Engine shuts off when put in gear. What's wrong?

pull plugs and if difficult plugs probably struck piston and will need to pull head off[look for replacement head or motor ] if they come out check if piston did strike if did will need to do compression test as to fuel if engine ok drop tank and empty

Buick LeSabre Questions - Running without a Mass Air Flow ...

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CarGurusJun 11, 2013 - It drives perfect then all of a sudden while im driving it stallsoccasionally. ... it idles ruff an stalls until it warms up does the same in gear its throwing engine ... If you are installing a idle air control valve in a 1990 Buick Centuryyou ... I put in my 3rd MAF sensor in 2 weeks and now the car is no longer stalling.

Sigh. The most likely answer is the MAF Boot (Air Mass Sensor) has a crack in it, AND, your motor mounts are worn out. When you put it in gear, there is a significant torquing of the engine, and it tends to try and "move" or twist to get the tired turning, but until you drive that isnt happening so the transmission clutch or torque converter eatst he rotation and allows it to idle. All find and well, HOWEVER, if your motor mounts are bad, then the initial twisting force when it goes into gear causes the engine to move enough that a crack in the MAF sensor will open, causing a massive air leak that tells the computer the wrong amount of intake air, it then sends the wrong amount of gas...engine only runs on right air and fuel. Engine stalls.

When putting in 1st gear, the lower gear coupled with quicker takeoff can sometymes prevent the initial stall and as the engine untorques when it moves back, the crack closes and returns to normal (ish)..
There could also be some electronic answers...but as its very common and easy/cheap to check, Id start there,.

run fault codes and look for a problem with the TCM ( possible reading and engine over speed which shuts the engine down)and it should re-engage gears immediately upon reselection

On some of the newer card when your low on oil or transmission fluid the car will not move. Check your fluids and then try again. It worked fofor me, a mechanic told me this and I thought he was crazy but it worked

It can't both not engage and not disengage? If you can't put it in gear even with the engine shut off than there is something wrong with the transmission internally. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

hi did you find out what was wrong with it? i am having the same problem can start it and put it in gear and it is fine but if i leave it to warm up and then try to put it in gear it shuts down.

Check kickstand interlock. Kickstand should close switch when up. Either interlock must be closed or trans in neutral for ignition to power up.

bad clutch

sounds as if the torque converter clutch is locked up or has an electrical short inside the transmission pan, or a chafed wire.