Have a 2002 ford f-150 rwd with a 5.4 triton.

every time I take it to the garage it stops. Here is the issue

On really hot days even if I don't run the AC in when i have to sit and idle it will start dropping to a low idle and even if I put my foot on the brake it will try to lurch forward in self drive but If I am right behind someone and I can't take the foot off the brake at all it might stall all together or take off but have no power then a few seconds later the gas will work again. Because of it not doing it all the time though I have not been able to pinpoint the exact issue I thought it was a faulty gas cap and maybe rain got in the gas but I put a new cap on and it was fine after that until today. I also noticed every now and then I need to unhook my battery cables so the gas cap light will go off in the dashboard if not it will stay on. I am not a mechanic so it really is hard for me to explain the issues but maybe with more interaction I can narrow it done more.

 

What has been done to try to cure this?

Off hand , I'd 1st scan for codes and post what comes up.

Then , if this hasn't been done already, I'd change fuel filter and clean and or replace the IAC

 

If you don't put a Scanner on and see what code you have , you can't effect a fix.
From the description you give, my guess is you have a vacuum leak.
This has shifted the long term fuel tables 'RICH' and makes the motor run rich at near idle causing stalling.
This causes you to add throttle against the brake to keep from stalling.
Pulling the battery cable off reboots the computer shifting the tables back to a no shift point.
This temporarily clears the problem but it will go right back to the previous condition again.
Do it right.
Good luck.

 

Ironically everytime I have taken it to the garage it stops and runs normal so they can never get a scan on it when it is idling bad.

 

Thanks I will look into a vacuum leak is it a expensive fix?

 

Depends on what it is.

 

I noticed it only does this when it is real hot outside and we are stuck in bumper to bumper traffic.

 

Look closely at the connections for the vacuum lines. Something that might seem a little softer then the rest.

With stopped traffic and lack of air flow under the hood something may offer a leak.

Another thing maybe good to do is flush your radiator so it is cooling as well as it can. Also check the heater core hoses that might weep on the COPS during these "low air flow" times too. You might not notice coolant leaves dropping but just a little bit....but remain level during cooler times of the year. Installing a Fresh T stat might not be bad thought either, unless its been done recently?

 

I was thinking of putting a new thermostat in it because the temp inside doesn't show it getting hot. The truck will just start dropping it's idle and and vibrating and wanting to take off or lurch forward even if i have my foot on the brake pedal.

 

I had a similar issue with an older F150. It turned out to be a cracked intake manifold. Normal temps the thing would run fine. As it got hot, the crack would open up and the idle would go to crap.The hotter it got, the worse it got. This could be your vacuum leak.

 

Probably should have a scanner hooked up and try to reproduce the situation where is idles and runs poorly so the scanner can read those conditions. This will help pin point the cause and avoid just replacing parts in hope something cures it.

It can be several things....to weak fuel pump, or torque convertor, or shift solenoid, or something simple like vacuum leak or low transmission fluid. It does sound like more than 1 thing is playing into this.