QuestionQUESTION: For a while my wife's car has been having issues downshifting from 2-1 (bucking sensation). Took it in, was told bad motor mount, and replaced that. Still happens. Now the car has issues idling at lights and taking off. The bucking (which also drops engine speed ~50-100 rpm) goes away when you shift to neutral. It seems like it has a lack of power when you start to drive it while it's bucking, also. Doesn't do it all the time, though. I've checked fluid level, looks good, tried a bunch of different things and haven't been able to fix it. Any ideas? Speed sensors, possibly?
ANSWER: It's not clear from your description whether this is an engine performance issue or a transmssion issue. Since it has a lack of power, I'm inclined to think it may be an engine issue. If you can answer the following, I may be able to develop a better idea of what's happening and give reasonable advice on how to address the situation.
1. Do you have a manual or automatic transmission?
2. Do you have a four cylinder (2.4L) or V6 (2.5L)?
3. Does the check engine lamp come on to indicate a problem?
4. You say the bucking occurs on a 2-1 downshift. Am I correct in presuming this downshift occurs due to throttle application? If not, please explain the circumstances of the downshift.
One thing that comes to mind that's a quick check is a loose or torn air bellows between the air flow sensor and the throttle body. This can cause a bucking on acceleration.
---------- FOLLOW-UP ----------
QUESTION: It's a V6 with an automatic transmission. The bucking occurs while braking to a stop (no throttle), around 5-10mph. No check engine light pops on, and no codes are stored, either. I stopped by an auto parts store today and asked them to check it out, they think it could possibly be the torque converter not releasing fully.
AnswerI think the auto parts store is probably right. Unfortunately, the problem could be in the valve body, the torque converter, elsewhere in the transmission, or with the TCM. While eliminating the TCM is relatively simple, figuring out where the problem is in the transmission can be tricky. My primary suspicion would be a defective solenoid (in the valve body), but on your car this will be expensive to repair. Unless you weren't able to read the transmission codes, I'm surprised you couldn't get any. Stop by a dealer to have it checked, but brace yourself for needing a solenoid or transmission. I recommend the dealer in this instance because, in my experience, they have a lower tendency to recommend a transmission overhaul or replacement when not necessary.
