Jolting when revved - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-19-2010, 08:18 PM   #1
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Default Jolting when revved

In park or neutral, if I rev the engine anywhere beyond 2000rpm the car will violently jolt. If I hold the engine at a certain rpm it will shudder periodically (but with no discernible pattern). If I rev it then let go of the throttle, while coming back down to idle it will jolt at about 2000rpm.

Also if I give it a very sudden input of gas the engine will shudder, so I have to be easy with the throttle to get the rpms to rise.

But while in gear and driving, the engine gains and loses rpms with no jolts or shudders and sudden inputs of gas are translated into sudden gains of speed. I know something is wrong, and I thought I would have fixed it by now with all the maintenance I've done. The mechanic friend that did my bearings also noticed this jolting and suggested it was the transmission. Also note: idling is a lot rougher in park/gear than it is in neutral.

Where should I begin diagnosing?
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Old 04-19-2010, 09:33 PM   #2
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Start by checking your throttle position sensor.
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Old 04-19-2010, 11:52 PM   #3
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The connections look fine, but I'll need to borrow a multi meter to check the voltage so I'll get back to you. Is there anything else I can check without needing a meter or am I probably looking at a sensor problem?
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Old 04-19-2010, 11:55 PM   #4
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You need a DVOM to test the TPS, or another one that you can swap out. What you describe sounds like a bad TPS
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Old 04-20-2010, 12:19 AM   #5
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Right, thats what I meant by multi meter, I may be getting my terminology mixed up, but I do have access to one. I can see how the TPS might relate to the shudder at sudden throttle inputs, but the jolting? It concerns me how violent it is.

Also I remember reading in another thread that the boost is supposed to be at full -10 when there is no throttle input, but I see my boost hovering around -8 or -9 psi at idle. This might be related to whats happening but correct me if I'm wrong.
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Old 04-27-2010, 09:55 PM   #6
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Quote:
Originally Posted by Danthurs View Post
You need a DVOM to test the TPS, or another one that you can swap out. What you describe sounds like a bad TPS
So with the multi meter I got a voltage reading of 5.28 on the plug, and on the 20k ohms scale a TPS reading of 5.86 to 2.03 (at wide open). The ohms were consistent while moving the throttle.
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Old 04-27-2010, 10:20 PM   #7
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Is there any chance of getting a video of this problem?
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Old 04-28-2010, 12:27 PM   #8
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Dan... Where are you going with checking the TPS? If it functions fine when in gear, it will be good when not in gear, no?

My first thoughts are:
1. Stop revving your motor past 2k in park/neutral as there'* no good reasons to do so.
2. Check the motor mounts as they would be the only holding point during this quick rev.
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Old 04-28-2010, 12:39 PM   #9
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I've moved past the TPS, motor mount was next thought
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Old 04-28-2010, 11:12 PM   #10
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[/QUOTE]My first thoughts are:
1. Stop revving your motor past 2k in park/neutral as there'* no good reasons to do so.
2. Check the motor mounts as they would be the only holding point during this quick rev.[/QUOTE]

Technically I could completely avoid this problem by never touching the throttle while in park/neutral, but it just concerns me that the engine isn't smooth if I do it, which is rare, and usually just for testing purposes.

I've had the transmission mount replaced, and the torque axis mount (the big one right around all the belts) checked. Are there any others I should attempt to examine?

Last edited by zecc81; 04-28-2010 at 11:13 PM.
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