Hey guys, my name is Ryan and i'm new to the forum. I've looked on here before on how to replace certain parts and you guys are awesome and very helpful! So I have a 5 speed, 2005 base model, fwd Vibe with right around 120K on it. For the past couple of months or so, I've been trying to track down/fix an intermittent engine hesitation. When I start my car up in the morning, it start fine and the first few minutes and few shifts are fine. After that, once the engine is warmed up, it begins bucking/jerking when I upshift through the gears. It is really noticeable when I'm coasting say at 35 mph, and I let off the gas. With both feet off the pedals, my speed obviously decreases but then the engine bucks or hesitates pretty badly. Then, when I let back on the gas there's another hesitation. Also, the engine shakes pretty violently when I come to a stop and let it idle. It idles around 850 rpm, which seems normal I think, but for some reason it feels like there's a lack of power and the engine vibrates a lot. It's gotten to the point where my lower back is killing me from driving it because of the strong vibration coming through and from the random bucks/jerks lol. The lack of power throws off my shifts as well and makes it very difficult to execute nice, smooth shifts. Also, it's weird but it seems like when I rev it really high and shift (drive it like an ahole), the ensuing shifts on my way home are a lot smoother and better. Anyways, I was just wondering if you guys had any suggestions on what to try, I'm not throwing any codes. Here are some things I've tried/replaced already:
New spark plugs, intake manifold gasket, mass air flow sensor (cleaned and replaced), PCV valve, air filter, cleaned throttle body, checked for vacuum leaks under the hood. Also all of my fluids are up/fine, oil changed regularly w/ full synthetic, no leaks.
Basically the only things that I haven't replaced that I can think of are the valve cover gasket (not leaking any oil), fuel injectors, and 02 sensors/exhaust work.
Any help would be appreciated, thanks everyone!