I've posted wayyyyyyyy back about my engine "hiccuping" and sometimes stalled on me. I pulled the error code and found out that it was O2 sensor, however, when I took it my mechanic, he found out that my timing belt was loose. It was so loose that I could grasp the thing flexes until it hits the casing. So I had him replaced my broken water pump and the timing belt. This worked fine for awhile until one day, out of nowhere, my car ran like ****. When I press on the gas, the car just doesn't jump to speed as it used to, also, when trying to accelerate to speed from a dead stop, I had to hit the gas pretty hard to get it to speed, and at around 10-20 mph, the thing would flat out for like 5-10 seconds before something kicked in and it would rocket to 35 and beyond. Also, during normal driving, I notice like a grinding/grumbling noise that I'm pretty sure came from the transmission since if I put the car on neutral or when parked, the noise went away. Also when in Drive (yes I have an automatic), during stoplights, I can feel my car vibrating like crap, again, when put on parked, the vibrating is not as severe or as loud. to sum it up, my car feels like its loosing power when driven. Please give me some adivice on whether this is a transmission problem, engine problem (from the loose timing belt causing the piston to hit the intake/exhaust valves) or both or something else that I have missed. Also show me a way of testing them, approximately how much it would cost to fix them with brand new parts (I'm through with rebuild, my car's previous owner had it rebuilt once already). Thanks a bunch.

Also, would dirty/broken O2 sensor cause the engine to lag and hesitates during acceleration? I went to my local pepboys store to find out that the O2 sensors are $109 a pop, and with me being a student, I don't want to pay for anything that is not nessesary.

One quick note, I also am having trouble getting the RPM to go above 2.5 (as indicated by the gauge) even though I'm hitting hard on the gas, didn't have this problem before either.

 

Check your EGR Valve.

 

Yeah... AND the O2 sensor. If you've never had any of these items changed, then it's high time you DO get them changed.