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Unread 07-21-2013, 01:32 PM   #1
Charley3
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Join Date: Mar 2013
Location: Longview, WA
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My goal is 25+ mpg in my 99 XJ with 30 x 9.5 R15 ATs, 3.55 diff gears, auto tranny

Sounds impossible, doesn't it? I think it is possible.

When I bought my XJ, it was getting 15 mpg highway, even though my XJ showed no codes and ran reasonably smooth.

Plugs, cap, rotor, and plug wires needed replacing. Did those things. Those things got me from 14 to 16 mpg highway. A little improvement, but not much. I was disappointed.
Edited in Later: I wish I had used the Kolak tune-up kit aka ignition upgrade kit for Jeep 4L engines. I think it sounds excellent and would be a great set of tune-up parts that would help gas mileage and power for sure, and might even help more than other brands of tune-up parts would. See post 1032 and 1993 for more info about Kolak tune-up/ignition kit and where to buy.

Edited in Later: I bought Kolak tuneup/ignition kit. It's excellent. Gap plugs to 0.045.
Then Motorvac, new oxygen sensor, and 205F Stant thermostat. Those 3 changes were done on same day. So I don't know how much each thing helped, but the combination improved my gas mileage substantially.
Motorvac is a top-end engine cleaning treatment done at mechanic shops where they use a Motorvac machine/pump to pump (under pressure) Motorvac cleaner through top-end of engine to clean out carbon deposits. Works much better than Seafoam.

After Motorvac cleaning, change motor oil (just in case some of cleaner got into bottom-end of engine and contaminated oil).
This is why I waited until my motor oil was due to be changed anyway, then did the Motorvac, then changed motor oil.
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On an LJ I used to have, it gained 1 to 2 mpg from 205F thermostat. So I think that's a good indicator of the 205F stat on a 4L engine (in my moderate climate).

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Edited in Later: As discussed later in this thread:
See post 1125. for more info about thermostats, including which I recommend for each climate, brands, part numbers, where to buy from, etc.
===

A 205F stat is helpful in moderate climates and cold climates because it allows engine to run at (or near) full 210F op temp year round (instead of under-heating at 185F to 195F). IME, this is a big help in a Northern climate and does improve gas mileage noticeably. Over the last 20 years, I've done this on 3 Jeeps and 2 cars in a climate that varies from moderate to cold. It also works fine here in Summer, but we usually have 70s to 80s days in Summer, and sometimes low 90s. Very occasionally it hits 100F here in Summer. I find the 205F stat helps my a little in on cooler Summer days, more help in Spring and Fall, and a great help in Winter. On our occasional hot Summer days it is no help, but doesn't hurt anything.

If you live in a frequently hot climate your engine is already at, or near, full op temp with stock 195F stat. If you live in a hot climate, a 205F stat offers you no benefit, and might even be a detriment. If you're in a hot climate, use stock Mopar 195F stat or upgrade to Stant 195F Superstat. They open at stated 195F temp. If your stat is to hot for your climate, your fans work harder, which reduces gas mileage. It takes power to turn a mechanical fan, especially when fan clutch engaged. It also takes power to turn an electric fan. Turning fans creates parasitic drag on engine.
The ideal situation is to keep engine as hot as possible without triggering the fans. 208F to 213F is ideal for this since the fans trigger around 215F. The ideal temp (for Jeep 4L engines) is 210F. To accomplish this, people in cooler climates need a 205F stat, while people hotter climates (regularly 95F+) need a 195F stat.
My XJ (with 205F stat) runs 208F most of the year. On exceptionally hot days my XJ runs 210F. My fans trigger at 215F. So my fans seldom come on, except when off road on hot Summer days. Keep in mind I am in a Northwest USA climate that is cool most of the year, and a hot Summer day here is 90F to 95F at most, except for the occasional 100F day.

When it occasionally gets above 95F here, my 205F stat is with regard to protecting my engine from overheating because my stock fans never let it get above 215F (damage doesn't occur until above 230F). However, my gas mileage suffers when fans run all the time on those occasional 95F+ days, but in this area we only get 0 to 5 days a year that are 95F or hotter.
Replacing mechanical fan with electric fan is a great efficiency improvement mod in a moderate to cold climate (because fan seldom needed and seldom comes on), but offers no efficiency benefit in a hot climate (where fans have to run often).
The downside of converting the mechanical fan to electric is it's expensive to hire it done, and complicated to do yourself. The biggest complications are getting it to fit, wiring it properly, and figuring out an appropriate and reliable temperature sensor to automatically trigger it. For these reasons, I might not do this mod, and I recommend caution to others considering this mod. A lot of people have difficulty getting this mod done and ongoing difficulty getting it to function properly.

So although replacing mechanical fan with electric is good in theory, in actual practice it's not easy to get working properly.
===

Recently my cousin installed an AFE "Cold" (actually slightly cooler air, high volume) Air Intake and I have noticed more power and gas mileage increased 1 mpg. It's a great high flow air intake, but not it's not true cold air. It's warm air drawn from the same place as the stock air intake. However, high flow warm air is good for gas mileage in a moderate climate, or a climate with seasons. I bought the dry filter for it because I don't like oiled air filters (I fear oiled filters might dirty sensors). See post 151 for more info on types of air intakes.
This 1 mpg improvement is compared to a K&N air filter in stock air box. So that'd be a 2 mpg improvement compared to a stock filter in stock box. IMO.
I've been reading about guys installing modern 4 hole fuel injectors from Dodge, Ford, Bosch, and others. I want to try some of those more efficient injectors. See post 210 for injectors explanation and part numbers.

Edited in Later: I'll get an efficient aftermarket cat back exhaust in future. It will help power and might help gas mileage. There are some good cat back systems available for very little cost. The Dynomax cat back is reputedly good breathing, easy to install, quality product, low cost, and quiet. I'm undecided whether to get the 2.25" or 2.5" Dynomax cat back, but I want one of those. Cost $107 to $149 with free shipping.
Edited in Later (after gaining more experience): A 2.25 cat back will improve gas mileage and performance. A 2.5 cat back will reduce gas mileage and low rpm torque. The only thing 2.5 cat back helps is high rpm horse power. Do get a 2.25 cat back. Do NOT get a 2.5 cat back.
I recommend Dynomax 2.25 cat back. At $150 incl shipping from SummitRacing.com it's a great price and easy direct bolt in.
Edited in Later: Best prices I've seen for Dynomax cat backs are at SummitRacing.com and AdvanceAutoParts.com.

Edited in Later: The stock downpipe between header/manifold and cat has a dent in it that restricts exhaust. I plan to replace with a new down pipe without the dent. This cheap mod will help power and possibly gas mileage when a high flow cat back exhaust is installed. Make sure to tell your exhaust shop that stock routing for dent free downpipe won't clear the driver side UCA during off road articulation. They need to figure out a new routing that will clear the UCA and they need to locate the oxygen sensor where it won't get hit by front differential yoke during suspension uptravel. I learned these things the hard way. Ask Cruiser54 for more info about routing.

Edited in Later: I want to make or buy an adjustable grill cover for the colder months. Like the colder 6 months of the year. Adjustable grill covers help gas mileage 2 ways. They help engine reach full op temp in cooler weather, and they improve aerodynamics by keeping unnecessary air from entering engine compartment and creating drag.

===

It's possible to get 25+ mpg driving 65 mph. I'm working towards it.

Does anyone have other gas mileage suggestions?

===

Edited in Later: I am using all synthetic oils in my XJ. This is mostly to protect engine and auto tranny from heat and wear, but also to increase gas mileage. All synthetic oils in all areas increases gas mileage an estimated 1 mpg, and reduces wear substantially.

0-40 motor oil is excellent/ideal in 4L Jeep engines for use year round in all climates.
Mobil One 0-40 engine oil is excellent. Mobil One 0-40 is what I'm using (although I'd be just as happy with Lucas 0-40).
Lucas 0-40 is excellent
Valvoline 0-40 is excellent (maybe even the best, widely used in Europe, but very difficult to find in USA)

Synthetic auto transmission oil in my auto transmission, and 242 transfer case.
Mobil One and Valvoline synthetic auto transmission fluids are great.
231 transfer case uses 75-90 or 90 gear oil. I recommend Mobil One or Valvoline synthetic gear oils.

Mobil One and Valvoline synthetic gear oils are great for differentials and for a 231 transfer case, BUT do NOT use these gear oils in a manual transmission because their limited slip additives makes them to slippery for syncromesh shifting to work properly.

Do NOT use any brand of gear oil with limited slip additives in a manual transmission.

In a manual transmission, use gear oil that does NOT contain a limited slip additive.
75-90 synthetic gear oil front diff (I recommend Mobil One or Valvoline)

75-90 or 75-140 synthetic gear oil rear diff (choose viscosity according to climate and severity of use (towing requires 75-140 or 80-140)) (I recommend Mobil One or Valvoline)

I like Valvoline best in 75-90, and Mobil One best in 75-140 based looking at their spec sheets and their viscosities cols vs hot, and their flash points.

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Edited in Later: I have perfect alignment and perfectly balanced tires, which helps gas mileage.

My tire pressure is on the low end of acceptable to soften ride for my bad back. This is NOT helping my gas mileage. I am going to install softer springs to soften ride. Then I will start running more tire pressure, which will gain me at least another 1 or 2 mpg and put me at, or very near, 25 mpg.

I've already increased to 23 mpg from fine tuning and a few of the mods mentioned above. I still have many more mods to do. l'm sure I will get substantially more than 25 mpg when I am finished.

Edited in Later: My XJ has auto tranny, 3.55 diff gears, and 30x9.5R15 AT tires. Later in thread a member with same setup regeared to 3.73 diff gears and his gas mileage improved.

Edited in Later: Later in thread it was pointed out that my auto tranny puts me at some disadvantage for gas mileage. The manual tranny XJ have a built in advanatage for gas mileage.

I will get 25+ mpg with my auto tranny, but if I had a manual tranny I could do even better. Manual tranny 4L XJs are known to get 24 mpg highway when completely stock. So with some economods a manual tranny XJ could potentially get 27+ mpg at 60 mph.

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Unread 07-21-2013, 02:39 PM   #2
98muddyjeep
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yea i have heard about people using the 4 hole injectors from a neon to replace the stock 1 hole injector,don't know how much benefit there is, i guess there should be some as the spray will be finer and burn better than a drip.
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Unread 07-21-2013, 03:21 PM   #3
JMH21814
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You've piqued my interest. I'll follow this post with anticipation. Good luck!
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Unread 07-21-2013, 03:38 PM   #4
TheBlueDevil
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you should certainly look into a bored throttle body, not a straight through muffler but one with a little more free flow, ditch anything that sticks out on top of your roof (roof racks), certainly run dual elec. fans, a new coil (accel makes a great one) and definitely do the injector upgrade.
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Unread 07-21-2013, 03:42 PM   #5
Newtons3
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Instead of CAI you might look at hot air. This is the way the hypermilers get the computer to lean out the fuel mix even more. I dont find the 4 hole injectors are worth anything over and above quality precision single hole. Look at the flow balance between them instead and get it as close as possible. mine are within 0.3 percent of each other. Wrap your exhaust front to rear. Swap cams to a tamer grind (under .375 lift and around 180 duration at 0.050). Couple that with a 50-52mm throttle body. Swap to the late-model head with raised exhaust ports. Start closing off and smoothing the front end. Reduce the grill opening as much as possible. Belly pan to direct air helps. Run as light as possible.
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Unread 07-21-2013, 03:53 PM   #6
mschi772
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Go to EcoModder.com and look-up FastPlastic he/she has done some nice aerodynamics work with his/her XJ in addition to other MPG mods. FastPlastic has also discussed some very promising ideas that he/she has not yet implemented. You'll have to do quite a lot of work to get 25+ MPG, and you'll find that a WARM air intake will be more beneficial in this regard than cold.

With grill covers, stock to small lift with lightweight belly pan for reduced undercarriage turbulence, keep the air dam, warm air intake, no armor (keep the vehicle as light as possible, roof rack delete, electric fans, quality tune-up, quality O2 sensors, stock-sized tires, no AC (even better if you have the guts to remove the AC compressor completely)... You can hope for MAYBE 25 mpg highway. I'm using 4-hole injectors, but I wouldn't call them an MPG booster.

The 4.0 can be improved a little bit, but it's not the most efficient engine, and there isn't a ton of room for improvement. The AW4 has goofy shift points that certainly don't help either, but not much can be done there other than to swap-in an AX15 manual or something. Using a 2wd XJ would also help quite a bit. The XJ's aerodynamics are the real enemy, and where the biggest gains can be had if you have the fab skills.
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Unread 07-21-2013, 04:20 PM   #7
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The TransGo kit can help with the AW4 shifting. Lose the passenger side mirror and radio antenna. Push the wheels out where the tires are flush with the fenders. Flat wheels help. Run minimal fender clearance. Pull the front driveshaft when not needed. Trade out your 300 cpi cat for the shortest one you can find with a cell count around 100 cpi. Vent the hood (especially across the rear) AND vent the fenderwells into the engine compartment. Isolate the timing (via Msd) and play with advance. Drop pulley ratio on alternator and P/S. Mod the MAP- I like the Apexi controller. This is less beneficial on OBD-II systems.
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Unread 07-21-2013, 08:22 PM   #8
Charley3
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I did remove rook rack and running boards.

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both gas and fuel economy.

I'm also considering having intake and exhauat ports ported and polished, and high flow headers (insulated with ceramic paint) and exhaust.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a V8.
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Unread 07-21-2013, 08:39 PM   #9
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The headers wont help either significantly- you already have tubular exhaust. The head work helps a lot but to realize power gains you have to change cams, and there went your mileage. In other words, theres no free lunch. You can do one or the other. But 25mpg on 30s in that 4wd while increasing performance is an unrealistic expectation. You cant have your cake and eat it too.
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Unread 07-21-2013, 08:40 PM   #10
Charley3
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I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both fuel economy and power.

I'm considering having intake and exhauat ports ported and polished, and JBA high flow headers (insulated with ceramic paint) and high flow exhaust. I think that would gain some more fuel economy and power.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a 300 cu or 5L V8 would be quiet and high flow on a 4L.

I'm using mid grade gas. That was the case before and after the mods I've done so far.

I think I'm going to have the fuel map sensor cleaned or replaced next.

What about EGR valve? Do 99 4L have that? Would it affect fuel economy?
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Unread 07-21-2013, 08:56 PM   #11
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Quote:
Originally Posted by Charley3 View Post
I did remove rook rack and running boards.

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both fuel economy and power.

I'm considering having intake and exhauat ports ported and polished, and JBA high flow headers (insulated with ceramic paint) and high flow exhaust. I think that would gain some more fuel economy and power.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a 300 cu or 5L V8 would be quiet and high flow on a 4L.

I'm using mid grade gas. That was the case before and after the mods I've done so far.

I think I'm going to have the fuel map sensor cleaned or replaced next.

What about EGR valve? Do 99 4L have that? Would it affect fuel economy?
A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.

There is no EGR on your 4.0l.

What on your map sensor are you going to clean? Until the much later years in the Grands and Wranglers where they were mounted on the throttle body, the map sensors sit on the firewall and gets manifold signal via a hose. They generally don't get dirty as long as the tiny reference hole isn't plugged.

Mid grade gas, unless need to counteract preignition or detonation is not beneficial. Within the same type, style, brand etc., lower octane fuel has more btu energy and will achieve more horsepower and mileage than a higher octane rating.
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Unread 07-21-2013, 09:13 PM   #12
mschi772
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Indeed, 87 octane is all you need. Save your money.
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Unread 07-21-2013, 09:15 PM   #13
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It's a Jeep......if you want good mileage get a Dart.
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Unread 07-21-2013, 09:24 PM   #14
Charley3
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I just got home from a road trip.

I gained another 1 mpg from high cooler flow air intake, and noticeably more power on hills.

So now 22 mpg highway. Pretty good for 30 x 9.5 R15 all terrain tires.

I will switch to 87 octane gas and see how it does. Could be a real economy boost considering cheaper per gallon.

Thanks for suggestions.
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Unread 07-21-2013, 09:26 PM   #15
Charley3
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Quote:
Originally Posted by Newtons3 View Post
A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.
I meant a muffler from a V8. That'd be high flow on a 4L wouldn't hit?
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