Car wont idle at all

If its been without the line for over a year, I Highly doubt it. Look into IAC valve. Most are installed with 2 screws, easy to adjust. Idle Air Control Valve. Look that up, for a start.

look at your fuel filter

Most likely need the idle readjusted. Your vehicle has a high idle rev limiter that will shut down the motor to keep it from getting damaged. Make sure your accelerator line isn't being pulled on. This will pull on the gas pedal just like having a foot on it and will rev the motor as well.

ok..you must have more than one car???? you don't drive the 300 atall im guessing. #1 start car drive up the road and \back to your house. #2 let it idle till problem starts. #3 once it does start with the problem keep it alive with the gas pedal. #4 once your have it safely from dying in higher rpms "rev it" like a kid would, you know like if you were gonna race some one lol. be careful not to over rev or damage engine. you should do this for five minutes no longer than that. #5 let it idle for another 5 minutes. #6 repeat every day until problem with idling decipates. it took mine 3 300e like 3 days of this. if your not going to drive the car once you get the problem out of the way. turn the car on everyday and let it idle for 5 minutes atleast and take it for a short drive once a week. they get lonelly you know lol. later on you can just turn the car on every 2 -3 days and let it idle but still take it on a short drive atleast once a week

You need a 10 mil and 8 mil spanner, carb cleaner and mabie some wd40.
Remove the big hose which goes from the throttle body to the air filter housing. Under the throttle body there is a idle control valve, unplug it, spray some wd40 on the 8mil bolts and take them off. Clean the idle control valve and housing and anywhere else which looks dirty, dry it with a lint free cloth and then put everything back on.
remove the battery negative terminal and leave it off for 30 minutes, this will reset the ecu to factory settings.

On the top of the throttle body there is a black circular rubber screw cover, take that off and there should be a plastic screw.

Connect your battery and start your engine, unscrew the idle speed control (plastic screw) so it revs at 2000rpm cold. When the temp needle reached the middle so it revs at 1000rpm hot.

This should solve the problem, if you drive around and find that you in 4th gear, with your foot off the accelerator and the idle is going up and down madly then the idle control valve is worn out and it needs replacing. Cost around £125 on ebay.

I don't know the year but it could be a defective IAC solenoid. If its an older truck then it could be a throttle cable.

have you cleaned the air filter?

send your car to authorize dealer to scan the fault,avoiding guessing of replacement of part,ty

you need to have the fuel pressure checked

yes checked fuel pressure and replaced the fuel filter, checked the mapp sensor,tested throttle position sensor, has strong spark, 50psi fuel pressure, fresh tune up, runs real rich and rough idle but wont rev up like fuel injectors arent speeding up, also checked exhaust. thanks julius