So, last week, during the hottest days of the year so far, my car had three separate occasions where it jolted forward over 4 days, and I am wondering if anyone can help me figure out what could be wrong.

What pretty much happened was that the car would be at a low speed and when I'd start to accelerate, the car would jolt, this happened twice going uphill, and one on level ground, as I was going under an open fire hydrant (if that matters?).

I imagine it could be any number of things, but it hasn't done this since, and it only happened in the city. Between the occurrences, I drove on the highway and there was no problem for 50 miles. The day after that, I go through stop and go traffic in 100 degree weather and the jolt reappears. Any ideas?

This could be caused from an oxygen sensor, or a faulty spark plug or wire. If its on a Chevey impala it may be a solenoid in the transmisson that maybe faulty. If it only happens in the city my guess would be oxygen sensor but I would have a shop check them out because they are not cheap to replace on a guess. I would change the plugs and wires if you havent had a tune up in awhile. Change the fuel filter, air filter plug and wires. Thats only if you have'nt had a tune up in awhile

The plugs and wires were changed last fall and the rear o2 sensor was changed a year and a half ago, but all of the other things haven't been. I've been wondering if it was all those things as well, but I also want to throw in the possibility that it could be my catalytic converter? Theres a code for it not reaching the threshold or whatever not being reached, could the cat be done and/or fouling the o2 sensor? I'm tired and have work to do tonight, but I'll go out tomorrrow and see what codes there are.

Is the SES light flashing? Do you notice a lack of power when trying to accelerate?

I really don't think an O2 sensor could cause such an erratic condition that your car is exhibiting. Usually they just fail and you will notice a difference in the smell of your exhaust(rotten egg) maybe and a difference in fuel mileage.

Also, if you do have codes, we pretty much need them before we can continue troubleshooting.

I agree with Mike if its an 02 sensor it would be faulty all the time and not just everyonce and awhile. Also you could check your coil packs terminals for rust or corrosion which would indicate a loose connection causing loss of power/

Another thing that can caused jolting when accelerating from a low speed is the throttle position sensor. Here is an excellent article on trouble shooting the TPS:

Part 1 -Throttle Position Sensor (TPS) Test: GM 3.8L (P0121, P0122, P0123).

Sorry its taken a while to get back to you guys. The codes I have are po128, po420 and po446

P0128. Coolant thermostat. The PCM is detecting that the engine is not reaching optimum operating temperature. Possible due to thermostat stuck open.




A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• A damaged or failed oxygen sensor (HO2S)
• Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• Leaking fuel injector or high fuel pressure
• Cylinder misfire
• Oil contamination

And your P0446

Faulty vent valve
Open, short or excessive resistance on Vent valve control circuit
Blockage of vent valve

so i would change the thermostat and coolant, clear the codes and see. the o2 and running weird could be from not being up to temp.

the vent valve probably needs cleaned and shoot some wd40 in and actuate it with a battery a few times see if it works