I have engine vibration in 2004 Trailblazer. Vibrates when at a stop. Some vibration when in nutrel, but really bad when put into drive. Had motor mounts replaced. Replaced the plugs and coil packs. Also had decarbonizing liquid and injectr cleaning fluid run through engine. There is a kinda of shudder when you turn motor off.

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There are several reasons this could be happening.could be engine or transmission mounts are bad,could need a tune up possible idle control motor,how does the car run going down the road.

Misefire.

mai Lor

exhaust resonance is unlikely
vibration is caused by something spinning that is out of balance
it can also be from a misfire or a mechanical problem
considering the mechanical problems
harmonic balancer - replaced delete reference unless loose bolt
loose clutch pressure plate --no information as to auto or manual--possible
cracked flex plate--no information as to auto or manual ---possible
failing cam shaft lobes --cam nobes wearing off causing valve timing problems --very possible
faulty hydraulic lifters --pumping up and holding a valve off the seat -possible
hydraulic follower failure --excessive valve clearance altering valve timing --very possible
ignition timing too far advanced -- tune up should have checked that but is a possibility
failed water pump bearings --noisy but a possibility
viscous fan hub ( if not electric fans )-- a distinct possibility--bearing failure and fan running off center-off balance
injectors --- any injector squirting but not the same amount as the others will cause a harmonic unbalance --possible
except for the cam shaft as mentioned , the engine has done 30.000 miles so any fault in the rebuild would be shown up before that mileage was done
drive line problems ---unlikely as you state it vibrates when stationary and that confines it back to the engine and transmission
only parts spinning are the engine parts and the input shafts , torque converter or clutch

First step is to see what error codes (DTCs) are present using a code reader.

Check the harmonic balancer

THIS IS MOST OFTEN CAUSED BY RUBBER MOTOR MOUNTS GETTING HARD FROM AGE AND ENGINE HEAT, REPLACE THEM

Check the idle control system, also check if all the injectors are working. Provided you already changed the spark plugs.

You have a shot front wheel bearing which will sound like a whining rumbling noise, if the bearing is really bad it will have started to break up inside and the streering wheel can snatch.

Guy with a trolley jack can tell you in a few minutes how bad it is

sounds like it has a bad motor mount, once the rubber wears out, it is metal to metal and there is nothing to coushin the vibrations from the engine.