My car wont start unless its in N and I half to have my foot on the gas till rpms go up it will idle own But when put R or D sometimes die down?

There could be a few reasons why your car won't start unless it's in neutral and you have to keep your foot on the gas to keep the RPMs up. It's possible that there is an issue with the idle air control valve (IACV), which is responsible for regulating the amount of air that enters the engine when the throttle is closed. A faulty IACV can cause the engine to stall or idle roughly, and it can also cause the car to hesitate or stall when shifting gears.

Another possibility is that there is a vacuum leak somewhere in the engine. A vacuum leak can cause the engine to run lean, which can lead to stalling and other issues. You can check for vacuum leaks by spraying a small amount of starting fluid around the engine while it is running. If the engine speed increases when you spray starting fluid in a particular area, that is likely the location of the vacuum leak.

Finally, it's also possible that there is an issue with the transmission. If the transmission is not shifting properly, it can cause the engine to stall or idle roughly. You can have the transmission checked by a qualified mechanic to rule out any issues.

Here are some additional tips that may help you troubleshoot the issue:

* Try cleaning the IACV. You can do this by removing the IACV from the engine and spraying it with a carburetor cleaner. Be sure to let the IACV dry completely before reinstalling it.

* Check the vacuum lines for any cracks or leaks. If you find any leaks, replace the vacuum lines.

* Have the transmission checked by a qualified mechanic.