I have a 99 f-250 superduty 4x4 with the 5.4. At speeds between 30 and 45 mph, when you let off the accelarator, the rpm's drop as normal, then dip to about 400 rpm once or twice before leveling off at 750 rpm. Ive replaced the tps twice, cleaned the maf, iac, egr valve and throttle body. Ive changed the gasket on the throttle body houising, and changed the fuel filter. The cel doesn't come on, and I even had it checked for codes anyway, none were found. If you let off the accelerator at speeds over 45 mph, it idle's down normally with no dipping. Im out of ideas and would appreciate any help.

 

double check/ clean the IAC, might end up replacing it....& check for vacuum leaks. the Elbow for the PCV on the rear of the throttle body is a known spot to leak and the small vacuum lines behind the battery for the ventilation.

Have you replaced the PCV...do so.

Did you remove the throttle body housing and clean the interior of it....couple of EGR ports in there that can get clogged up.

MIGHT want to make sure you don't have a bad bearing in the serpentine belt system...Idler, Alt,A/C , water pump....etc. Might be providing extra drag

 

Maybe converter unlocking?

 

What are your idle rpm's cold ? What are they warm ?

 

Sorry for the delay in responding, my computer was down. Thank you for responding. I have replaced the pcv and the elbow on back of the tb housing. I have removed the housing, the ports are clean. I then cleaned the housing and throttle body. The egr was not sooted up, and the iac was tested by a friend of mine whos familiar with the electronics on these motors, he said it was fine. I don't have the readings. I have replaced all rubber vacume hoses as well as the tps. I also replaced the serpentine belt and checked the idler arm. It has good tension and makes no noise. The ac and water pump seem free as well.

 

I took the truck to the muffler shop. They said the converter isn't clogged and is fine. Also had a transmission tech drive the truck. He says the tranny is operating as it should.

 

The rpm's cold start at around 1150, then slow down to 800 with in a few seconds. At total warm up, it idles at 750, and fairly smoothly.