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Steve, ASE L1 Master Technician
Category: Jeep
Satisfied Customers: 5494
Experience:  25+ Years experience as a professional working automotive technician; ASE L1 master technician
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Jeep Cherokee Good luck on this one!!! 1998 Jeep Cherokee

Resolved Question:

Good luck on this one!!!

1998 Jeep Cherokee 4.0 (my own vehicle):

4 front O2 sensors and 1 rear O2 sensor failures in the past 7 months (happens within a few hundred miles of driving after replacement of each unit).

When units fail, Check Engine Light comes on with Code P0171 for ?Detected a Lean Condition?, Engine misfires at idle and at cruise speed, sparkplugs get carbon fouled and the (failed) O2 sensor is found to be carbon fouled upon removal. Oh, and mileage drops to about 9 mpg.

In trying to determine cause of O2 failure, we?ve:

Checked for vacuum leaks (none found).
Checked for exhaust leaks (found none).
Cleaned the Throttle Body (it wasn?t really dirty).
Replaced plugs, wires, cap and rotor.
Replaced Catalytic Converter (with stock-flow aftermarket unit).
Replaced MAP sensor (vehicle does not have a MAF sensor).

Checked Fuel Line Pressure and performed Leak down test. It had a full 49 lbs. of pressure at idle, but it FAILED leak down test: Upon shutting off engine, pressure dropped quickly down to less than 10 pounds ? should have stayed at 30 lbs. or above for 5 minutes. When supply line clamped off, pressure did NOT drop, so it?s leaking at the back, not at the injectors ? I think fast leak down indicates the combination Fuel Pressure Regulator / Fuel Filter has gone bad. Could this be causing the problem?!?!

First O2 sensor to fail had at least 50,000 miles on it and I think its failure was caused by Power Steering Fluid that had leaked all over the outside of it. Leak was fixed when first failed O2 sensor was replaced.
Next three replacements were Bosch brand exact fit O2 sensors.
Current (4th) front O2 is a OEM Jeep/MOPAR unit and it's already on its way out (the symptoms are returning and the Check Engine Light has come on, but then gone off after I ran the car at 3000 rpm for three minutes to clear the carbon fouling off the thing - the CEL light reset itself after this heat-cleaning of the O2 sensor, but it's only a matter of time.

Symptoms: For the first hundred miles or so after O2 sensor replacement, the car runs perfect and gets great mileage (17-19 mpg). Then, I start noticing a slight hesitation - when I take my foot off the gas at cruise speed there comes a slight jerking feeling as the engine rpm drops a little quicker than normal. And as I pull up to a stoplight, instead of smoothly going down to idle rpm, the engine dips down to about 400 rpm, stutters a bit, then climbs up to the normal 700 or so rpm. Then I hear it misfiring while idling.

I had read that Bosch O2 sensors and the Jeep electronics don't play nice together, hence the current MOPAR sensor ? but it?s now failing.

One last note: When a installed the new MOPAR O2 sensor a few days ago, I also installed another set of new spark plugs. After driving about 150 miles, I pulled them all out and checked them ? they were in perfect condition ? no fouling at all. Drove about another 50 miles, then I felt the symptoms coming back, and at about 235 miles post O2 sensor installation, the Check Engine Light came back on again.


Thanks for reading all this!
Submitted: 5 years ago via OnlineAutoRepair.
Category: Jeep
Expert:  Steve replied 5 years ago.


It seems you have some underlying rich running condition going on there, and you have checked most of the common things that can cause a rich mixture. Your fuel pressure while running is right on spec, so it is probably not a fuel delivery issue. Since the pressure loss after shut down is back through the supply line you probably do not have any leaking injectors causing specific cylinders to flood with fuel. The pressure loss IS something you should be concerned about, since it probably indicates a badly worn fuel pump, but I do not think it is related to your present concern.

On these engines, the 2 things that come to mind that could be causing a long term rich condition are:

(1) Coolant temperature problem. If the thermostat is out of calibration and not keeping the engine temperature up to 195F the computer will respond by increasing injector pulse width because it thinks the engine is still warming up. Likewise, a coolant temperature sensor that is out of calibration, a low coolant level in the engine, or high resistance in the CTS electrical circuit can all result in the computer commanding a richer than normal fuel mixture.

Try measuring engine coolant temperature directly with a thermometer, and compare it to your scan tool's temperature reading to make sure they agree and that coolant temp is above 195 when the engine is warmed up.

(2) The exhaust manifolds on these engines commonly develop cracks as they get older; this can allow air to enter the exhaust stream ahead of the oxygen sensors. The extra oxygen in the exhaust then will be picked up by the front O2 sensor as a lean condition, causing the ECM to increase injector pulsewidth to bring exhaust oxygen back into normal range. So what happens is that the O2 sensor ends up reporting a normal fuel mixture, when it is actually too rich. Carefully check for ANY tiny exhaust leaks upstream from the front O2 sensor; a smoke generator tool works well for this.

Steve, ASE L1 Master Technician
Category: Jeep
Satisfied Customers: 5494
Experience: 25+ Years experience as a professional working automotive technician; ASE L1 master technician
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