Hi everybody,

My 1998 Cherokee 4.0 consistently achieved around 15 - 17 mpg for years while commuting to school or work. This past winter it dropped down a lot (to around 11 - 13). I blamed it on the weather and heavy traffic, but now my wife has been driving it to work (about 22 miles each way, 80% expressway, good traffic) and it is still just getting around 13 mpg. I changed the oil and filter, air filter and spark plugs about a month ago and this has had no effect. The old spark plugs were worn (they had a solid 30,000 or more miles on them), but their color was the normal color.

When I talk to my mechanic about this he just shrugs his shoulders and points out that we have more than 230,000 miles on it and can't expect better mileage anymore. But the engine runs smooth, there is no smoke in the exhaust and I go through only about a 1/2 quart of oil every 1,000 miles.

I would really like to get it back to the 16 mpg range. I am wondering about the O2 sensor. I'm not getting any codes, but I've read that an O2 sensor can go bad before throwing any codes. But on the other hand, this problem has been going on now for at least 6 months and I'd think by now if it was the O2 sensor, a code would have been thrown.

Note that upon start up, there is a little bit of misfiring and hesitation as one accelerates away just after starting, but this disappears within 20 seconds of start up.

Is there any way to test spark plug wires and/or distributor cap? I don't want to just throw parts and $$$ at it. Note that it's had a recent front end alignment, the tires are almost new (Goodyear TripleTreds) and properly inflated.

I welcome any and all suggestions!

Thanks,

John

 

John,

I recommend new spark plugs at 30K for the 4.0 engine. They're cheap. Get some Champions, gap them to .035 and install them as well as quality plug wires, distributor cap and rotor (I go 60K on those 3 components). These components just plain wear out and become inefficient over the miles. Inspection of plugs can tell you how the engine is performing but at 30K, they should be replaced regardless of how they look. Money spent on tuneup hardware is possibly the best money you can spend on your XJ.

Yes, oxygen sensors can get a bit "lazy" before they trip a code. Oxygen sensors have a finite lifespan depending on the miles on them and condition/efficiency of the engine. If you have 100K or more on your current O2 sensors, replace them with NTK sensors (no other brand) if you plan on keeping your XJ.

Other than those things, in the absence of codes.....your mileage pretty much will be what it is.

 

Definitely what he says ^^^ about the O2 sensors. Had mine go out on a road trip. Went from 20+mpg to ~13. No codes or CEL. Just the drop in mpg.

 

Seeing as you gave it a tuneup a month ago, the plugs and such are probably fine. You mentioned the o2 sensor, which is a possibilty. As tjwalker posted, they get "lazy". Fuel mileage suffers, which is often gradual so many dont even notice, but it can affect the engine quite a bit. If you have never replaced them, do so. Like said, stick with NTK. The upstream o2 tells the PCM if the engine is running too rich, or too lean, and makes any adjustments accordingly. The downstream o2s only real function is to monitor the cat, but if you want to replace that one as well, go for it. Injectors get dirty over time and the engine may not be getting the fuel it needs, so the PCM adds more, killing the fuel mileage in the process. You can also check the fuel pressure. This wont tell you if the injectors are dirty, but its a good idea to keep tabs on it when mileage starts to suffer. I check the transmission fluid, coolant, and tire pressure as well.

When i gave the 97 its last tuneup, i also checked the battery and cleaned the grounds.

 

Your next step is to get a BAFX bluetooth OBD2 scanner (cost about $25 on Amazon). That way you can see what your fuel trims are doing. My guess is that your fuel trims are higher than normal (positive) because of an O2 sensor that is failing. Your long term and short term fuel trims should be hovering around 0. Combined, they should not be more than 10% (positive or negative). This will greatly help take the guess work out of things. If you find that they fuel trims look great (close to zero), then you will know that it is not so much a air/fuel problem, and is more of a horsepower problem where you are having to drive the motor harder to get the same amount of performance out of it (due to age and miles).

 

Thanks for the tips guys.
TJ I think you missed where I wrote that I just replaced the Spark Plugs (and the ones I put in are Champions by the way).
I will go ahead and replace the front O2 sensor, as it is more than 7 years old now and probably has around 80,000 miles on it. BUT: It will be a MOPAR one bought at the dealership. If you look up my old posts you will see a nightmare I went through when I tried using aftermarket O2 Sensors (Bosch I think). They would fail every few weeks. Turned out that even though the Bosch one is supposed to be a direct fit replacement, the resistance in its heater unit is wrong, causing it to heat up too slowly, causing it to fail. When I finally spent the extra bucks for the MOPAR one, it has lasted just fine for years (until possibly now). Perhaps the NTK is the same as the MOPAR sensor but I don't want to take the chance.
And I suppose I should pony up the money for new plug wires, distributor cap and rotor. If the old wires are worn out would I see arching at night under the hood?

 

Thanks Jordon! I have an OBD2 scanner. This one:
I am not sure if the one I have can read fuel trims or not though. I'll have to try when my wife gets home tonight and see.

[Edit: Just opened up the app and figured out how to get to the PID for fuel trims - so I'll check it tonight when SWAMBO gets home with the Jeep.]

 

Yup, I missed that; didn't have enough coffee this morning! So plugs are new so that is done. Spring for the other tuneup components of wires, cap and rotor. If you do the installation yourself, the cost is minimal. Looking for arcing with wires is not a good primary test for plug wires. While seeing arcing would indicate a bad wire, you really can't be sure that wires are good if you don't see any arcing. Spark plug wires can be tested for resistance, but if they're old, just get new ones.

If you buy oxygen sensors at the Jeep dealership, then yes you are getting NTK. You can save some money though by buying NTK at a parts store. They are the same part. An NTK oxygen sensor purchased at Rock Auto or anywhere will perform perfectly in your 4.0 Jeep engine. I agree completely with you on why you shouldn't use Bosch oxygen sensors on the 91-01 Jeep XJ (and I love Bosch products in general). If memory serves me they work okay on the 87-90 Renix XJ. I have seen some HO 4.0 engines tolerate the Bosch O2 sensors, but many have the exact issue you describe.

 

Thanks for the further info TJ - when the time comes for new O2 sensors I feel confident now I can save some money by going with NTK.
Jordon, I was able to check my fuel trims last night after my wife got home from work. They were great - neither short or long term trims rose or dropped by more than about 1.5%, with the short term peaking momentarily at around 3% when I changed RPM. And the O2 sensor was varying its voltage as it should - I saw fluctuations of 0.1 volts, 0.4 volts and 0.8 volts. I tested at idle, 1500RPM and 2500RPM.
So my next step is to replace the wires, cap and rotor. Then, if that doesn't cure it I will probably go ahead and buy a new O2 sensor and put it in. I've read up on the subject and I know that though I could see the voltage fluctuations with my simple Engine Link App and Scanner, I couldn't tell how fast the fluctuations in voltage are actually occurring. As I learned from the articles I read, the only way to see if the voltage changes are happening as quickly as they should (about every 150 milliseconds) is to use a computer based scan tool and graph the fluctuations. And an aging 02 sensor can become sluggish in those voltage changes and that causes... poor fuel economy.
Having older vehicles is a wonderful thing - I learn so much more than my neighbors will ever know...

 

Don't forget the coolant temp sensors. The one for the dash isn't important but the one for the ECU can cause bad mileage.

 

I'm pretty sure there is just one coolant sensor on the 96+ 4.0.

 

Yes, I'm finding only one type of engine (coolant) temperature sensor on both the AutoZone and the NAPA websites for the 98 Jeep Cherokee (there are also ambient air temperature and battery temperature sensors but I don't think these apply to the situation). And coolant temperature is reading in the normal range on my OBD-II Scanner (between 196.5 and 200+ degrees).

 

 

The trim number you look at is the long term. Ideally it's 0. This tells you which way you are...wait for it....leaning.

 

I see what you did there.