unusual engine problem

[5speed406]

My wife has a 2000 Grand Am with a 3.4L and just loves the car. It has had an unusual engine stumble since we purchased it with 184k miles on it. The problem occurs in every gear at 2000 rpm if the throttle is held steady or just barely backed off, no problems with acceleration through that rpm range. No codes are set while the problem occurs and the stumble is hard enough to shake the entire car. Mary describes it best as "it feels like the engine fell out and was run over".
To date I have replaced the coil packs, wires, plugs, harmonic dampener, crankshaft sensor (and had it synchronized with the cam sensor) and the intake gasket. None of these repairs have affected the problem and not all of these repairs were in an attempt to cure the problem.
The mass air flow sensor was unplugged and the car driven about 20 miles; this seemed to lessen the problem but oddly did not turn on the SES light or set any codes. So I suspect the MAF sensor may be bad but not setting a code while unplugged is odd.
Any help with finding and correcting the problem will be greatly appreciated. The car now has 220k miles on it and other than this one issue it looks and drives like new. Mary has no plans on letting me replace it, my directive from her is to "fix it" Thanks for any help you can give us.

 

[melsg5]

Could it be related to the lockup torque convertor? Do you have the tool to read transmission codes?

 

[5speed406]

I do not believe it is a transmission problem since it happens in all the forward gears, but will try to find someone that can read the transmission functions. My scanner is engine only. Is there a plug that can be disconnected for the torque convertor lock up to see if the problem goes away? The problem will occur in all 4 gears at 2000 rpm, isn't lock up only in 3rd and 4th?
My thinking it is rpm related but do not know enough about the engine management functions to find the problem. The mechanic at the dealership that synchronized the cam/crank sensor said he could fix the problem but would not tell me what it was. Given that he poured antifreeze under the car on the floor to scare me into having the intake gasket replaced I did not trust him. If he was smarter he would have used old nasty coolant not new coolant to convince me it was leaking.

 

[Zimgxp]

Maybe a throttle position sensor? You need a milti meter to check that.

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[5speed406]

I will check the TPS too, thanks for the suggestion.

 

[94 Junk That Moves]

I had a friend with the same problem.Pretty much did all you did with no change. I know it sounds really stupid simple but we replaced the air filter and fuel filter and it runs like a new car...Go figure...

 

[msmith1956]

To check your Throttle Position Sensor, follow these instructions. http://easyautodiagnostics.com/gm_tps_testing/tps_test_gm_1.php

 

[5speed406]

I had a friend with the same problem.Pretty much did all you did with no change. I know it sounds really stupid simple but we replaced the air filter and fuel filter and it runs like a new car...Go figure...

Air and fuel filters have been changed a couple of times since we owned the car, the fuel pump went out and it was changed too. None of this affected the problem.

To check your Throttle Position Sensor, follow these instructions. http://easyautodiagnostics.com/gm_tps_testing/tps_test_gm_1.php

Some good tips there but I disagree with probing wires. Once the insulation is compromised moisture can get in and start to corrode the wire changing the resistance of the signal to the PCM. I have small jumper leads that allow the circuit to be functional with a test lead for attaching a meter, I can also read the voltages with my scanner and will try again while having someone tap on the TPS to see if that affects the reading. There are no codes set right now and this issue has never set any.

Mary will be home for 2 days this weekend, that will give me time to check out a few of these. I will post the results. I will drive the car and capture some freeze frame data to check out as well. Thanks for the help and if you think of anything else let me know.

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