Lately it seems that my idle just isnt the same as it was a while back. THis is a 99 f-150 5.4. When i start it up my idle goes down to about 680-720 (I'm using the digital tach on the LED display). WHen I shift to reverse or drive the tach drops to about 550 and then it goes up to about 600. If i keep my foot on the brake and hold it there while still in gear the idle continues to stay around 600. In neutral or park the idle is around 700. These seem low to me but i dont remember what the idle was when i first got it but i thought it was around 800. DOes it sound like there is starting to be a problem with the iac or maybe something else? Any help would be appreciated. I seemed to notice this a few days after i replaced my pcv valve but i dont know for sure if that would affect my idle at all.

 

if your PCV was bad, yeah it will make a difference. when you shift into gears holding the brake the engine will bog for a split second, you are adding a load onto the engine, at idle it has no load. also engines are programmed to run a little high when first started, it helps them warm up. unless your engine is stumbling and hesitating badly it sounds like you are fine. may just be time to change plugs and wires and filters etc...

 

When I come to a stop sign the tach drops as low as 550 and then bounces back up to around 700-800 before settling back at 600. I also feel a slight hesitation when accelerating from the stop.

I changed the plugs about 4,500 miles ago and changed the fuel filter a while back. I said i had the 5.4 so Pheonixx there are no plug wires.

 

Some of what you mentioned is normal, it should go to 550 or so before resting at 600 in any gear. To put it in neutral or park should go to 800, then 750ish. I suppose a IAC cleaning along with a t-body cleaning might be a good idea. If anything, changing the PCV would give higher idle speeds. I'd be changing things to perhaps eliminate them as culprits, things like plugs, fuel filters, IAC cleaning as mentioned and some other stuff. It's probably telling of it bieng time for tune up.

Whoops, just saw you changed plugs and fuel filter. Try cleaning the t-body and IAC, even the MAF sensor.

 

Well, I finally figured it out. It only took two weeks but my service engine soon light finally came on. REad the codes and got p0401. I have a DPFE sensor ordered for tomorrow and will put it on tomorrow too and the problem should be solved.

 

Make sure you get one from the dealership - they have the updated ones - know one else will have them..