: 2005 STS with Northstar is lunging under acceleration

Dr. John
11-05-12, 10:56 AM
I have a 2005 STS with the Northstar engine. It is not accelerating properly. This problem started with hesitation and bogging down under rapid acceleration. So I replaced the spark plugs with DENZO brand recommended by my auto parts store. I also replaced the fuel filter. Since then, the lunging under acceleration has gotten worse. It generally happens in the lower gears (1st and 2nd). The car will bog down under acceleration and then kick forward with an awkward lunge. This pattern continues repeatedly under acceleration. Once I reach road speed, and the transmission is in 4th or 5th gears, the lunging ceases. I need some help!

11-05-12, 11:46 AM
Failed torque converter. Theses an updated tcm cal that helps or get it tuned to help solve it.

11-05-12, 08:07 PM
Use only OEM sparkplugs. If you suspect the Torque Converter, try shifting manually.

I'd suspect a dirty throttle-body and/or faulty wiring at the under-hood connector. Early models were prone to it; there's a TSB on this as well.

Dr. John
11-08-12, 07:03 PM
My mechanic found a disconnected vacuum line on the air filter housing. Reconnected. Running like a scalded dog!