2004 impala engine runs rough, cold engine

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kwmech
Posts: 2
Joined: Thu Dec 06, 2012 7:39 pm

2004 impala engine runs rough, cold engine

Unread post by kwmech » Thu Dec 06, 2012 7:47 pm

3.8 supercharged. Start up engine, runs about 30-40 seconds. Scanner shows going into closed loop. Short term fuel trim shows that the computer is taking fuel away and of course engine starts stumbling, then the Mass Air Flow starts dumping in air. As soon as the temp goes above 142 degrees, and you blip the throttle to clear it out the engine smoothes out and runs great. Shut off engine, wait a few minutes for the engine to cool off....same thing. If the temp stays over 142-144 still runs fine.
Temp sensor show ok thru the scanner and shows correct.......


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ProTech
Posts: 651
Joined: Wed Oct 17, 2012 10:08 am

Re: 2004 impala engine runs rough, cold engine

Unread post by ProTech » Thu Dec 06, 2012 9:56 pm

Probably a Mass Airflow Sensor, these are notorious for causing problems like this on all of the 3.8 engines. Maybe keep an eye on the grams per second that the sensor is reading and give it a few taps with your fist or the handle of a screwdriver and see if it glitches out. Even if it doesn't, it can still be bad.

Otherwise, check for the obvious such as vacuum leaks, fuel pressure regulator leaks, and misfires due to ignition components or other.

If you do end up replacing the MAF sensor, do yourself a favor and pay the extra money for a genuine GM part. If you get a MAF at Pep Boys or Autozone or the like, you'll end up replacing it over and over again....trust me.
GM Dealer Technician For 18+ Years
In the automotive industry for 20+

kwmech
Posts: 2
Joined: Thu Dec 06, 2012 7:39 pm

Re: 2004 impala engine runs rough, cold engine

Unread post by kwmech » Fri Dec 07, 2012 11:14 am

Cleaned out the old one, no difference. Did replace with a factory unit P.N. 19179715. Actually made the condition worse/more severe. With the new one it ran the short term/trim up to -27. So what can be causing the computer to run the engine lean under 142 degrees. Fuel pressure is constant. Engine has 104k on the speedo. I might add that this vehicle has been doing this for about 3 weeks and has yet to trip the check engine lamp. We did a major service/tune-up at 100k thinking it would eliminate the problem------needed it anyways.

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ProTech
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Joined: Wed Oct 17, 2012 10:08 am

Re: 2004 impala engine runs rough, cold engine

Unread post by ProTech » Fri Dec 07, 2012 7:00 pm

Cleaning the old one isn't going to make a difference if it's bad. And if a new one made the problem worse, maybe you got a bad new part in the box?!

Also, you've got it wrong...if the short term fuel trim is going into the negative, it's showing RICH, not lean. Plus, there is no way that problem would not set a trouble code, maybe your scanner is not adequate for the vehicle.

Have you taken a fuel sample? Maybe it's bad gas. How about a fuel pressure reading? How about checking the fuel pressure regulator? Since it's so rich, how about a restriction in the air intake somewhere, like a clogged air filter or something blocking the tube? Maybe you've got a fuel injector dumping gas constantly...etc..etc..etc...

Obviously lots to check, you just need to understand what you're looking for....and that's a RICH problem, not a LEAN problem!
GM Dealer Technician For 18+ Years
In the automotive industry for 20+

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