98 Grand Cherokee 4.0L TERRIBLE gas mileage

About a month ago, I've posted a thread about how my Grand Cherokee Laredo with a I6 was getting terrible gas mileage. I was probably only getting about 9.5-10mpg. So I did some upgrades. I replaced the spark plugs with Bosch Platinum IR Fusions, A/C Delco Distributor Cap and Rotor, Autolite Professional Series Wires, new exhaust and intake manifolds, new tires, new Flowmaster Super 40 Series muffler and a Magnaflow Catalytic Converter, new fuel pump and fuel filter, K&N Air filter (not the entire kit, just the filter), new wheel bearings, and changed all the fluids for the 4WD (transfer case and the front and rear differentials). The only thing I DID NOT upgrade was the oil. I still use conventional oil and I am very hesitant with switching to synthetic. But even for oil, I use Mobil1 Clean 5000 mile oil and a ToughGuard Oil Filter. But I still change my oil every 3000 miles.

I am STILL only getting about 10mpg and this Jeep now runs like BRAND SPANKING NEW. Before it used to run a little rough. Now, there is not even the slightest engine hesitation. Acceleration is smooth and I feel that I don't have to press the gas pedal as hard to get the Jeep going. I can definitely feel I have more power than I used to. Now the Jeep does have 152,000 miles on it. However, I do not race it and I usually don't go faster than 75 on the highway. But thats the thing, too. On the highway going 75 I'll get between 20-23mpg. But around town, the gas just SUCKS! Is there anything else I can do to get at least 12-13mpg in the city like I used to when I first got the Jeep (I've owned it for almost 4 years now). Thanks.

One more thing. Driving the Jeep from a full tank to a 1/4 of a tank, I used to get 180 miles around town. Now I only get 150. IT REALLY SUCKS!

i too have very poor gas mileage in my 98 5.9L, I get 10.6 mpg average (mostly around town) and just figured out I get about 12 mpg on the highway. which also is very poor.

Let me know if you find anything that helps your mileage.

i too have very poor gas mileage in my 98 5.9L, I get 10.6 mpg average (mostly around town) and just figured out I get about 12 mpg on the highway. which also is very poor.

Let me know if you find anything that helps your mileage.

well i also forgot to mention i replaced the O2 sensors too. i asked my buddy whos a mechanic for jeep and he told me to reset the computer to factor settings so it can re-learn how i drive with all the new parts i put in. heres what to do:

1. Disconnect the battery by removing the positive battery cable.

2. Ground the positive cable to the negative cable for 30 seconds to erase the electric charge in the capacitors.

3. Reconnect the positive cable to the battery.

4. Turn the key to on but do not start.

5. Turn the headlights on.

6. Turn the headlights off.

7. Turn the key off.

The ECU has now been reset to its factory settings. For the next 50 warm-up periods the ECU will re-learn the engine and driving habits for your driving conditions and the newly installed parts. So I tried it on mine. I'll let you know what happens.

I've always had luck resetting the PCM by disconnecting the battery then turning the ignition to Start and holding it there a few seconds.
#2 probably does the same thing.
But I don't get #4 thru #7. Did he tell you what that does?

I've been getting poor gas mileage in my 4.0, '01 Grand for a while now; around 11-11.5 mpg city. Runs great but without codes all can you do is start replacing/upgrading performance related parts like plugs, wires, air filter etc.
I have heard that failing O2 sensors can cause poor gas mileage since they don't trigger the MIL lamp/fault codes until they almost totally malfunction.
Interested how it works out for you.

I've always had luck resetting the PCM by disconnecting the battery then turning the ignition to Start and holding it there a few seconds.
#2 probably does the same thing.
But I don't get #4 thru #7. Did he tell you what that does?

I've been getting poor gas mileage in my 4.0, '01 Grand for a while now; around 11-11.5 mpg city. Runs great but without codes all can you do is start replacing/upgrading performance related parts like plugs, wires, air filter etc.
I have heard that failing O2 sensors can cause poor gas mileage since they don't trigger the MIL lamp/fault codes until they almost totally malfunction.
Interested how it works out for you.


well so far it has gone up slightly...the computer is reading 10.7 mpg now. I am taking a 200 mile trip this Friday so we'll see if the highway mileage is better too. ill update you guys in a week and see how i make out with city driving.

I've always had luck resetting the PCM by disconnecting the battery then turning the ignition to Start and holding it there a few seconds.
#2 probably does the same thing.
But I don't get #4 thru #7. Did he tell you what that does?

I've been getting poor gas mileage in my 4.0, '01 Grand for a while now; around 11-11.5 mpg city. Runs great but without codes all can you do is start replacing/upgrading performance related parts like plugs, wires, air filter etc.
I have heard that failing O2 sensors can cause poor gas mileage since they don't trigger the MIL lamp/fault codes until they almost totally malfunction.
Interested how it works out for you.

Does this apply to GC Laredo 1995 with 5.2 liter?
My gas mileage is a real pita.

Does this apply to GC Laredo 1995 with 5.2 liter?
My gas mileage is a real pita.

yes reseting the computer to factory settings so your Jeep can re-learn your driving habits will work for you 95.

Does this apply to GC Laredo 1995 with 5.2 liter?
My gas mileage is a real pita.
You mean about the O2 sensor going? Nothing documented, I'm trying to find out where I heard about it.

yes reseting the computer to factory settings so your Jeep can re-learn your driving habits will work for you 95.
Thanks Sean, I will try it this week-end.

You mean about the O2 sensor going? Nothing documented, I'm trying to find out where I heard about it.
Unless you think O2 has something to do with gas mileage, I was asking about resetting computer vs performance. Thanks.

Unless you think O2 has something to do with gas mileage, I was asking about resetting computer vs performance. Thanks.
The O2 sensor has a lot to with gas mileage - it's responsible for the fuel/air mixture. Some Jeeps have more than one, my CA. emissions Jeep has FOUR; 2 pre cat. and 2 aft, but only the pre cat. sensors feedback mixture to the PCM.

Here's an update guys. My computer is reading 14 mpg now around town. I think it worked. I even calculated the gas mileage using math to see if the computer was accurate and with the miles I've driven from full to 1/4 of a tank (which was now 205 instead of 150 miles) and I got 13.899 rounded to to the third decimal spot. I will let you guys know how I make out Friday when I take a 200 mile highway trip to see if highway mileage is better.