Chrysler Jeep P0171 and P0172 simultaneously

[justbrosing]

Lean and rich at the same time? I can believe rich because my 2.5 is probably averaging 8mpg rather than a best of about 15. I performed an ignition service, installed new NTK oxygen sensors, replaced the manifold gasket, and went through the vacuum system. The fuel tank is clean, the pump is a recent replacement from Carter, at least that's what the box said, and both fuel and air filters are recent as well. The converter is the free flowing type now, and everything seems to be in order, but still I get terrible fuel economy despite being perfectly reliable to drive.

I've had maybe six months without a check engine light and never had better fuel economy than a 4.0, just with half the power. Bonus code today is a P0133, slow response, O2 sensor circuit, bank 1 sensor 1. Probably packed with carbon.

It leaves a nice black spot in the driveway on cold starts where the tailpipe points towards the ground.

I have a Snap On Solus but I'm always learning more about how to use it.

I'm not interested in spending much on it, just waiting for a 4.0 donor to come along for the right price, but it would be nice to make it run right in the meantime. It is a zippy little engine to the top of second gear. If I can be pointed in the right direction to understand better with the scan tool, I'd sure appreciate it.

Otherwise, I can only offer a sense of pride in answering something the Jeep forums could not.

[1project2many]

An oxygen sensor generates voltage based on the difference in oxygen level between the exahust and ambient air. If there's more unreacted oxygen in the exhaust the sensor generates a lower voltage signal. The source of unreacted O2 can be an exhaust leak, unsufficient fuel, or poor or nonexistent combustion.

Lean and rich from the same sensor seems contradictory. But they are both possible. There are a few different scenarios where this can happen. If the AFR for an engine is correct or slightly rich when an engine is cold, but becomes more and more rich as the engine warms, you will see rich O2 readings as the sensor warms and begins to send a signal. If the AFR further deviates from correct its possible a spark plug will begin to misfire. Each misfire causes a higher concentration of unreacted O2 to pass the sensor. The sensor will report the lean condition and the ECM may attempt to increase fuel delivery. The AFR is already rich so the spark plug continues to misfire. The ecm could set a rich code before the misfire and a lean code after.

You haven't given the year of the Jeep but combination of code plus Solus indicates OBDII. I have not used Solus but I have used Modis, the predecessor. If you are able to see the datastream I believe it would be good to do basic data validation.

A faulty coolant temp sensor could create the scenario I described above by failing to report temperature rise properly. Start the engine from cold and compare coolant temp reported by ecm to coolant temp at the engine. If you don't have an infra-red thermometer you may be able to measure by feel at a heater hose. 100 deg F feels neither cold nor warm to the skin. 130-140 F is a little uncomfortable to most. 190-200 F feels uncomfortably hot for most poeple. You may be able to hold a part at that temp for a short time but you'll likely feel discomfort after letting the part go.

A faulty O2 sensor can also generate incorrect voltage. You might also try disconnecting the O2 sensor before starting the engine. This should prevent the ecm from commanding closed loop forcing the engine to use "basic" programming. If you're seeing black smoke once the engine is warm then you can conclude the O2 sensor is not the issue.

I believe MAP sensor failure is somewhat common on Jeep products. a faulty MAP sensor could report too little vacuum which will be interpreted by the ecm as high engine load. While it is possible to measure barometric pressure and engine vacuum and work out MAP voltages, many techs like to keep a known good sensor in their toolbox to make a quick check.

Regardless of what you find, it's very likely the plugs are heavily carbon fouled. Carbon will definitely affect the plug's ability to spark by providing alternate paths to ground. You may want to remove them for cleaning or replacment. If there is another problem this will only help temporarily but if you do find a root cause then having new or clean plugs will really help the engine run at peak performance.

[justbrosing]

It's a 96, surprised I left that out. It's late, but I have a mind now to clean the plugs in the morning, along with the upstream O2, and replace the manifold donut which I suspect but have never inspected. Being a cast manifold, I don't expect it to be cracked, but it isn't hard to wrap a hand around the junction after a cold start to feel for leaks prior to heating up. I expect fouled plugs, but somehow it has been running like this since January without any real change or misfire codes. It's a backup vehicle I really only drive when I absolutely have to, and at times I've put substantial amounts of work into it, so when this popped up, I kind of gave it the finger.

No clue where my IR thermometer wound up, but prior to misplacing it, I verified the gauge in the dash is surprisingly accurate when read at the back of the head. The temp sensor is not a recent replacement, but within the last four or five years. IR reading at the thermostat housing was the same as the scan tool within a few degrees, but that's been a while now.

I drove my first 96 for fourteen years, started driving a few different ones over the past few years, and never encountered a MAP issue beyond a broken vacuum source. This is and has been my only 2.5, but I have at least three 96+ 4.0 XJs I could use a known good MAP sensor from, if it will serve adequately on the four cylinder. That said, I don't drive anything newer than 96 so my experience kind of ends there. Jeeps got a pretty heavy Dodge Neon treatment after that, and in my opinion went downhill fast from there. The MAP was relocated to the side of the throttle body around late 95 and sourced with a 90° elbow, which I have seen those break on later Chrysler vehicles.

Otherwise, it's past my bedtime and I'm probably forgetting a lot. I appreciate the input and will return tomorrow with some items eliminated and data stream figures fresh in my mind.

[steveo]

usually when you get two conflicting codes that don't make sense you start looking for root causes like a crappy ground somewhere, especially on a 4.0 jeep that usually has a really odd ground configuration. it's probably not going into closed loop anymore with those codes set which could explain why you have awful fuel economy. if it is going into closed loop, it would be good to see what your trims are doing while driving.

i have a 2000 jeep with a 4.0 and i've been forced to learn a lot about it but it's not a lot of fun to work on the computer side of things, that's for sure

what are you using as a scanner/datalogger?