Car seems to stumble a little, like flat spot in throttle

[cougar694u]

My mom has a 2002 Impala with the 3.8l and it runs really funny. I drove it and it sucks! It starts fine, idles fine, accelerates fine under Wide Open Throttle (WOT).

The problem occurs when you're cruising. When you take off from a stop, it does fine in first, but you can feel some hesitation, almost as if it's missing just a little. Then, after the 1-2 shift, it seems to fall a little, and has very little acceleration. Then, after the 2-3 shift, it falls on it's face. You can't accelerate unless you go to WOT, at which point it down gears and hauls ass.

My initial thought was the TPS (throttle position sensor) was bad because it seemed like flat spot in throttle response. After hooking it up to my dad's snap-on diagnostic computer, the TPS checks out fine by showing complete smooth sweeps from 0-100%. We did a full tune up on it, changed the MAF (Mass Air Flow) sensor, had the PCM flashed, but nothing.

The dealer told my mom the cats were bad. Under my experience, when cats go bad, the motor has trouble at high RPM's not low RPM's. When it's stumbling, you can go to WOT and it'll down gear and take off, running just fine. Where as with clogged/bad cats, it would build up too much back pressure at high RPM's.

I read a lot of posts with similar issues where people changed the cat and it took car of it. I just want to know if those people were having the same issues as what I described above.

Any ideas are greatly appreciated!

TIA!

[ogre73]

Sure sounds a lot like the cat may be a contributor. See my post here: http://www.automotiveforums.com/vbul...d.php?t=439933.
Jim

[cougar694u]

Thanks for the info, I'll check into it. Unfortunately that car's got 106k miles, so no dealer replacement. It just didn't seem like the cat to me because at WOT it still hauls butt.

[ogre73]

Originally Posted by cougar694u

Thanks for the info, I'll check into it. Unfortunately that car's got 106k miles, so no dealer replacement. It just didn't seem like the cat to me because at WOT it still hauls butt.

Yeah, mine still went fine when I gave it the gas, and even seemed to "come out of" it's fit and downshift. When it did it went like a bat out of hell. I know that my car sees a lot of highway mileage, and little idling while still, but I think what contributed to my problem was a spark plug issue a while back (see http://www.automotiveforums.com/vbul...d.php?t=354316 for details on that). Something like a misfire problem will clog up a cat real quick, so if you have had that or something else wrong in the last 30-40K miles, I would start wondering.
Jim