Dodge, Chrysler, Jeep PCM Strategies, sensor inputs

Thomas Diagnostician Cleburne, Texas Posted   Latest   Edited  
Discussion
Driveability
2007 Jeep Grand Cherokee Limited 4.7L (P) 5-spd (545RFE)
P0300 - Random/Multiple Cylinder Misfire Detected
Runs Rough
Misfire
Bank 1 Lean
Bank 2 Lean
Black Smoke

Hello all,

Have a 2007 Jeep grand cherokee with the 4.7L V8. It will set p0300 random misfire and when you check the what cylinder is misfiring in the scan data it shows bank 1 and bank 2 misfiring with a pretty high count on both sides. Before we do further testing, the one thing I can't seem to find enough information is the pcm strategy for fuel on this vehicle. I see that it does not have a MAF sensor. But on the scan data it shows mass air flow g/s. Even though I know this vehicle does not have a maf sensor only a map. With key on engine NOT running the MAP sensor reads between 12 inHg to 16 inHg. I even used a vacuum pump that I have to try and trick the sensor reading. Shouldn't the key on engine off there should be ZERO vacuum. Or roughly said HIGH pressure? Since there is no vacuum shouldn't I read Low vacuum High pressure? Meaning 0inHG at least? there is no manifold vacuum with the engine running only atmospheric pressure. If I apply vacuum to the Map sensor, the signal coming out going to the PCM does change according to the vacuum applied to the sensor the voltage does decrease to almost 0V the more vacuum I apply to it. The only reading that does not change is the vaccum inHG it stays the same. I've read somewhere roughly that for every liter over 3.0L its 1 gram per second of airflow. So this should flow between 5-6 g/s at 500 RPM. Now when the vehicle is running the g/s reading I get is between 9-11 g/s at 500 RPMs. To me that seems a little higher than it should be. I'd like to not throw Oxygen sensors, and PCM and just parts at it. The airflow coming in just seems a little to high for me. The vehicle runs rough pretty much all the time. Tried a new map sensor for testing purposes and still the same. It will start to run rough, and bank 1 and 2 will rocket to +25 to +35 positive fuel trims. And the exhaust is black smoke when you give it gas. I feel as though the PCM thinks more air is coming than it should and therefore dumping more gas. Its just an observation could be totally wrong about what the airflow at idle in g/s should be in this vehicle. I've done so many mass airflow sensor systems that those systems just seeming easier to pin point the rough running. This Jeep is just showing a MAP sensor and tps for the inputs and o2 for the fuel strategy as well. I'd like to eliminate first and keep testing until we have eliminated all possibilities and one is left! I forgot to upload the scan data but I will do that soon. Thanks in advance. 

UPDATE: So I have put in the readings from the scan tool. 

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Hi Thomas, Many times vehicles use inferred values to represent sensors not present. In this case based on the manifold vacuum compared to barometric readings and RPM a calculated value for MAP is derived. For fuel trims I would carefully look at both front and rear O2 values. If the rear O2 sensors are whacked it will effect the fuel. The MAP readings are based on an absolute value. Meaning…

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Randy Diagnostician
Glendora, California
Randy Default
 

Subtract the koef to the keor to get your true vacuum readings

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Thomas Diagnostician
Cleburne, Texas
Thomas Default
 

Thanks Michael, I really appreciate your input. That makes sense. So is it safe to say that the MAP reading in inHG is more of a calculated value than an absolute real time value? The calculations are based of barometric pressure readings and rpm and tps readings? I have attached a picture from my scan data. Shouldn't my barometric pressure and map vacuum be the same 29 inHG only when the…

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Michael Mobile Technician
Clinton, Utah
Michael Default
   

Thomas, What I meant was that the MAF numbers were inferred by a combination of sensors. So if it is showing 4.5 g/h that is a calculated value. The MAP value should be the same as baro with the engine off. The MAP will change based on throttle position and RPM when running. Sometimes the PCM will substitute values if the sensor gets too out of line. If the MAP is reading as you have…

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Thomas Diagnostician
Cleburne, Texas
Thomas Default
   

Thanks for the helpful insight.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
   

Like Mike said "The MAP readings are based on an absolute value. Meaning you have 29.92 inHg at sea level without engine running". If yours says 12 or 16 Hg" it is broken, or there's a wiring problem, or your scan tool is confused. Most Jeep don't show a MAP value in the data list though, they show "Manifold Vacuum". Switch to Generic OBD2 and see what it reads for MAP.

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Robert Mobile Technician
Grand Rapids, Michigan
Robert Default
 

Lots of good comments here. Personally being your fuel trims are all positive in nature and you do not know how long this is going on and we don't know the full story. I would start over on your diagnostic approach. Reset the fuel trims either with the scanner of a global disconnect. Do a clear flood on the vehicle and listen to the cadence of the motor to give you a base line if there are…

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Jason Technician
Hawthorne, New Jersey
Jason Default
 

Hey Robert I'm a little confused on the disconnect part u suggest. Why not just give a couple of rapid snap throttles while watching the 02 sensors to see if it reacts to rule out that maybe the 02s are stuck. Maybe the 02s are seeing a lean condition and computer is reacting to adding fuel. Sometimes these 02s are not comfortable to disconnect on a hot exhaust. But Thomas, as Mike and Goeff…

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Thomas Diagnostician
Cleburne, Texas
Thomas Default
 

Thanks Geoff, Now correct me if I'm wrong but shouldn't the barometric pressure readings and map readings be both the same with the key on engine off? Since there is no vacuum and its just atmospheric pressure. I live in Texas which is sea level. My map vacuum is reading 0 inHG and my barometric pressure is reading 29 inHG. Both of these values should read the same? I have checked the 5V on…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
   

That's the thing with Chrysler, they say "MAP Vacuum" which confuses everybody used to reading "MAP" on every other car. The "MAP Vacuum" should actually read the same thing as a good old regular vacuum gauge. So 0" Hg at KOEO and probably 18-20" Hg if the engine runs well. Also note that the "MAP Voltage" is showing a true MAP (as opposed to vacuum).

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Randy Diagnostician
Glendora, California
Randy Default
 

True . So what is ecm calculation for this . To eliminate ecm you should check engine cranking vacuum on a gauge and scan data

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

This engine you are working on is a speed density engine. The MAF value you see on the scan tool is an inferred value. All speed density engines must calculate MAF to come up with the air mas for fuel calculations. KEOE should show a MAP pid of baro. and is read in inches of HG. Once the engine is started this value, of 29 or so, should drop down. If your engine vacuum measures 20"HG, then the…

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Thomas Diagnostician
Cleburne, Texas
Thomas Default
 

See now that makes perfect sense. My scan data is showing barometric pressure at 29 inHG and map vacuum at 0 inHG with the key on Engine NOT running. And I'm getting a airflow reading in g/s of about 1.7 g/s. Don't know if this is normal or the PCM is corrupt or the software has lost its ability to calculate properly. I have attached a picture of my scan data with the engine not running. I have…

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Any time you suspect a PCM error, test the voltage at the sensor, and compare the real voltage with the scan tool voltage. If they are not the same you have a problem. This sounds a little confusing (MAP vacuum) I would imagine it means manifold vacuum. Any time I suspect something askew with PCM calculations, like you are having, I will pull up the fuel trim data and let that give me a…

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Randy Diagnostician
Glendora, California
Randy Default
 

Have you tried using a regular vacuum gauge to compare scan data to it . Looks like computer is interrupting wrong vacuum

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David Diagnostician
Candler, North Carolina
David Default
 

What are your rear /downstream O2 sensor(s) reading?

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Thomas Diagnostician
Cleburne, Texas
Thomas Default
 

Now they are switching but on my scan data it showing voltage from 0-1V and then its reads around 3.2V but where I get confused is I get two readings of voltage for the o2 sensor. Do I pay more attention to the 0-1 V or the 3.2V? For the higher voltage is anything close to 5V lean? and anything blow 3V is rich?

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Jj Technician
Quincy, Massachusetts
Jj Default
 

Chrysler O2 sensors have a 2.5v bias voltage. So the 3.2v is actually .7v on the 0-1v PID. I believe the 0-1v PID just subtracts 2.5v from the actual sensor reading, so if you wanted to just read the o2 pid just keep in mind that you're starting at 2.5v so that would be your zero mark. I'm sure there's some kind of reason they put both in there, I don't know it, but I'm pretty certain they show…

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Caleb Technician
Mishawaka, Indiana
Caleb Default
 

Jj is correct on the 02 operation. One thing I wanted to point out though. The ground side of the signal circuit for the 02 sensor is 2.5 volts above ground (0volts). This allows for proper 02 sensor operation even if say there is a voltage drop on a main PCM ground. The 2.5 volts on the 02 sensor ground circuit never changes. The signal circuit for the 02 sensor is a 5v ref to the 02 sensor. If…

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Caleb Technician
Mishawaka, Indiana
Caleb Default
 

And just a tip man. Your killing your eyes by not selecting certain data pids. Select the four 02 sensor (0-1v) pids and both long terms. This will make it a lot easier to read and understand. Also on the MAP issue. Chrysler displays MAP in 2 ways. They display the actual MAP value( BARO minus actual intake vaccum as read with a gauge.) Then they display the actual Manifold vaccum as you are…

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Jeffrey Mobile Technician
Winthrop Harbor, Illinois
Jeffrey Default
 

So far everything in your scan data looks ok to me. Your MAP vacuum is just calculation and shows manifold vacuum. You may or may not be able to get actual MAP on the scanner. Your gps reading looks normal also. Every Chrysler I have seen has a higher grams per second reading than a vehicle with an actual MAF. I tend not to use that pid too much. With the gps reading koeo that also may be…

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