10mpg???

[GordoSmasho]

So I have a puzzling situation involving fuel economy with my 1997 2door 4,0 AW4. It has a 3" lift and 31" all terrains. The axles are 4.10 geared. The motor has about 150k on it and the trans has about 240k. I got this jeep really cheap and it had issues. I did a simple rebuild and got it running again but had really bad milage. This was when it was still completely stock. So I replaced the CPS, MAP, both o2s, the cat, Pickup coil, and did the usual tuneup stuff. No change in economy. Also started to notice really low oil pressure so I went ahead and did a motor swap. All the sensors were swapped to the new motor as I had just replaced them all recently. The new motor has about 150k on it and ran superb in the previous vehicle. With the swap I put on an apn header and a 99' intake manifold and a Cowl intake System will some precision 4hole injectors and a bored throttle body. Also put on a Super40 Muffler with a turn down and switched to an electric triple fan setup for cooling. So with all those upgrades and a fresher motor that was proven I am getting about 10mpg. I've checked the TPS, MAP, CPS, and i have no codes on my scan tool. Filling up every 180 miles is really sucking especially with the price gas is now.

Just wondering if anyone would have a list to start checking even if its redundant to what
I have already done. It would be easier I think to have a list with links to diagnostic
procedures and such. Starting with the most important.

Most of the threads i read on fuel economy are just full of people saying what they have
and how great or horrible their Mileage is. This is not helpful in my case as I already
know I'm getting bad mileage. (sorry for the short rant. I'm done)

Also wondering if the trans could be robbing any power and thereby fuel economy. My 1-2
shift lasts about 2 full seconds even under full throttle and the rest are very sluggish.
I have adjusted the kick down cable several times and it is set properly.

I do not have a heavy foot as my 5" lifted four door with 35s and 410s accurately measured gets 17mpg regularly.

 

[solomon7]

Have you swapped in the appropriate speedometer gear yet?
With larger tires and lower gearing, if the gear is stock your mileage is way better than the instruments are showing.
Based on this: http://jeep-xj.info/HowtoSpeedoGears.htm

You likely started out stock with a 34 or 35 tooth gear. You need a 37 tooth speedo gear.

 

[Ecomike]

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test

 

[GordoSmasho]

you are right about the speedo gear however my speedo is reading roughly 10% faster than what my actual speed is. That would tell me I'm getting better mileage that what I'm actually getting. It has the stock gear in it that is a 34 tooth. I used a GPS to verify my trips and calculate my current mpgs.

Ecomike that is a great threat on o2s. Ill give that a read. I wasnt originally considering them as the culprit because they are fluctuating normally on my scan guage. I will run through a few of the tests tomorrow to see if anything shows up.

 

[Bdiddy11]

If I remember right, even though the o2 sensors appear to be normal they can actually be running on the crappy side. Even with the larger tires and not the right speedo than is a way off from what it should be. I've got a 3" lift on 245/75/16s which is about 30.5", the last tank I just got was about 270 miles give or take with mixed driving.

160k miles, new o2 sensors, tune up, air filter, and recent oil change. Calculating for the larger tired it puts me at about 302 miles / 20 gallons is just over 15mpg mixed driving. I could be getting slightly more but my fuel pump/sensor is finicky and sometimes doesn't read right. I was at 250 miles when the gas light came on. I didn't want to try the gas gauge before getting more gas because sometimes it drops low and sometimes reads normal when I'm near E.

Have you done an oil change recently.

 

[GordoSmasho]

I have done a recent oil change. One more change I forgot to mention was a fuel pump assembly. Just wont to make all info known though that didn't change my fuel economy any better. I have found after this though that my gas guage is very inaccurate on the lower end. However when I fill up at the 180 mile mark i put in about 18 gallons every time.

I would suspect this issue was present before the engine problems started with the previous owner. It is running very rich. I can smell gas out the tailpipe and I had to replace the cat as there was almost no honeycomb left in it when I took it out.

I'll post up my findings on the O2 sensors when I get them. I have to pick up an analog volt meter tonight.

 

[Ecomike]

Also check the actual engine operating temperature. The newer jeeps seem to be more finicky about get up to temperature on MPGs. T-stat stuck open in winter or bad CTS on the new ones are a possible issue if not the O2 sensor operation.

 

[GordoSmasho]

Will double check the temp sensor. I seem to remember it working properly. I will report back in a few days as I'm out of town for work right now. Thanks for all the replies guys.

 

[weebur]

What O2 sensors did you replace the originals with? I got a nice bump in mileage when I replaced my upstream O2 sensor (Bosch with NTK) and fixed a minor upstream (at the donut) exhaust leak. I picked up a 9.2% increase. Just a thought.

 

[Ecomike]

Will double check the temp sensor. I seem to remember it working properly. I will report back in a few days as I'm out of town for work right now. Thanks for all the replies guys.

Be sure to test it with an ohm meter, or an infrared tester, don't trust the OBD-II data, as it is based on the sensor, which if bad, is giving bad data. best test is in ice water, and boiling water, then measure resistance, ohms, and compare to factory spec.

Then, with a known good CTS, see what temp it reports, or use the IR tester to check the T-stat opening temp..if the T-stat failed open, it will knock the hell out of MPGs on an OBD-II 97 jeep.

 

[MrAnderson]

You do realize your driving a jeep right? I had similar concerns when I noticed I was getting about 9mpg outta my girl so I did some usual maintenance things like plugs (which were about a 16th away from being flat), oil, put new intake and exsaust, and seafoamed my induction and fuel system and I bumped it up to 10mpg (still plan on doing sensors but for now I'm out of money ). I then thought about my driving habits. I have a very light foot so it wasn't that. It turned out to be the area I drive in, Pismo Beach, which is riddled with hills. I filled up ( nettin me the afore mentioned 10) hoped on the freeway and drove to slo and back (about 40 miles rnd trip) filled up again and got 23mpg! Could this be the culprit for you as well?

 

[GordoSmasho]

I don't think that's the problem. I used to commute in my 4door with 4.10s and 35s an a 5" lift. Worst traffic around 150 miles every day through various hill ranges. Hauling about 600lbs of tools and gear with ladders strapped to the roof and I accurately measured 15mpgs regularly.

I know what it's is capable of getting just don't know what's broke yet. Got some good leads from previous posts. I'm going to give them a go.