Not PT. Mom's DD is acting up... Need help!

[ej68bird]

Hey guys. Not sure if this is in the right forum but it seems close enough (Mods, you won't hurt my feelings if you move it).

My mother has a 2002 Chevy Impala, 3.4L V6, 4spd auto, 133K miles. Lately the car has been running very, very strangely.

Sometimes the car seems to "chug" under light acceleration around 40-60 mph. The car has no tach, but based on feel I'd say that the torque converter is locked and the engine is well under 2000 rpm. It feels similar to trying to acceleration a manual transmission car in too high of a gear. If you punch the gas pedal and unlock the torque converter it smooths out.



Likewise, at certain times the car has very little power. You have to mash the pedal at highway speeds and force the car to downshift into 3rd just to climb a shallow incline. Accelerating from a dead stop can be equally as frustrating. In addition, the transmission is sometimes very reluctant and hesitant to downshift. And of course the transmission sometimes shifts poorly as well. It's firm and jerks and jolts the car on both upshifts and downshifts (doesn't slip though).

The car has been pigging down gas. We usually get 30+ MPG freeway, even with the A/C on. On a recent trip I observed 18-20 MPG.

I've been pretty thorough with maintenance. Full tune up at ~100K. Replaced the battery roughly 6 months ago. Changed the fuel filter about 1500 miles ago. Pulled a few plugs the other day and they didn't look terrible. The resistance on all three ignition coils was fairly consistant (I don't know what the factory specified range is). The fluids look fine also. I am just not sure what it could be because there isn't a check engine light. I figure if it were an O2 sensor, MAF/MAP, coolant, cam/crankshaft position sensor, etc. the light would come on.

And to make things more fun, the car sometimes runs perfectly. It can be smooth as silk (for a Chevy V6) and run just as well as the day we bought it new. Other times it may exhibit any or all of the aforementioned symptoms. Because this problem is so intermittent and widespread, I think the problem may lie with ECM. But I'm really not sure and don't want to plop down a lot of cash for a new computer and fix nothing. And I don't really trust a mechanic to not rob me before figuring out what the actual problem is.

Any help and advice is greatly appreciated. Thank you.

[MrQuick]

Its very difficult to diagnose electronic controlled vehicles without getting scan data or seeing the car in person.

No check engine light is a big problem cause its either correcting itself or the failure is clearing the PCM.

I'd look to an egr valve control or a catalytic converter issue. I'd suggest shopping around for the best deal in a full scan diagnostics.


good luck