2000 Chevy Impala 2000 Impala Poor Mileage, Bad Accel, Har

Tiny
T0PWATER
  • MEMBER
  • 2000 CHEVROLET IMPALA
Engine Performance problem
2000 Chevy Impala 6 cyl Front Wheel Drive Automatic 100K miles

My 2000 Impala was bought used 4K miles ago and now has 100K. 3.8L engine.

Recently my average fuel mileage has dropped about 25%. The engine is louder at low speeds than it was before, and the acceleration is worse. I originally thought it was a clogged Cat and the backpressure was causing the sluggish engine. I also got a Cat efficiency code. I replaced the Cat last week and it half-corrected the problem.

I fear the problem was not the Cat but a rich fuel mixture or bad timing, which caused the plugged Cat. If I don't find the problem soon I'm afraid my new Cat will suffer the same fate.

There is another problem that I think is related; the shift to 2nd is delayed even on light acceleration, but I think this is just because the engine is producing low power. Other shifts are sometimes harsh as well. The trans is not slipping.

I get NO trouble codes and the problem is worse some days than others.

Things I've already done:
I've checked plugs and wires, coils and module, and they all seem to be fine. There is no hesitation or bucking (just steady low power), so I don't think fuel pressure/delivery is an issue. I checked the O2 sensor with my DMM and the reading was normal (0.1V-0.9V with rapid fluctuation). I listened to all the injectors with a screwdriver and all are clicking steadily and clearly. I've tested intake vacuum (at the fuel pressure regulator) and it's good at idle and with throttle changes when the car is still cold. After the car is warm, the vacuum reading begins to wiggle tightly about 1 inch. In both cases, the level is good at 20 inches at idle, 0 when throttle is snapped open and 25 when shut. I can hear the Idle Air Control valve clicking somewhat when the key is ON but engine off (possibly normal?).

I don't know what could be causing the poor mileage/ bad acceleration. Please let me know what other things I can check to pinpoint the problem.

Thanks,
Steve
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Sunday, October 5th, 2008 AT 5:04 PM

1 Reply

Tiny
JACK42
  • EXPERT
Aside from actually verifying fuel pressure witha gauge, the cat may have fallen apart and caused a restriction in the exhaust syetm farther down. Try loosening the head pipe before the cat and driving it
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Monday, October 6th, 2008 AT 8:06 AM

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