When I go to start the car when it has already been running and the motor is warm, the motor will crank alittle slow and will start with a hard start. You can hear a piston slap or knock. I did have problems with my opti, but i just replaced it and it did help alot. Also the wiring harness to the alternator needed to be replaced because it was not charging the battery and it was cranking real slow before i replaced that. Any ideas or help.


Thanks Mark Tisdale

 

While the fuel injectors have a distinct tapping noise (usually under higher throttle), at a light load and low speed there seems to be "piston slap" on some people's cars. There doesn't seem to be anything in the service manuals. Most dealers seem to acknowledge the problem and say it's normal, which it seems to be. For some reason, the pistons shrink enough in the cold weather so as to "slap" around a little. Once the car warms up, the noise should go away. This is also a noticeable quirk of the LS1. Some peoples cars is it more noticeable than others.
useally the piston slap comes when you first start the motor, i get that once in a while when its cold outside, but if it warms up i never hear it, but your saying when its warmed up it slaps, dont know dude but that doesnt sound good, im sure guys will help you out on this one

 

Yea the knock will only will happen when the car cranks slow and it will start hard with a knock.(Only when the car has been running). But when the car it cold the motor will crank very smooth. Also when the car is running it will not knock under accleration or idle, car has no problems.

 

TTT

 

On my old 82 Camaro, if the motor was warmed up and I shut it off, the starter wouldn't want to turn it over, and when I did get it started again, I would heard a "knock".

It turned out to be my starter...

You might try replacing the starter and seeing if that doesn't fix it.