Bad gas mileage just got worse

SBpunk

SBpunk

NAXJA Forum User
Location
Enid, OK
1998 XJ Classic less than 100k miles 4.0 4WD tires not big enough to make a difference and a 4 inch lift.

What I've done:
New CPS - TPS - Coil - Dis Cap - Dis Rotor - Spark Plugs - Spark Plug wires - Oil change - New injectors - 63mm Throttle body - Thor Snorkel (goes into the firewall) - 136a Alternator upgrade - im sure this is more but i cant think off the top of my head. My tires are at 26 to 28 psi and I don't drive like an idiot.

Alright now thats all out of the way. I was getting 11 mpg around base on a good tank but I was hitting P0123 P0301 codes. Solved that by putting in a mopar TPS. Got rid of the codes but notice while driving around today I was hitting 7 mpg on base. I took it on the highway today and it hit 10.3 or so and I could almost see my fuel gauge going down (not really just went thru and 1/8 of tank in a very short distance). Any suggestions? Oh my motor is still jerking pretty hard when I'm at a stop light and my engine bay smells like I'm running rich.
 
What are you using to calculate mileage? The only reliable way is GPS and using the same pump. Also, tires "not big enough to make a difference" that aren't stock size is going to throw off your speedo, miles traveled, and any factory fuel mileage monitor.
 
I understand but they're the same tires that have been on it since I picked it up. Im using the on board computer in the overhead. If I had upgraded the tires to a larger size I would understand the sudden drop in MPG.
 
How about your upstream O2 sensor? They typically have a service life of 100,000-150,000 miles. I didn't see it in your list of replaced parts and it could definitely cause your mileage drop. How is the Jeep at throttle tip in? Any hesitation? Could also point to O2.

Can you get a scan tool on it to make sure you're in closed loop?

Also, are you underinflated on your tires? 26 psi seems a bit low. Are you getting tire wear on the outside edges? That could point to underinflation and that would effect mileage.

I get about 16-17 in town and 21-22 on the freeway consistantly.
 
Last edited:
98NWCherokee said:
How about your upstream O2 sensor? They typically have a service life of 100,000-150,000 miles. I didn't see it in your list of replaced parts and it could definitely cause your mileage drop. How is the Jeep at throttle tip in? Any hesitation? Could also point to O2.

Can you get a scan tool on it to make sure you're in closed loop?

Also, are you underinflated on your tires? 26 psi seems a bit low. Are you getting tire wear on the outside edges? That could point to underinflation and that would effect mileage.

I get about 16-17 in town and 21-22 on the freeway consistantly.
Click to expand...

I was thinking an o2 but wouldn't it throw codes? I have and obd2 scanner if that's what you mean. If not hook me up with a dummy guide lol. I keep the tires low here because of the roads. Usually ill do 28 to 32 on 30s but the main roads are like country roads in Hawaii.
 
Tires and tire pressure are not your problem. I forget the whole story on yours, but the P0301 code had nothing to do with the TPS sensor. A problem with the O2 sensor, or the wiring or the 12 volt feed to the heater in the O2 sensor are highly likely.

Did you ever disconnect the Battery for a while to drain the PCM memory, and then take it through the relearn procedure after changing the TPS, and fuel injectors? You may need to do that, then use the OBD-II scanner to monitor and verify that there are no pending codes (they do not set of the CEL, and it requires a tester that shows pending codes), and to verify that all the emission monitors are locked in, as it will not set some codes or CELs properly until the emissions monitors complete their relearn procedures in a special driving mode listed in the FSM.

It is the front upstream O2 sensor you need to test. Test the harness wires, 2 grounds, 12 volt input, and

NO, O2 sensor problems seem to rarely set OBD-II codes, except if you disconnect them totally, or have a huge overvolatge short to the output wire. A bad heater in the O2 sensor, low heater voltage to the sensor, bad ground or old worn out sensor can cause idle problems and use tons of gas!!!

Here is my old how to test jeep O2 sensors thread:

http://www.naxja.org/forum/showthread.php?t=1012701

Even though these codes have not come back YET, I would keep the problem list from them in mind, like O2 sensor, and the PCM, and wiring issues. Just replacing a part, like a TPS can move wires and interrupt a short from bad wire insulation!!!! O2 sensor wires often get eaten by the drive shaft or exhaust manifold!!! I was suspicious when you said the dealer TPS worked and an aftermarket new TPS did not fix the problems!!!!

http://repairpal.com/obd-ii-code-p0123

http://www.obd-codes.com/p0123

http://www.obd-codes.com/p0301

OBD-II is not perfect, I chased my tail for weeks with no OBD codes on a 96 Ford, that had a rough idle. All else was great!!! Turned out the OBD-II was stuck in relearn mode and was still too dumb to issue a miss fire on cycle 3, that did not have the spark plug wire attached!!!:doh: It was deep in the rear and you could not see it.
 
EcoMike was 30 seconds faster than me.

Jerking, stalling, and/or backfiring often indicates an O2 Sensor issue. A sudden 30-40% drop in gas mpg's often indiactes an O2 sensor issue. As already stated, O2 sensor have a service life, and either become slow to react or simply fail. Many times they will not throw a Check Engine Light codes.


If your OBD-II scanner has live data, check you O2 sensor function, or test your O2 sensors with your volts/ohms multi-meter.
 
Eco and Tim I was hoping to hear from you guys today. Eco I did reset the PCM but I will look into the O2 sensor testing once I'm done testing for promotion today. Thanks for the quick response. One of these days I'll get my engine running correctly. Thanks again everyone I'll report back soon as I have results.
 
Ok so my O2 sensors are good to go. BUT I was changing my coolant today and decided to pull my injector plugs with a wild hair. When I pulled number 1 nothing happened. No stutter in the engine or anything. Number 2 it bogged the engine down a bit same with the others I pulled. So I took a testing light (just turns on a light if its getting power) and I was getting power to the number 1 injector. Im guessing the injector is bad and could be causing some of my problems? Thoughts would be great.
 
Oh I ordered two new O2 sensors just for giggles since I'm getting close to the 100k mark.

56041212 O2 sensor and 56041213AB O2 sensor... I know there's a third after the cat I remember right but I'm hoping these will work at least for a bit. One in the intake manifold and the other before the cat right?
 
No O2 sensors in the intake. Your year I think has 2 in the exhaust manifold. Those are the ones you need to change, the one after the Cat does not really affect engine performance, it is an EPA sniffer to verify the Cat is working more than anything.

You need to use a NOID test light on #1 to see if the light flashes. If it flashes, the injector is bad. If no flash, either the wiring is bad (bad ground wire?) or the PCM has a bad injector firing circuit, which does happen. I was already starting to suspect the PCM a few posts back, but I like to eliminate the simple cheap, usual suspects first.

I have tested loose injectors with a 9 volt battery and alligator clips, to listen for the clicking!!!!
 
Ive never heard or a NOID tester but I'll look into it. Damn I was hoping I got off easy with just needed a new fuel injector. If the light isnt flashing I'll just start probing back to the PCM to see if its a bad ground or a crappy PCM. Damn I hope its the wiring or fuel injector.
 
You still have the other injectors right? Known good ones, just too large a flow? Take one and attach it to the connector you want to test, run the engine, if you can hear it clicking, you know it is a bad injector in the engine and not the wiring or computer!!!!
 
Ok so getting blinking lights on all of them so looks like im in for new injectors... on that note I will never buy from PercisionInjectors again. Garbage
 
I got mine at fiveomotorsports dot com.

SBpunk said:
Ok so getting blinking lights on all of them so looks like im in for new injectors... on that note I will never buy from PercisionInjectors again. Garbage
Click to expand...
 
If you've got some extra money kicking around, maybe a ScanGauge II or Ultra Gauge is in order I have one of each, the SG II permanently installed in me '96. The UG usuallually lives in the Outback, but with its suction cup mount, I use it as a portable diagnostic tool. Both show real time MPGs, open or closed loop operation, and can show OBD II codes and reset them. As my fleet ages, I like to be able to see what's going on. They are available on Amazon, and there's a ton of Youtube videos showing them in operation.
I have a vehicle-specific printout of OBD II codes in each vehicle, hopefully never to be used.
 
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