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Poor idle mpg

Tiny
HAZBRO
  • MEMBER
  • 1998 JEEP CHEROKEE
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 165,000 MILES
I have a 1998 Jeep Cherokee Limited with the 4.0 engine and four wheel drive with 165000 miles. It recently started running rough at idle and went from 16 mpg to 9 mpg. The car runs fine otherwise acceleration and power seems ok. The CEL codes that popped up were a lean code and a high amp code for a sensor.
I thought maybe the injectors were dirty so I put Lucas oil injector cleaner in. The car smoothed out and the mileage went back up for three quarters of the tank after the fill up. It started to go back to running rough and poor mileage with a quarter of a tank left. Any ideas?
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Saturday, March 5th, 2011 AT 3:15 AM

12 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,740 POSTS
Usually a rough idle and a lean mixture is caused by a vacuum leak. There are quite a few vacuum hoses that can leak on the 4.0L. Do you still have the code when the computer was scanned? It would help me.

If you could, let me know the code so I can better help. Also, check for vacuum leaks. Aggain, they commonly cause a rough idle and they can lead specific sensors to force the engine to run richer. Thus, you lose MPG.
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Saturday, March 5th, 2011 AT 5:23 AM
Tiny
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The codes are PO138 and PO171. Thanks for the quick reply. The thing that confused me is when I put the injector cleaner in it started to run like it is supposed to. Thats why I didn't think it was a vacuum leak. Whats the best way to check for leaks?
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Saturday, March 5th, 2011 AT 5:30 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,740 POSTS
The PO138 is an O2 issue (shorted to Voltage)
the PO171 is the lean mixture.

The sensor is where we need to start. First, this is a heated O2 sensor (HO2S). I need to know if this is a grand cherokee or one of the original body types so I can identify wiring schematics. What you need is a scanner that can read real time information. You will start the engine, allow it to warm up and then check to see the voltage that is produced by the HO2 sensor at an idle. If the voltage is 4.97 volts or less, replace the sensor. If it is more, we need to check for a short. NOTE: The wiring harness or wiring to the HO2S can be damaged, melted, corroded. And causing the problem. This is the 1/2 HO2S. Let me know which model vehicle you have and I can tell you where the sensor will be located. Also, let me know if you have access to a live data scanner.
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Saturday, March 5th, 2011 AT 5:56 AM
Tiny
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  • MEMBER
  • 7 POSTS
This is not the Grand its the original. I don't have a live data scanner. Would an auto parts store have one to lend out?
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Saturday, March 5th, 2011 AT 6:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,740 POSTS
If you have a nationally recognized parts store near you, they should have one. What they do is charge you for the tool and then refund the money when the tool is returned. Many of the stores will actually check it for you while you wait.

Let me know if you can get your hands on one. We need it to check what is happening.

Joe
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Saturday, March 5th, 2011 AT 6:10 AM
Tiny
HAZBRO
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Heated O2 sensor (HO2S) is this the one that is after the converter or downstream I mean where is it?
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Saturday, March 5th, 2011 AT 6:19 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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It will be before the converter. IT is an XJ body so the wiring to it should be tan / white.
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Saturday, March 5th, 2011 AT 6:29 AM
Tiny
HAZBRO
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  • 7 POSTS
I can't find any store that has a data scanner for loan. Should I just replace the sensor and see what happens?
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Saturday, March 5th, 2011 AT 5:39 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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It won't hurt anything, but I hate to see you spend the money if you don't need to. If you plan to do it, check the wiring first to make sure nothing is shorting out or the plug is corroded or damaged. If you do replace it, make sure to get some anti-seize on the threads of the new one. If you don't and have to replace it at a later date, you'll never get it back out.

If you decide to replace it, let me know the results.
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Sunday, March 6th, 2011 AT 3:29 AM
Tiny
HAZBRO
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Replaced the upstream sensor and the car runs great now. Perfect mpg and idles right. Thanks. It took about 3.5 minutes to replace. Theres a special tool to do it which they would lend to me for $29 instead I bought a 22mm combo wrench for $12 and cut the wires off the old sensor(the connector is to big to slide through) so I could get the closed end of the wrench on it so as not to strip it and it worked like buttah.
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Tuesday, March 8th, 2011 AT 8:49 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Great! Let us know if you have questions in the future.
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Wednesday, March 9th, 2011 AT 1:03 PM
Tiny
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  • 7 POSTS
Apparently I celebrated to early. After clearing the codes the cel came back on after 10 miles of driving. The 138 code popped up again.
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Wednesday, March 9th, 2011 AT 1:39 PM

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