2005 KJ 3.7 Rene with poor MPG

[Malicesin]

Before you type in get a car if you are concerned about MPG hear me out.

When I first got my KJ I was getting about 18-21 MPG and that was fantastic for me. Lately, I've been getting about 13-15 MPG. I have always done the recommended maintenance items like diff fluid, t-case fluid, ****** fluid, coolant, oil changes, PCV valve, spark plugs, air filters, ect. Most of those items have been changed in the past 15,000 miles and since this poor MPG, I went ahead and changed ignition coils and new spark plugs too (NGK factory .44 as my emissions label says in the engine area).

I do use a a K&N filter that has been on the jeep now for like 4 years and I take it out and beat it on the drive way to knock out the dust but I have not bought their recharge kit as it's never really gotten that dirty. Other than that, what other items should I clean or replace to get back to the MPG I used to have? I've read some posts about cleaning IAT, MAP and throttle body but no one has reported back if that actually increased their mileage or not. Any help would be greatly appreciated.

I also always use 89 Octane since i'm in the desert where its 100F+ most of the year

 

[RenKJ]

same oiled filter for 4 years without cleaning it? Did you wheel with that?

 

[Damotee]

Id start with a new air filter. Regarding the fuel, in Australia we generally have 4 types of petrol; E10 (contains 10% Ethanol and is the cheapest), 91 Octane, 95 Octane and 98 Octane. Where I live the best i can get is 95 Octane so thats what i normally use, but when i go to the big smoke for supplies I always put 98 Octane in it and I get ALOT more mileage. Maybe try that if you have access to 98?

Edit; also O2 sensors?

 

[Malicesin]

same oiled filter for 4 years without cleaning it? Did you wheel with that?

I've gone wheeling but every oil change I always take the filter out and it's just as red as the day I got it and I always smack it on the driveway to knock out the debris.

Id start with a new air filter. Regarding the fuel, in Australia we generally have 4 types of petrol; E10 (contains 10% Ethanol and is the cheapest), 91 Octane, 95 Octane and 98 Octane. Where I live the best i can get is 95 Octane so thats what i normally use, but when i go to the big smoke for supplies I always put 98 Octane in it and I get ALOT more mileage. Maybe try that if you have access to 98?

Edit; also O2 sensors?

Well, the highest octane we have here in Arizona USA is only 91 octane. Also, our owner's manual states to only use 87 Octane unless certain conditions exist like hot weather or towing.
https://i.imgur.com/Qja2DR7.png

 

[tjkj2002]

Trash that K&N,might be to late though as it has already damaged your engine internals.

They do not filter worth a damn and has been widely known for 15+ years.They are for racing only on pavement.

 

[Damotee]

Well, the highest octane we have here in Arizona USA is only 91 octane. Also, our owner's manual states to only use 87 Octane unless certain conditions exist like hot weather or towing.
https://i.imgur.com/Qja2DR7.png

Hmm interesting. I just know i get better fuel economy using the richer fuel. My Jeep carries a bit of extra weight though (bullbar, winch, skid plates, lift, roof rack etc) so maybe its good for heavier vehicles.

 

[Malicesin]

Trash that K&N,might be to late though as it has already damaged your engine internals.

They do not filter worth a damn and has been widely known for 15+ years.They are for racing only on pavement.

Glad to see you responded in my thread as I always look for your post when I'm looking for information. I'll throw it away and go back to using a paper filter. Really think my internals could be bad? My oil doesn't even get black before I change it.

 

[RenKJ]

I always take the filter out and it's just as red as the day I got it and I always smack it on the driveway to knock out the debris.
https://i.imgur.com/Qja2DR7.png

if the fine debris dont get trapped by the filter i guess that's one way to keep a clean filter

you may want to switch to using a paper filter like WIX when wheeling if you plan to keep that engine

 

[tjkj2002]

Glad to see you responded in my thread as I always look for your post when I'm looking for information. I'll throw it away and go back to using a paper filter. Really think my internals could be bad? My oil doesn't even get black before I change it.

Could be,without a teardown you would not know.

Just use a good paper filter.

Proper fluids(correct weights for engine oil,diffs,and such) is key,worn wheel bearings,sticking brakes,bad u-joints,tires and there pressure,and a few others things can and will effect MPG's,sometimes drastically.Tires will drastically effect mpg's also(bigger,heavier,and such).

High heat will reduce mpg's,this summer has been starting out so much hotter than years before.

 

[Tog]

Hmm interesting. I just know i get better fuel economy using the richer fuel. My Jeep carries a bit of extra weight though (bullbar, winch, skid plates, lift, roof rack etc) so maybe its good for heavier vehicles.

I'm with you Damotee I only run 98 and it gives the best mileage.

2004 3.7 limited

 

[JasonJ]

I'm with you Damotee I only run 98 and it gives the best mileage.

2004 3.7 limited

Probably in AU, I'm not surprised. But guys, don't go automatically assuming that higher octane rating means "better" or "premium quality" fuel.. it simply means that it resists pre-ignition (spark knock) better than a lower octane rated fuel.

Of course, in high heat, or in Damotee's case, with a heavy load, that is what you need/want.

For normal driving, not towing, hauling, weighed down with 1000lbs of extra "stuff", high or low altitude, etc... rarely a need to vary from the recommended 87-89 octane unleaded fuel.

Higher octane fuel normally has less BTU's of energy as well, so often drivers will find they get worse mileage or engine performance with it unless engine compression or air density is increased.

To the OP, trash that K&N, it's not good for on-road use, track and strip only. They filter hardly at all. Best is an OE style paper filter. It will still flow more cfm of air than your engine needs or is physically capable of pumping.

The 3.7 in the KJ barely breaks 400cfm of air flow through it at 6000 rpm, not counting exhaust restrictions or throttle plates getting in the way... a paper filter will do about that and upwards of 500cfm depending on filter. Plenty fine.