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EDIT March 9, 2016

So I took the car to a guy who specializes in ATX gears, and he felt that it's not a tranny problem. Got a second opinion from a Chevy tech who concurred. So due to that and the negative fuel trim at higher rpm's I'm leaning either toward a bad / plug or maybe an injector that's sticking open. This video from EricTheCarGuy describes really well what I'm hearing on hard acceleration. However, as there is no safety issue and no pending issue of catastrophic failure, it seems that my neighbor probably won't pursue this for now, although if the problem gets worse it might get picked up again.

Original Post

I took a look at a neighbor's car ( 2000 Chevy Cavalier 2.2L 4Cyl ATX ) and among other problems, it makes this horrible vibratory kind of noise under hard acceleration from a stop or slow speed. It's fine under slow steady acceleration and once the car is already moving above maybe 30 kph no matter how hard you accelerate. I can also induce the noise by lightly accelerating in a traffic circle.

The first thing coming to my mind is the torque converter:

enter image description here

This is from my ATSG manual for a different trany, but I think it's relevant.

The other problems I found were a slight wobble when going to WOT in park and then rapidly letting off the gas ( all engine mounts have been replaced recently, more than once ), the thermostat is either stuck open or has been removed as the car never gets above about 60*C, oil leaking from the valve cover gasket has filled up the spark plug holes, and Long Term Fuel Trim goes to about -11% when revving the engine at about 2500 rpm. There is also a weird PID shown by OBDWiz SEA 0x15 Short Term Fuel Trim stuck at 99.22% which is in addition to the regular STFT PID 0x06 which looks normal staying just slightly negative. I've seen allot of posts on the net which also have this same 0x15 PID stuck at the exact same value, which is really strange, but that's a different question.

In short, am I in the right direction for this noise?

  • While you may be on track, I'd get the little things you know are wrong before you tackle the "big thing". Two reasons for this: 1) you know the little stuff and they do need to be fixed; 2) The "big thing" may become self evident as you are working to clear the little stuff. This really isn't an answer, so will leave it here. – Pᴀᴜʟsᴛᴇʀ2 Mar 6 '16 at 13:58
  • @ᴘᴀᴜʟsᴛᴇʀ2 Any idea what that weird 0x15 PID is? Is that maybe the after cat sensor? It says Bank 1 Sensor 2 after it. Or should I ask this as a separate question? – Robert S. Barnes Mar 6 '16 at 14:04
  • That's what I'd assume, but I was under the impression the after cat sensor had no affect on the fuel trims, but was just there to monitor the cat when it got beyond a certain threshold. – Pᴀᴜʟsᴛᴇʀ2 Mar 6 '16 at 14:27
  • @paulster2 it depends on the car, some manufactures use it to correct fuel trim. – Ben Mar 7 '16 at 8:50
  • @Ben See this answer to this other question: mechanics.stackexchange.com/questions/26931/… – Robert S. Barnes Mar 7 '16 at 10:29
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So I spoke to the lady with this car a while back and she said she took it to someone who changed the plugs for her ( as I suspected the oil leaking into the plug wells from the worn out valve cover gasket had fried one or more plugs ) and they changed one of the motor mounts. After that the problem seems to have gone away. Apparently, whoever had previously replaced the motor mount used the wrong one ( according to these guys at least ) which had or lacked some kind of knob, which was theoretically throwing the engine off balance in tight turns and causing it to vibrate.

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