Ok something has to be wrong, MPG dropped massively!

inv3ctiv3 · Jun 21, 2013

So I posted a thread a while back about my 2001 XJ dropping MPG after putting on OEM sized General Grabber AT2's. With my old tires I averaged 19-20mpg per tank (this was calculated by filling up, reseting trip, driving and then filling up again and dividing the number of miles driven by the amount of fuel put in).

It dropped to like 16mpg average but now I am averaging 13-14mpg just highway driving it's really annoying because I sold my Land Cruiser to get away from the 10-12mpg life....I've heard O2 sensors can cause MPG drop but don't know of a good way to test them, and I had a check engine light on that I had a mechanic read and he said it was an EVAP code which was most likely a vacuum leak or my gas cap if that helps.

I really need help diagnosing this because it's got to get fixed, I have stopped driving my XJ because of it....

8Mud · Jun 21, 2013

I'd go with the usual suspects, your Cat is slowly disintegrating and restricting your exhaust flow. Bang on the side of your (cold) Cat with the palm of your hand and listen carefully, It may be subtle at first, later it may sound like a tin can full off rocks when you start up the motor.

O2 sensors get flaky way before they throw a code. I usually replace by mileage or for me around every few years.

Vacuum leaks, many of the vacuum nipples are on one end of the intake or the other, the extra vacuum (air) makes for more O2, the PCM compensates. They may not be large enough to increase your idle and still be large enough to mess with your motor management. Your codes may indicate this as a possibility.

Basic tune up.

And just because you brought up lower fuel mileage after the tire change. A buddy showed up the other day and complained about his fuel mileage going down. During the test drive I noticed a swishing sound coming from the rear. To make a long story short his rear drums were full of sand. You said new tires, hopefully they used a torque wrench on the lug nuts and didn't warp your rotors. Dragging brakes can sure enough mess with your fuel mileage.

inv3ctiv3 · Jun 23, 2013

I'll look into some of that, what brand O2 sensors are best for the money (I have an 2001 so I have 4, don't wanna break the bank too much....)

joe_peters · Jun 23, 2013

Three part series on XJ fuel mileage--interesting reading.

http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html

http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html

http://www.jpmagazine.com/projectbu...cherokee_xj_project_mileage_master/index.html

inv3ctiv3 · Jun 24, 2013

joe_peters said:
Three part series on XJ fuel mileage--interesting reading.

http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html

http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html

http://www.jpmagazine.com/projectbu...cherokee_xj_project_mileage_master/index.html

Yea I've read that before, good article for sure just figured something has to have broken in order to cause such a sudden drop in MPG.....

GrimmJeeper · Jun 24, 2013

I have the exact same problem with my 2000 4.0 2wd. popped a CEL for a failure of the "Leak detection pump" (the pump that pressurizes the gas tank to make sure it's sealed) and now my gas mileage went to crap I get about 160 miles out of a full as it will go tank.

I haven't been able to verify it but i'm assuming the CEL being on is putting the PCM into some kind of limp mode and making it run rich. :dunno: I have a new leak pump coming in so if my mileage goes back up once I replace it I'll report back in on this thread.

inv3ctiv3 · Jul 8, 2013

I'm starting to wonder if maybe the AC condensor is causing drag or something? Whenever I turn on the AC it starts squeeling really bad and is rev dependent but will then stop and the AC seems to work but the engine seems to run rough or something when the AC is blasting.

CobraMarty · Jul 8, 2013

All the above.
I had a bad alternator bearing that finally seized and prior was dragging so badly that it got bad mileage.

bluejeepkid · Jul 8, 2013

Is there a leak In your gas tank that fuel is possibly evaporating out of?

old_man · Jul 8, 2013

Run a can of BG44K through the fuel. Most drops in MPG I have seen are caused by either clogged injectors or plugged CAT's. Do a visual and make sure you didn't hit the exhaust some where, causing a restriction. Air filter clean? Did you happen to go to a thicker oil?

Kingkong0192 · Jul 8, 2013

8Mud said:
You said new tires, hopefully they used a torque wrench on the lug nuts and didn't warp your rotors. Dragging brakes can sure enough mess with your fuel mileage.

Is this really that important? I've never used a torque wrench and have never had a problem (or so I think.)

What is the actual torque spec?

joe_peters · Jul 8, 2013

Kingkong0192 said:
Is this really that important? I've never used a torque wrench and have never had a problem (or so I think.)

What is the actual torque spec?

Reasons using a torque wrench is important on brakes:

1. Over torquing can lead to lugs snapping off--not fun.

2. Under torquing can lead to lug nuts backing off--not fun, ruins wheels, spoils your day.

3. Over torquing can lead to warping hubs/rotors--not fun, etc.

Do you have to use a torque wrench on everything? No.

Should you always use a torque wrench on certain things? YES.

inv3ctiv3 · Jul 9, 2013

One thing is different and that's that I put a FRAM oil filter in (big no no I know now) and will change it out for a WIX this week. It just sucks it could be any number of things haha, I don't want to replace the alternator, PS pump, AC condensor, o2 sensors, etc....Is there a good way to diagnose the O2 sensors?

joe_peters · Jul 9, 2013

You can remove the serpentine belt and move the belt driven accessories by hand--power steering pump, alternator, idler pulleys, water pump--and see how they feel.

inv3ctiv3 · Jul 10, 2013

Well I was gonna re-do the cooling system as it appears to have developed a small leak from the radiator overflow outlet on the radiator so that'll be a good time to inspect all the pulley's and everything. I have a feeling it could be the O2 sensors because a lot of people say they can cause huge drops in MPG but I have 4 O2 sensors and that's like $200 for something that may not fix it....

joe_peters · Jul 10, 2013

The "downstream"--after the CAT--o2 sensors don't have to be replaced unless you need to clear a DTC specifically for them. They just provide data on CAT performance.

The "upstream" o2 sensors are what will affect your engine performance.

inv3ctiv3 · Jul 10, 2013

Oh ok cool! What brand do you guys recommend? I've found bosch for a decent price.

joe_peters · Jul 11, 2013

inv3ctiv3 said:
Oh ok cool! What brand do you guys recommend? I've found bosch for a decent price.

Well, Bosch is the granddaddy, but I have moved to NTK (NGK).

(Note: on a lot of the very late model vehicles the PCMs HATE aftermarket o2s.)

CobraMarty · Jul 11, 2013

NTK for me on 2 sets and no problems.

Jonner · Jul 11, 2013

I have NTK on my 98 4.0L (upstream) and no issues. I still need to replace the downstream, along with the cat I have in the garage.

Thanks for the tip on checking pulley resistance by hand with the belt off. Don't know why, but I never would of thought of that.

Isn't there a way to check O2 sensors using an analog multimeter? I've never tried it, but remember something about this.

Ok something has to be wrong, MPG dropped massively!

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