04 Impala Issue...need help

I have a 04 impala with the 3.8 engine that I’m having some issues with and could use some help. Issues started about 2 weeks ago when the CEL came on for code P0107 Manifold Absolute Pressure/Barometric Pressure Circuit Low Input. Car was running fine with no issues then yesterday while I was on lunch break car justed died it would start back up but cuts of immediatly and will not run. I call a buddy to come and pick me up and I go back to work for about 5 hours. When I get off work go back to car and it runs great so I start home and after about 10 mins of driving samething happens. Luckly I was close to a parts store so I purchased a MAP sensor and installed it and reset the code P0107 but still would not run. Left car sitting in a parking lot over night and go back the next day and car starts right up and runs fine. I start home again and car doing ok for several miles and then the CEL starts to flash I get it home and check codes and it has the P0300 for multiple misfires. Now I have it home where i can work on it.

Car runs fine for about 15 miles then starts to act up. I have done some research and symptoms point to clogged Catalytic Converter. To test this theory can I just pull the upstream O2 sensor to releave the back pressure form the CAT ?

I have graphed sensor data for O2 sensor, MAP sensor , and also MAF sensor. Could not figure out how to post picture directly so uploaded a slide show to youtube and attached the link. https://www.youtube.com/watch?v=8LSPzXwSGzY&feature=youtu.be

Engine Cold…MAP sensor with engine off key turned on reading 1.0 ATM. Then started car and reading went to .42 ATM at idle and when I increase RPM to 2500 drops to .28 ATM I did not take pictures of this picture are from after car had warmed up.

Engine warm…MAP sensor at idle .35 Atm increase RPM goes to .28 Atm

O2 sensor car was warmed up to operating temp graph is in slide show but noticed when I increased RMP graph changed dramatically and graph was all over notice in picture one picture is at idle and the other is at 2500 RPM.

MAF sensor was also showing some funning readings …see pictures


I guess I’m showing my American upbringing here but I can’t help you unless your data units are in inches of mercury (inHg). I have no idea what an atm is in car-related terms. Also your O2 sensor data looks ok but your graph is meaningless without a voltage and time scale. MAF sensor readings looks like a good sensor on a poor running engine, but again without voltage and time scale hard to tell anything.

More important than individual sensors are fuel trim numbers, but I don’t think you’re chasing a fuel trim or exhaust issue. Your P0107 didn’t tell you the MAP was bad, just that it was reporting a reading outside of normal operating conditions.

Tell us which cylinders are misfiring and under what conditions.

Not sure what ATM is myself that is the units that was displayed on the graph. Code for miss fires states multiple miss fires so not sure what cylinder and only happens after driving for several miles aprox 15 to 20 miles then wont run at all. have to wait 5 to 8 hrs then will run fine for another 15 to 20 miles.

ATM…ATMosphere…maybe, I think, my guess is.


One atmosphere=14.7psi

I’m not interested in codes, codes don’t really help fix the car.

Your car will tell you which cylinder(s) is misfiring and how often/how many times in the past that cylinder has misfired. I would want that data, long term fuel trim, short term fuel trim, and other sensor data when the car is acting up. Does your scan tool (or whatever you’re using) allow you to gather all that info on one screen and give it to us? We need some more info to go on before deciding which direction to go.

One ATM, or atmosphere is 14.7 psi absolute. Since the reading is ATM absolute 0.33 atm absolute is 4.8 psi absolute or 20 inches/mercury vacuum. ((14.7-4.8) /0.491). 0.23 atm would be 23 in/hg vacuum.

Thanks @Mustangman . I know what an ATM is, but in automotive applications pressure/vacuum is measured in inHg or kPa. And those are numbers with which I’m familiar and can think about. I know that 10-12 inHg as reported by the MAP at idle is normal, I have no idea what normal ATM is.

I was right!!!

I win the free stack of pre-shrunk shop rags…just pay the
$19.95 shipping and handling.


I will check to see if I can get the log of misfires, and I can get the short and long term fuel trims but not sure if I can get it all on one screen. Can’t get much info when car is acting up because once it starts to act up it will not stay running, cuts off and will not start back for 5 to 8 hours.


If the vacuum line to the map sensor is clogged or cracked you will get bad readings which will lead bad running when the computer tries to correct the reading.