88 Jeep Cherokee Bad MPG

jerkstore · 03-03-2007, 04:58 PM

Never posted in this forum and only had my jeep for a few months and i love it. I bought an 88 Jeep Cherokee 4.0 w/4WD that ran rough, a few months ago. I've replaced the Plugs, wires, rotor, cap, air and fuel filter, EGR valve and it runs A LOT better but I still get bad gas mileage. I average about 8 MPG in city in 2HI. My tire pressure is fine....I am just really stumped on what my problem could be. I dont believe i have a gas leaks as i have never smelled gas and have never seen any leak from the jeep. The exhaust always smells like the engine is burning really rich. Any help would be really appreciated as i am about to sell my daily driver and drive my jeep full time for a few months.

fredjacksonsan · 03-03-2007, 08:50 PM

O2 (oxygen) sensor. I've heard of them failing, and mileage going down the tubes.

8 mpg is waaaay too low. I'd also check your air filter.

1996Chreokee · 03-04-2007, 09:31 AM

O2 sensor

jerkstore · 03-04-2007, 03:47 PM

thanks, ill try replacing the O2 sensor....never replaced one before, should i use the bosch universal (cheaper) or one that in designed for the car (twice as much)

gtmud · 03-04-2007, 08:07 PM

I would recommend going with the MAnufacutre replacement if you can afford it. takes the guess work out of weather or not its the right one. Also, if you can get your hands on a pressure guage for the fuel system. once your engine is warmed up, see what your fuel pressure is, with teh engine running unplag and cap the vacuum line to the fuel pressure regulator, your pressure guage should spik to about 40 psi. If it doesn't, then there is a possibility that your regulator is bad. ( will not set a Engine light, and O2 sensor should) Even on the old OBD system.

jerkstore · 03-05-2007, 10:37 AM

thanx, ill try that today!

wagonbldr · 03-05-2007, 10:25 PM

Just had to replace the O2 sensor on my 95' Cherokee with 4.0 engine, auto trans, 4X4 and 187k miles.. I was starting to get Check Engine light, and it was for the O2 sensor. My mileage had dropped to 16mpg. Changed the sensor and now my mileage is back up to 20 in 2H.

jerkstore · 03-10-2007, 05:49 PM

i checked the fuel pressure like you said and came up with about 37 PSI. This is not enough of a differance to cause the bad MPG is it?> I have not had the money to buy the new oxygen sensor yet. I will update forum with result when completed.

rlrobian · 03-15-2007, 02:25 PM

Going through the same type of problem with my 89 4.0L I6. I think Ifound the problem, will know for sure in a couple of days. A cracked vacume line between the throttle body and the MAP sensor. Cost $6.00 to fix.
Good Luck

jerkstore · 03-15-2007, 09:08 PM

just replaced the oxygen sensor, we'll see what happens! tell me if you figure anything out about the vacuum line between the map and throttle...ill check mine out tomorrow.

citizen@large · 03-16-2007, 02:35 PM

Any updates fellas? I'm hella' curious about y'alls results! My milage is crap... Though it might just be the 33's on 4.10 gears. who knows!

jerkstore · 03-17-2007, 10:34 AM

the o2 sensor seemed to smooth out the engine a little, but the exhaust still smells like its burning to rich....ill update once i have the mileage results....anyone have any other suggestions on helping improve my performance?

gtmud · 03-17-2007, 09:35 PM

DOes it act like its loading up on fuel? If its burning rich, and the fuel pressure (connected) is at 30-31 psi than I would look at the injectors, I too have an 89 XJ with simular problems, pulled the fuel rail today and found a corroded connection on one of the injectors and a leaky one. I have already ordered new #19's at about 250 us. I would recommend checking for a leaky injector or one that is not spraying as it should. Also you could pull your plugs and see which one is black a sooty. this indicates the rich burning cylinder.

jerkstore · 03-18-2007, 12:11 PM

yeah the o2 sensor did not help that much im getting about 10 mpg. i guess ill start looking at the injectors...thanx

jerkstore · 04-08-2007, 08:13 PM

I replace two injectors and still i am getting no where....I am about ready to give up.....anyone have any suggestions?

RECAP:
I have replaced:

Airfilter
Fuel Filter
Plugs
Wires
Cap and rotor
2 Injectors
O2 sensor

Started at 8mpg...went up to 10 after all that.

Something i noticed the other day is that when i rev the engine and the rpms start to get up there(i have no tach), i have a bunch of black smoke. do you think that as the RPMs rise the fuel pressure regulator starts to lets too much fuel in? At Idle they are normal? I dunno, but any help would be appreciated

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