I have a rather interesting issue. I have been researching the forums here and found some great information. However nothing I have done helps.
I have a 99 F250 SuperDuty I bought a few months ago. When I start the truck the rpms go up to 1800 and then slowly back down to normal idle. It does this whether the truck is hot or cold. Also while driving I feel the engine surging and can see the rpm needle move between 100 - 200 rpm difference. While idling it fluctuates rapidly. I have tried a number of things including...
IAC replacement,
pcv valve replacement
DPFE replacement
throttle body cleaning
new TPS switch
EGR Valve replacement
I have even run seafoam through the motor.
Short of the intake tuning valves I am not sure what other route to take. I had the engine smoked for leaks but it was tight as a drum. The mechanic suggested removing the intake Manifold to see if anything was wrong but at $400 for just the removal I'm a little skeptic.
I know there are numerous threads on this issue so I am sorry for the new one but I can not seem to get anything to work with the previous solutions I read. Has anyone else had this issue continue and if so did you find a solution?
Any info would be greatly appreciated.

Thanks,
Scott

 

Have you checked your vacuum hoses for leaks? The corner of the tube from the pcv valve seems to be a common issue around here. For the money, I'd replace hoses blind before spending $400 to see if the intake is the issue. Does sound like some kind of a vacuum or intake leak though.

 

Actually yes I have checked the hoses for leaks and elbow for cracks. Everything looks good but maybe a replacement just to rule out the possibility like you said would be a good idea.
Forgot to mention before that it is the 5.4l V8 motor.

Thanks,

 

Well all new hoses and vacuum lines and still have this high / erratic idle problem. I now get a high pitched whistle while accelerating between 1500 and 2000 rpm. Also have a low humming sound that I can only hear in the cab while driving.
The truck is stock with the exception of a k&n filter. Just the filter not the performance kit. The only thing left That I can think of is the intake tuning valves. What symptoms would I get if one or both of those where sticking or not working properly?

Anything would help?
Thanks