99 4.0 rich, black smoke p0171 code - JeepForum.com
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post #1 of 5 Old 11-07-2013, 01:51 AM Thread Starter
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1999 WJ 
Join Date: Feb 2012
Location: Miami
Posts: 26
99 4.0 rich, black smoke p0171 code

Okay, here we go. Jeep was having intermittent stalling, especially at speed. Swapped out the CPS and it fixed that problem. At the time I also had a code for the front O2 and the MAP sensor. So I reset the computer and the P0171 code came back. Jeep is using a lot of fuel, there is a lot of black smoke coming out of the exhaust with hard acceleration and it's missing at idle. I had recently changed the MAP sensor so I swapped the old one back in to see if there was a difference, there was none, I ran a 1/4 can of throttle body cleaner through the intake, a bottle of fuel injector cleaner, and reset the PCM. P0171 came back, so I swapped out the front o2 sensor that I changed about 4 months ago and it was black of course. Reset PCM, stomp on gas, still have black smoke.

I checked fuel pressure and it is around 45PSI and seems to stay there when shutting off engine. Right now I'm leaning towards faulty or leaky injectors. Let me know if you guys think this is the direction I should take (gonna check plugs when I get a chance to try and narrow things down)... I also banged on the cat and heard no rattling, it is not discolored and there is a strong exhaust (doesn't seem clogged). Jeep has 185k miles so don't know if the rings are bad but I could probably borrow a compression test gauge and do a wet comp. test. But I'm pretty busy and my girl needs the WJ for work too.

-Some maintenance done in the last year:
Put in a new tranny a year ago and have changed many many things since then. No fuses are blowing, electrical connectors seem OK. Fresh oil change with rotella syn, pure one oil filter, spectre air filter, new IAC, TPS, MAP, CPS, plugs, front o2, fuel pump, fuel filter, let the injectors sit in carb cleaner overnight just in case and put new o-rings. Checked and replaced any bad vacuum hoses, checked and cleaned all electrical/sensor connections and checked for corrosion. The list goes on and on, radiator, condensor, starter...

Anyway help a brotha out! What could it be now?? A couple of the sensors are cheap off ebay but voltage readings were fine, no codes, and aside from maybe TPS I don't see what could be causing it.

So in order of what I think could be wrong
Fuel Injector
Exhaust leak (I checked obvious places but will inspect further)

What sayeth the forum?

madmanmarz is offline  
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post #2 of 5 Old 11-07-2013, 01:34 PM
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1998 XJ Cherokee 
Join Date: Jul 2013
Location: Coleridge
Posts: 133
Idk dude, but you are heading in the direction that i would. Start looking for exhaust leaks 1st. Injector could be bad to though, since missing at idle. Is it a consistant miss? Or random?
2tall2slow is offline  
post #3 of 5 Old 11-07-2013, 02:35 PM
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1992 XJ Cherokee 
Join Date: Oct 2012
Location: umatilla
Posts: 1,961
I agree with 2tall injector stuck open. If its #1 then just simply try swapping it with one of the others. That'll tell you if the injector itself us the issue or if the ecm is bad.
Had this exact problem on a 90 xj and a 99 cavalier. Ecm on both.
You have just 1 o2 sensor right?
Go over you vac lines.
check the egr valve.

uberxj92 is offline  
post #4 of 5 Old 11-07-2013, 06:44 PM
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1999 XJ Cherokee 
Join Date: Dec 2011
Location: Kingston
Posts: 306
P0171 JEEP - Fuel System 1/1 Lean

Possible causes
- Intake manifold leaks
- Exhaust manifold leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Incorrect fuel pressure
- Lack of fuel
- Incorrect Positive Crankcase Ventilation (PCV) hose connection
Based on what I can find about P0171, the code is thrown when the front O2 consistently reports a lean condition. Since you are blowing a ton of black smoke this is obviously not the case. Is the wiring from the front O2 in good shape? Fuse for the O2 heater circuit good?
holysirsalad is offline  
post #5 of 5 Old 11-07-2013, 10:22 PM Thread Starter
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1999 WJ 
Join Date: Feb 2012
Location: Miami
Posts: 26
Misses are random

Have gone over vacuum lines. There are 2 o2 sensors but only the front should cause adjustment in the mixture. Have not checked EGR, I'll take a look.

Fuse is good, wiring is good.

It is my guess that the PCM thinks there's a lean condition and is dumping fuel.
I will take a real good look at the exhaust manifold on Saturday for any cracks but a quick check found nothing and exhaust isn't loud either.

Update, o2 sensor swap seems to have fixed the problem but the random shutoffs are supposedly back. Gonna check the PCM connections and maybe replace CPS with mopar.
madmanmarz is offline  

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