99 f150 5.4 idling problems

[jpbrown]

my 99 f150 is not wanting to idle right, it starts without a problem everytime but then it drops to 500 rpm's and acts like it wants to die. it drives fine and has no problem as long as im pushing the gas, but when i let off it goes right back to the low idling. ive replced the TPS and IAC and its still not running right, any ideas? im thinking it could be spark plugs but i hear these trucks have problems with the spark plugs breaking off into the head, and so im kinda scared to try and get them out. all answers are appreciated.

[galaxie641]

I'd look for vacuum leaks first.

[Ron W.]

The 99 5.4 doesn't have the problem of spark plugs breaking off in the hole, that would be the 3 valve 5.4s.
You don't state how many miles on the truck or how many miles since the plugs, fuel filter, air filter have been changed. If these haven't been done it could be your problem, you know basic tune up.
While a vacuum leak is a possibility that would/should turn your check engine light on, of course the bulb has to work for it to do so.
Is your CEL on? Have you checked for codes? Did you use Motorcraft products when you replaced the TPS and IAC? Load of bad gas maybe, try fuel injector cleaner.

[jpbrown]

i replaced the fuel filter and air filter, i didnt do the spark plugs cause i was scared of them breaking off, ill take them out now and check them, thanks for letting me know that bit of information. i had never heard of the problem before, but then i got on here and saw where people had posted stuff about it..
I put fuel injector cleaner in it and some 93 octane gas. the check engine light has come on and then codes are reading P0171 and P0174 which mean its running too lean.
i dont know what to do to fix that though??

[SMok]

vacuum leak. Check the pcv hose from the passanger valve cover that runs to the back of the throttle body. They are bad about getting brittle and leaking.