2000 Chevy Impala open/closed loop

2000 CHEVROLET IMPALA
186,000 MILES • 6 CYL • FWD • AUTOMATIC
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MLIPHART
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I have been working on my daughters 2000 Impala, and it runs fine at an idle, or at speeds under 50 mph. When you attempt to get on the freeway, or even on a 55mph road, the car soon begins to cough & choke. I have checked the fuel pressure, and it is within in specs for the KOEO and at idle.

I borrowed a scanner, and hooked it up while driving the car. The only thing that I have found is that when the car begins to act up, it switches from Closed Loop to Open Loop. I put the car in neutral, and let it idle, it comes right back to Closed Loop, and the car drives normal for a bit. But, if I try to continue to drive the car, it stays in Open Loop, and continues to choke & misfire.

The car has new plugs & wires, a new catalytic converter, and the fuel pump was replaced a few months ago. I have been told that it may be the ECM. Is there a way that I can be more sure, before I start spend her money on a wild goose chase?

Thanks,

Matthew
Apr 20, 2009 at 9:09 AM
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DOCFIXIT
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Where they any DTC's present? if so what are they?
Apr 20, 2009 at 11:32 AM
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MLIPHART
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No there were no codes present.
Apr 20, 2009 at 2:58 PM
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DOCFIXIT
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Why was cat replaced rich fuel, miss ? Where O2 sensors replaced? Going out of loop indicates car goes into limp mode. With no codes need to have a diagnostic scan done what you spend on that could save $$$$
Apr 20, 2009 at 3:07 PM
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MLIPHART
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The cat was replaced because it appeared that it was at least partially plugged. I had a local exhaust shop tell me to disconnect the exhaust from behind the cat (if possible). Fortunately, there is a 2 bolt flange directly after the cat. He said that I should notice a marked increase in the sound of the vehicle with it not going through the muffler. I could tell very little difference.

Then he suggested that I drill some holes in the front of the cat to decrease the back pressure to see it that made any difference. It did make a difference - because before doing that - we could barely get 40 mph for very long. After replacing the cat, I looked inside of the old cat, and it was beginning to disintegrate.

We have not replaced any of the O2 sensors yet.
Apr 20, 2009 at 9:58 PM