project mpg resurrection

alright my 99 automatic 4.0 xj is getting about 11 mpg. i know my 31 dont help out much but i dont think they are the biggest factor right now, so i think ive narrowed it down to exhaust. my hexhaust manifold is cracked and my cat is rattling.

anyone have any experience with these headers? mine has a nice crack in it and ive tried thermosteel twice and still has yet to fix the problem

http://cgi.ebay.com/ebaymotors/JEEP-EXHAUST-MANIFOLD-HEADER-cherokee-wrangler-comanche_W0QQitemZ200162508766QQihZ010QQcategoryZ33632QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

cheap but i want something to last, i know everyone is have great luck with apn but these would save some more money for this cat

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=320168249215&ssPageName=STRK:MEWA:IT&ih=011

another cheapy but would i have to weld a bung for o2 sensor? or can i cut off that part of the pipe, saving the bung and exhaust clamp it back on the cat and rest of piping..??

also thinking about replacing the muffler with just a cheapy from autozone,
http://www.autozone.com/R,APP1431919/vehicleId,2548302/initialAction,partProductDetail/store,369/partType,01917/shopping/partProductDetail.htm
that link may not work but its the cherry bomb turbo muffler
or would the dynomax cat back help more or do i really even need the tail pipe to improve mpg?
also can i just exhaust clamp all this stuff together, i have acess to a mig welder but would rather clamp it instead.
and one more would moving and modifying the intake to the other side of the engine bay away from the exhaust manifold lead any benfit? kind of like the wranglers, maybe move battery to other side... thanks :wave1:

Did you make sure to compensate the mileage for the fact that you are running larger tires? should make a small difference.

yea i figured about 10% error, dont know if thats right

Well Im getting around 13/14ish city. I dont know yet the highway milage.
1998 Sport 4x4 235/75/15. It had a full tune up almost 1 1/2 yeas ago b4 I got it.

I get 12-13 in town with 33's and 3.55's also have cracked manifold(very small leak) and a rattling cat.... thats going by the odo.

off topic; sjbond67 were in mo are you?

I get 11-12mpg city on 30's.

My Jeep:
2000 XJ Cassic, 4.0HO, AUTO, NP231, D30LP/D35c (both open) 3:55, 31X10.5X15 on stock wrangler steel rims, removed plastics on front bumper and rear sway bar, High Lift jack and two KCs mounted on front stock bumper, 4.25" now 4 inch homebrewed lift, and a few tweaks under the hood including some sensor tricks and a K&N filter for street, cheap stock paper filter for the trail.

I once had 13-15 mpg city, 17-24 mpg hwy before mods, pure stock .

I then had 9-11 city, 12-14 hwy After getting the 31s from stock 26s, and all the suspension mods, and removal of aerodynamic plastic "rock hanger" front bumper pieces and spare on the roof, and all the rest of the outside stuff.

I now get 13-15 city, 16-18 hwy on 31s, after I bored out my throttle body and moved my spare tire from the roof to inside and some sensor tweaking. I'm not done yet, but should get almost 20mpg when I finish with the electric fan conversion and other things on the list to help mpg, including a gear change to ?:?? (not sure yet of what size tires I ultimately want to end the build with and with what axle upgrades).

I did an efan conversion and didn't notice a change. Of course it isn't winter yet, when the fan would be locked up and keep the engine cooler until it warmed up which would take longer due to the locked up and freezing fan. We'll see.

I did the e-fan conversion this summer, at the same time I put new wires and plugs... so with that added variable its hard to say if the added mpg were from the e-fan alone. But I now get an extra 50km (35miles?) to a tank of gas.
I was getting 375km per tank, and now see over 400km everytime, usually around 425km. I didn't think there was anything wrong with my old clutch fan, but this added mpg has made me think it was always on, causing that added drag on the engine.

Nice. I get less miles to a tank now due to the extra dents it has gotten over the past few months. If/when I replace the tank, it'll get a skid.

Do you have the correct threromastat? Anything less than the specified 195* will trick the cpu and make it stay in warm up mode and run richer thus using more fuel.

im from nixa, mo... you know, where jason bourne is from hahaha

i have replaced the tstat, im pretty sure i replaced it with 195* but who knows what i got from those oreilly guys, i might replace it again to see.

im also thinking about doing the e fan conversion. but first i think i need to address the exhaust. im getting to where i want to stick some 33s on. next ill probably get the bored throttle body.

I have the Cherrybomb turbo muffler from Autozone on mine, and it is a nice sound. If you are looking for a loud performance sound, it is not the muffler for you though.

I have that manifold, but it didn't have the girl in the ad when I bought it. Its been on about 18 months and maybe 5000 miles and it hasn't cracked so far. I picked up a cheap cat from Summit racing for about that same price, it wasn't a magnaflow one though. And I got a flowmaster 40 series on ebay for $60 shipped. I currently get 16mpg city and 20 mpg highway with a 4.7 stroker, 6" lift, 32" tires and 4.56 gears

I have that same manifold as well, and I seemed to have gotten worse mileage from it than the cracked factory manifold it replaced. Mileage from an XJ seems like it's hit or miss - some good, some horrible.

As for that cat, unless you are also replacing the muffler/tailpipe, that universal cat is a 2.5" in/out - the stock cat is 2.25" out. Yes you can just clamp everything together. Yes, you will need to drill and weld in an O2 bung if you go with the universal. Magnaflow/Carsound makes a 'direct fit' model which is nothing more than a correct size universal with an O2 bung.

I'm guessing that none of these upgrades will get you any more mileage, as I just went through the same thing with my '97. I have the manifold with the expansion joints, the stock downpipe, the magnaflow cat, put on a flowmaster delta 50 with a 2.5" tailpipe, and also put on a Rusty's TB spacer and airtube with the smaller K&N filter. All that did nothing to improve performance or mileage. In fact, I just removed the flowmaster and tailpipe (for sale, btw) and put a stock replacement muffler on along with my stock tailpipe. YMMV.

The rattling cat needs to be replaced so the engine can breathe properly. It is restricting your exhaust, which hurts MPG and power.

What mileage are you getting on the highway? Miles on the entire jeep?

sjbond67 said:

yea i figured about 10% error, dont know if thats right

Click to expand...

I have 31's on my 99 and I used my GPS to figure out the exact conversion factor. Multipy .08 to your overall odometer reading at fill up and you'll be right on the money.

I simply multiply 1.08 to the odometer and I use that number for figuring MPG. I have been getting near 17mpg with my 31's, but I have a Conn Ferr rack that adds wind resistance. You must consider all factors for accurate reflection of your mileage.

Yesterday I installed my speedometer/odometer gear and my GPS now shows the EXACT mileage, so no more error consideration for me. Good luck with your pursuit.

What size tooth would I use for a 32x12" tire?

CustomCrawler said:

What size tooth would I use for a 32x12" tire?

Click to expand...

assuming you have a stock 4.0 automatic... 31 teeth, but it depends on the gear ratio in your differentails

http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

From what all I have read in various threads here, all the 4.0 jeeps going back to 1987 are capable of 14 to 16 mpg city and a good solid 20 to 22 mpg highway. If you are getting only 11 mpg on a 99 model then one or more stock items need fixing or replacing. I would take care of those first!!!!

Stop and go driving from traffic, stop lights, stop signs, and driving up hill or towing are murder on gas mileage. When you are stopped the engine is still drinking gas and you get 0, ZERO MPGs, and it hurts the gas mileage you calculate from the driving.

I have a converted 2.2 L diesel that gets me about 34 mpg, and it still gets great mileage even when I get stuck on a freeway doing 0 to 5 mphs for an hour in 5 o'clock traffic because it uses so little fuel to sit and idle at a stop.

When Ford and GM first built their own Hybrids, the gas mileage was worse than the straight gas engine version. They abonded the hybrid until Toyota solved the problem with a Toyota patented computer algorithm that solved the problem and allowed the hybrid to get better mileage than the non hybrid equivalent design.

I read recently that an electric car run off of batteries that are charged off the electric grid while parked, have an equivalent operating energy cost of $1.00 gasoline at current electric rates (source was MIT Technology Magazine). So adding some batteries, and an electric drive motor and turning off the gas engine might make some sense, and rechaging the batteries while idling and stuck in traffic might make some sense, but the real mileage killer for your 11 mpg is stock parts that have gone bad as you should be able to get 16 city and 22 highway on a stock jeep with everything working right.

The other mileage killer is the unproductive horseposwer used just keep the engine rotating at idle while at a complete stop. Most of that horsepower is used just to keep the engine itself rotating. I doubt that the water pump, alternator, clutch fan and power steering pump use that much HP at idle. The power steering pump uses very little power until you turn the wheel. The alternator only uses significant HP when recharging the battery or while running heavy loads like brake lights, head lights, the two electric fans, and / or a High power radio. A properly working fan clutch and water pump probably use about 2 to 6 HP depending on rpm and engine coolant temperature.

The problem with your idea is that the engine must produce the electrical energy from consumed HP to recharge the batteries used to run the accessories (fans, power steering, etc.). The added batteries add weight to the jeep, adding to the consumed HP to pull that extra load.

I would guess that 75% of the HP is used just to keep the engine turning over at idle. Even if I am wrong, most of the gas mileage losses are caused by slow moving traffic (too few miles travelled per minute) and acceleration HP needed to get back up to speed.

What might make sense is to install some kind of regenerative inline motor on one of the drive wheels and use and some batteries to accelerate and then recharge the batteries by using the electric regenerative motor to recharge the batteries durring deceleration.

If you want to try your idea, I would add batteries and wire them up to handle the load from the A/C clutch, A/C blower motor, electric coolant fan, radio, all the lights, and then remove the mechanical fan and clutch, replace the electlric fan temperature control circuit with a variable speed thermostatic control that varies the electlric fan speed to try and hold a coolant temperature of 195 F, and leave the existing electric fan controls for a full power on at the PCMs preset temperature as a safety back up. I would leave the power sterring and alternator alone.

Then set the alternator voltage regulator to kick on only after the spare batteries have been drained to some preset point. You would need to set the spare batteries up with an electrical output voltage regulator set to produce a steady 14 volts while the batteries are drained. You would also probably need to set up dual battery power systems!