Why does the car buck and shudder?

Tiny
GRAND AM
  • 1995 PONTIAC GRAND AM

The car works fine other than this one problem. It shifts beautifully and runs well until I get to a cruising speed on the highway. After letting up on the accelerator and coasting for awhile, when I try to re-accelerate the car bucks and shudders until I let off or punch it into passing gear, and then it is fine again until I cruise and re-accelerate again. It occurs most often in the 80 to 100 km/hr range. I am not quite sure if it has to do with the transmission and or controls, or the engine gas delivery or controls. I am aware that having codes checked would be a good place to start, but I was hoping to get an educated opinion first. The check engine light does come on but does not always stay on. What do you think, any ideas? Thanks!

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Thursday, August 16th, 2012 AT 12:42 AM

14 Replies

Tiny
RACEFAN966
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Ok so do you have a digital multi meter? I am thinking of testing the TPS.

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Thursday, August 16th, 2012 AT 12:46 AM
Tiny
GRAND AM
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I do have a multimeter but It might be messed up. I can never seem to get it to read correctly. If I get a good tester, how would I do the check of the tps?

Btw, thanks for the lightning quick response! I wasn't expecting that. :O)

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Thursday, August 16th, 2012 AT 12:59 AM
Tiny
RACEFAN966
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Ok so you will back probe the green wire put the positive there and then ground the other lead. Now with the key on engine off slowly open the throttle. You should read with throttle closed about.8 volts and it should increase steadly with no drops or glitches to 4.5 or 5 volts. Let me know what you find and we will go from there.

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Thursday, August 16th, 2012 AT 1:05 AM
Tiny
GRAND AM
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Ok, thanks! I'll have to do that later when I get a decent multimeter and get back to you. Probably another day.

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Thursday, August 16th, 2012 AT 1:12 AM
Tiny
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Ok that works let me know what you find.

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Thursday, August 16th, 2012 AT 1:14 AM
Tiny
GRAND AM
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I get.6 closed up to 4.42 open. If I disconnect the sensor and test the wire I get.8 but it drops when connected to.61

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Thursday, August 16th, 2012 AT 4:44 PM
Tiny
GRAND AM
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So do these readings indicate a problem or would that be normal? What would be the next step?

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Saturday, August 18th, 2012 AT 2:50 AM
Tiny
RACEFAN966
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What engine does it have? The sensor readings are normal on that one. Sorry it took so long to get back to you.

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Tuesday, August 21st, 2012 AT 2:42 PM
Tiny
GRAND AM
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It's a 3.1 litre v6.

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Tuesday, August 21st, 2012 AT 9:51 PM
Tiny
RACEFAN966
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From what it shows you should have a mass air flow sensor. What I would like to have you do is get a can of mass air flow cleaner and clean it. Let me know if that helps.

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Wednesday, August 22nd, 2012 AT 3:00 PM
Tiny
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I've looked and it doesn't look like there is a MAF sensor.

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Thursday, August 23rd, 2012 AT 4:20 PM
Tiny
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Ok so there is an air charge temp sensor and the coolant sensor then. So when the car is cold ohm them both, they should almost match. Let me know what you find there.

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Friday, August 24th, 2012 AT 12:19 AM
Tiny
GRAND AM
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The air charge sensor gives 5.53 and the coolant sensor gives 6.60.

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Thursday, August 30th, 2012 AT 2:41 AM
Tiny
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Ok that is close enough no to be a problem. Have you checked the egr valve?

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Sunday, September 2nd, 2012 AT 2:53 PM

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