Today I was out in my grandparents driveway fooling with what I have come to believe as a stubborn air bubble in my cooling system. The car wasn't on jacks, but the front end was up higher by about 8-12 inches than the rear due to the slope of the driveway. Which should have also helped in getting any air bubbles out the top of the motor.

Anyway I loosened the nut at the top of the thermostat housing and found a steady green stream of coolant, and i pushed the throttle down (I don't know if this was helping any or not but its what I did) about 1/4 to 1/3 of the way down and held it there for maybe 6 seconds or so. When I let off there was a loud 'crack' that came from the engine or even the tranny, the motor jolted (almost similar to a motor mount being broken) and the car rolled backwards about a foot and stopped. It was in park. :? I don't know what the heck that was, though I have heard this crack for maybe a month or two now but I have previously thought it to be an exhaust leak pop, from inside the car thats what it has sounded like, but this time i was under the hood.

I shut the hood then and got in the car and put it in each gear and didn't hear or feel anything, so i took it around the block and put the shifter in each gear and drove around and everything seemed fine.

Ideas/Opinions?

I do use the Parking brake when im on an incline, I didn't know it was bad for the trans or anything, but i hate it when it gets sorta jammed because of that. And the drive way wasn't really on an incline, there is gravel for half of it and concrete for the other, the gravel sits 8-12" higher than the concrete, its like a speed bump almost is how to describe it, i'll take a pic tomorrow.

The driveway is level but the gravel steps up, and the front was parked on the gravel. when i put it in park it didn't roll anywhere.

I guess what you suggested is probably the case, but i dont know the mechanics behind though, so i dont see how that happened.

first time its jolted and moved like this, I thought for a second something broke and it suddenly went to reverse, heading to my grandparent'* car. Its not like it casually rolled either, its almost like it was in gear...

but that still leaves the 'crack' sound, would that possibly be the prawl every time? because i have heard this sound before, i dont recall if it was in park every time though, and I never felt a jolt or anything...maybe its two things happening at once? Tomorrow i'll put it in neutral and see if i cracks again...maybe get some video...

thing is my computer isn't reading the video files from the camera...so i'll have to fix that if i get some vid of it...but anyway...

The prawl is a detent of sorts, it looks like half or so of a gear, its what hold the shifterin place for each gear or N or P. It stops you from hitting the shifter into N or another gear. Your shifter linkage bolts to it. Someone on here has to have a pic of a trans with the trans pan off. Easier to show you on a RWD trans. A lot of times, especially on a steep incline if you dont use the parking break to hold, the weight of the car puts a lot of pressure on this part and makes it hard to shift out of park into gear. More common on older cars I think(older than yours). I had a boss park his 80 F250 pickup on a hill and he had to get someone to pull with a strap and another truck up a lil bit cause he couldnt get it out of park, all jammed up. But if you say it acts normally while driving I think you are good to go. Just keep an ear out for anything unusual....

that'll sometimes happen when it doesnt fully seat when the weight is put against it. you'll know when it fully seats when you put it in park and it rolls, you will hear a little click, thats the spring pushing it into the locked in position. if you dont hear a click, its not fully locked in and if enough weight is pushed against it, it will pop out and then back in. thats why it rolled a foot and then stopped. but when parking on a grade, be sure to apply parking brake to put the weight on all 4 wheels.

Its nothing to worry about, just dont let it keep happening :P

My car did the same thing to me a few times scared the **** out of me..Im still lookin in to it as well.

isnt this also reffered to torque lock. ???
with the weight on the parking gear on an incline.

Back when i had my 99, my girlfriend took my bonneville to town and parked on a pretty steep hill, and i guess she heard a big snap when she put it in park she started to get out and my bonni started rollin down the street i guess it took her about a half a block to realize to hit the brakes..haha. brought it to the dealership and it said all the teeth were broken off the prawl 350 dollars later with my discount i was back in action.