Hey everyone. Im having some trouble with my truck. It has started to run rough. It seems to be ok when its cool, but after it runs for about ten minutes it starts to idle rough, have low power, blowing black smoke and is throwing a lean code which i thought was odd. Black smoke should be excessive fuel so why is it that I am getting a lean code P0171. I have checked the map sensor, it tests well within its parameters, tps is checking good, the upstream o2 sensor appears to be operating normal when looking at it on my buddies scan tool. The engine has plenty of vacuum, its running at 18in of vaccum. compression test shows all cylinders at 120 psi except one is at 112, it does have 199k on it, im at my wits end with it, any ideas would be greatly appreciated.

 

failed to mention that all of the spark plugs are covered in black soot as well.

 

Unplug the upstream O2, remove it from the pipe and drive it, see if that makes it run differently. I would think if you are blowing black smoke and have fouled plugs your O2 would be covered in soot as well and likely soon to fail.

Have you cleaned your throttle body lately? Could have plugged passages to the IAC (Idle Air Control) and MAP. I would think if you are throwing a lean code you have a bad reading on an O2 sensor, plugged passage to the IAC or MAP and it is dumping fuel to compensate, leading to the fouling of plugs and rich running condition. Any vacuum leaks? You mention having good vacuum per the code reader but you could be losing vacuum away from the intake?

Pull the IAC and clean it along with its port in the throttle body. If you can I would remove and clean the throttle body thoroughly, removing all sensors prior to scrubbing it down. Put it back together, replace those fouled plugs and see how it runs. If still rough I would point to a bad O2 sensor.

Have you done a tune up lately? Plug wires, cap & rotor? Cleaning the TB is the quickest and cheapest place to begin prior to throwing parts at it. If it starts to run better after that, replace the plugs and see where you are at. Then you can likely diagnose it further from that point and narrow down the issue.

 

I think the front O2 sensor has ALREADY failed.....

 

First off thank you for the speedy response to both of yall. I did remove the front o2 sensor and it changed nothing on the way it runs. The map sensor i did have removed, checked it using a mulitmeter and vacuum guage. it was reading normal at 5 in 10 in and 15 in. it has had a full tune up done about 20k ago, i did put a new set of plugs in it a few days ago since all of this started happening. the throttle bore i cleaned the other day when i put the plugs in it but did not check the vacuum port for the map sensor. the vacuum reading i was getting was on two different ports on both sides of the intake and one at the throttle body, vacuum was 18 at all ports. im scratchin my head alot at his point. was wondering if the computer could be doing this but i have no way of proving it so im a bit gunshy to just throw a computer at it.

 

Next step would be to have a look at the temp sensors. You have two, a one-wire, to the passenger side of the thermostat housing, that is only for the gauge. On the drivers side of the housing, is a two-wire sensor the PCM uses. If that has failed, PCM thinks the engine is -40 degrees, and positively DUMPS fuel. Since you have a meter, and know how to use it, test the temp sensor next.

 

No prob, HeyYou is a great resource on here and has some great info. Don't replace the PCM yet, that is an expensive "maybe fix" without doing a proper diagnostic. I am with HeyYou on checking the coolant temp sensor, certainly could be giving the PCM bad info and causing your rich condition. Especially since you have already completed all of the tune up info.

 

i will check into that and get back to you. Thanks again. Would make sense seeing as how it is certainly worse the warmer the engine gets. Runs pretty decent until its about half warmed up when its gettin close to running in closed loop is when it acts up the most.

 

Any luck my truck is doing the same thing. It's a 99 durango 5.2