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4.0 bad gas mileage

9

90Notch306

NAXJA Forum User
Location
Redford, MI
I just picked up a 98 XJ and the gas mileage is horrible. Put $20 in it and went 70 miles. CEL was on and i had it scanned. Bank 1 sensor 2 O2 sensor high voltage. Replaced the downstream O2 and everything was good for 10 miles now the light is back on. The truck runs good though. Before and after i changed the sensor. Seems rich though. Has some black smoke out the exhaust when i step on it. Ive been searching around for the last couple of hours about this. Everything i read is about it chugging though. Mine runs and idles fine. Will the coolant temp sensor throw a CEL?? Any other ideas??
 
Scan again, the trouble code might be different.

The Coolant Temp Sensor can be partially failed enough to cause driveability issues or bad MPG's, yet will not throw a code until it is fully failed.
 
It would be better if you could tell us what the actual gas mileage and use conditions (city, highway, etc.). 20MPG highway would be normal.

A good way to check for rich condition is to pull a spark plug out and check the color. Should be whitish tan. Darker indicates too rich.

What temperature are you showing?
 
You mention replacing the 'downstream' O2 sensor. I'm not familiar with the four sensor system (which you have?), but in the traditional, two sensor setup, it's the 'upstream' (of the cat) that controls fuel system operation. The only thing the downstream does is tell the computer if the car has a cat, and if it's working properly. Given the age, and how many miles?. it's due for new O2 sensors anyway.
There's a ton of 'I just got a new (fill in the blank) threads all over the Net, and there are certain things you need to do-tuneup, etc.-to get as close to a stock baseline as you can, and then start diagnosing any remaining problems.
 
winterbeater said:
It would be better if you could tell us what the actual gas mileage and use conditions (city, highway, etc.).
Click to expand...

Agreed. You need a larger gas mileage sampling before coming to any conclusions. At least three fills of gas, divide miles driven by gallons consumed.

Doesn't mean that you can't chase your codes and work on optimizing the engine but you need more gas mileage data points to make good decisions.

When is the last time it was tuned up? That goes a LONG ways towards getting better gas mileage. For your 98, that means plugs, wires, distributor cap, rotor and maybe a bottle of quality fuel injector cleaner (BG44K, Techron, etc) in a tank of gas.

The coolant temperaturer sensor can be tested for resistance values (cold engine vs hot engine). You can do this with the sensor installed to get a "basic" idea of whether it is working or not as the values should be vastly different when cold vs when hot.
 
It has recently had a full tune up plugs wires cap and rotor within the last 3k. It gets around 9mpg for city driving. Truck only has 86k on it. And it only has 2 O2 sensors on it. I just had it rescanned and it said bank 1 system too lean. But the bank 1 sensor 2 code is gone now. Is it possible that the upstream sensor is bad too but not throwing a code? And the plugs are black.
 
Post the exact code number as there are a handful of oxygen sensor/fuel sensor codes.

Google that code number + Jeep for good information on troubleshooting that particular code.
 
90Notch306 said:
p0171
Click to expand...

Goggle says - P0171 (M) 1/1 Fuel System Lean A lean air/fuel mixture has been indicated by an abnormally rich correction factor.



A cracked or leaky exhaust manifold will cause an O2 sensor to read lean. Cracked exhaust manifolds are very common on Cherokees.

A dirty or worn injector will also cause a lean condition. Test the injectors with an ohm meter to see if the injector coils are within range. It's more difficult to see if they are flowing correctly, but a mis and a lean condition would point to injector issues.

A miss and a rich condition would indicate an ignition problem (wires, plugs, cap, rotor).

The computer should throw a DTC for taking too long to reach operating temperature. There is one sensor for both the temp gauge and for the PCM input.


O2 sensor info - http://www.aa1car.com/library/o2sensor.htm
 
Last edited:
I have seen weak/failing o2 sensors destroy MPG without ever giving a code--Fords, Jeeps, all kinds of vehicles.
 
The exhaust is all new with a pace setter long tube. Fresh tune up. I am about to go change the injectors to some mustang 5.0 ones that i have that i know are good. Might as well upgrade right? If the injectors dont do the trick then whats left? Just the upstream O2?
 
The 5.0L injectors help atomize the fuel better as it sprays from multiple holes vs one in the stockers. Have you run any seafoam, mmo, fuel injector cleaner thru your tank yet? Air filter clean?
 
I was just eliminating the possibility of a stuck or clogged injector by putting the 5.0 injectors in. Air filter is clean and there is a bottle of fuel injector cleaner in the tank right now.
 
90Notch306 said:
Put $20 in it and went 70 miles.
Click to expand...
And what did $20.00 get you in gallons where you are at??

Have you checked you fuel pressure?

Have you checked the readings on your coolant temp sensor? Are/were they in spec?

Re torque your exhaust manifold for good measure. Yes I know it is a pain in the butt. But header or not they like to loosen a bit, especially the back bolts.
 
$20 was good for 7.5 gallons. Went 70 miles on that. 9.3 MPG. Checked the fuel pressure its right at 50psi. Coolant temp is new. Been thinkin about re torquing them. Just waiting for it to cool off
 
I'd like to know too. MPG has been pretty crappy on mine, but mine has 175,000 miles. I've been looking at replacing my O2 sensors too. I have no idea the history on there replacement or not.
 
Especially if you put in used injectors, run a can of BG44K in the fuel to clean out the injectors and carbon. It is very common for used injectors to have gummed up as they dried out being removed. My stroker ran like crap until I used the BG44k and before I got 10 miles home, it ran like a different vehicle.

Don't waste you time with STP injector cleaner and the like. BG44K is by far the best. Lucas is second best.
 
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