What would happen if you installed the impeller on a Mercury 150 fourstroke? Would it self correct?

Nope.

Would it still work? Besides tearing it apart, what are the signs that it might be installed wrong? I know you are suppose to turn the impeller clockwise when pressing it into the impeller housing. But what if someone didn't know what they were doing.

Installing it incorrectly may cause the rubber fins to break off and get sucked up into parts of your engine.

I had a impeller come apart on a 115 Evinrude in the early 1980's while fishing Lake Fork during it's stage 2 fill. When I rebuilt the engine several years later, I found bits of impeller in various cooling veins in the motor.

You should see the instant the thermostat opens. It might pump a little but it won’t be right.

I have seen several videos where someone put the impeller in backwards. I have yet to see one acknowledge their mistake even when it was pointed out in the comments section. I what would happen when they fire up the big motor.

It shouldn’t fit right at all. The key slot shouldn’t let the pump cover fit. But people will force things that shouldn’t be forced.

YouTube videos of mechanics “fixing” stuff can be pretty entertaining. I watched a few on rebuilding the lower unit on an Ultrex. It was fun to see the finished product and notice the new nose cones and brush plate housings. They gave up, ordered assembled parts, then edited the videos to look like they knew what they were doing. Haha

Another great one to watch is the Amazon steering ram seal installations. You never see the video of the cheap seals blowing oil everywhere at the lake the next day.

I asked the service guy at Rockwall Marine this question. He said the fins would flip over. If I remember correctly the statement was prefaced with most of the time.

Like I said previously, I have seen multiple videos where people put their impeller in wrong but they never mentioned it after the video was made. I have also seen cases where most of the impeller fins were pointing in the correct direction but 1 fin was pointing in the opposite direction. Maybe that was because the installer installed the impeller backwards. Mercury motors allows this to happen because the impeller grove and keeper are uniform in size and no taper.