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Have a 2004 GMC Envoy with 100k miles. Have a rough vibration in Drive. Put it in Neutral, vibration goes away. After driving on the freeway for 10 minutes and come to a light the vibration is gone, Motor mounts look good. Any idea on the issue. RPM stay the same when it idles rough. 4.2l v6


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Location: Oregon
is your check engine light on? It sounds like you may have a miss. you should hook up to a scanner that can monitor misfires. or see if you have a code for a cylinder misfiring. A cold engine will experience a misfire that a warm engine will not notice except under a hard accelerate.

the engine idle is controlled by the idle air control solenoid. if the engine slows down because of a miss the IAC will raise the idle to the rpm the computer has set in it's program.


   
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Thanks, I will see if that works and let ya know. No codes so it threw me off.


   
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xuv 110k plus mi. been running fine for months now battery keeps draining engine starts and dies after battery charge pro mechs could find nothing had it diagnosed the other day for engine light readout said it was knock sensor now diag computer can't communicate with obd,any suggestions?


   
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how old is the battery? when you charged the battery did you check to see if the battery was good? is the charging system working correctly? Is the security light on when it starts and dies?

If the battery, charging and starting systems are all good and working normally then you should check for a draw on the battery. Looking to keep the amp draw under 0.030 amps when all the modules are asleep. (cycled off)

Knock sensor won't kill your battery or make it so your engine won't start. (unless something extraordinary is going on) So don't worry about that code for now.

If you can't communicate with the computer now check that you have good voltage at the pin #16 in the OBDII connector in your car. Pin # 4 and 5 are both supposed to be grounds. If you have battery voltage on #16 (12+ volts) and good grounds at # 4 & 5 then start checking fuses.

a bad pcm is possible


   
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