even with the 3.7.Will not harm it.

 

[rockymountain]

History shows on this site that the only thing that helps mileage is re-gearing. Any "gimmick" thing has always proven to not do anything or make it worse.
Bad o2 sensors may be a cause too, but those tires are probably to blame and all the other stuff.

 

[uss2defiant]

Don't mean to jump on OP's thread but this is the most current MPG thread.

I just spent the last few hours going through the forum on what I could do to improve my KJ's MPG. (o3 jeep liberty 4x4 3.7L at ~140k) Averaging 13 based of the onboard computer but verifying it, if the wife can remember to print a receipt. (It's her car)

I came across one post that mentioned use of an engine restore. There wasn't any specifics on that product.

I did a quick google and RESTORE came up. It looks like it is used to "repair" cylinder wall to restore the compression amount. Seems pretty positive on Amazon.

My engine does consume oil (also trying to figure out how much it consumes per mile).

I'm going to change out the spark plugs when I install the replacement electric radiator fan.

Would this be something to try or is it snake oil?

Also I saw a few posts recommending techron.

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