I have a 99 f250 SD v10 it has about 115000 miles on it but it has been babied for all of its life. Im 17 and i work as kind of an apprentice mechanic so I can hold my own when it comes to repairs and if i cant i have plenty of reliable sources to call upon...except for this problem. For the last 4000 miles or so, I have had a wierd idle problem. It doesnt nessescarily "idle" rough but only in drive. For example, when i am at a stop light, the idle rpm's will drop from the normal 850 ish to about 600 or lower then immediatly return to normal idle. It will go through this cycle of brief "ups and downs" about 6 times a minute. Not only is it audible in my flowmasters (Super 40's rule) but a very slight, but obvious, tremor passes through the car. Furthermore, the problem re-presents itself at the 1500 rpm mark during acceleration and cruise around 52-58 mph. It feels as if the truck is going over tiny but successive bumps in the road. The only way i can describe it is a surging while going over pencils. (use your imagination). Additionally, there is no check engine light and no misfires/faults were dectected on our scanner and dyno. So here is what we've tested/cleared so far. Air filter, cleaned Mass airflow sensor, decarbonized throttle body (which wasnt nessescary), New PCV and PCV hoses, plugs, gaps and plug wires checked and all were good, vaccuum (visual inspection only), new fuel filter. Tomorrow I intend to check/clean the Idle air control and EGR valve assembly (which may cause it because i recently replaced a REALLY nasty lookin DPFE, in fact the worst one ive ever seen in my short time workin at the shop)... Long story short, someone else MUST have had this problem, its driving me nuts (you keep your girl clean and pretty and then she burps and farts...i dont like it) IF ANYONE HAS EXPERIANCED THIS PLEASE HELP ME! keep in mind though it only happens in drive; parked shes as clean as the day she rolled off the lot. Finally when i cleaned the mass airflow, the problem got slightly better...good luck and thanks!

 

PS...Does anyone know where i can get some exhaust bypass valves?

 

JC Whitney used to stock them, guess they still do.

 

The Idle air control may be the problem, I have had to clean mine a couple of times and it clears it up for about a year, then gets gummed up again.

 

I concur - this is like the second thread I've seen just today on this.

 

no body seems to do a site search before they post any more.

 

well I figured that would be A culprit but possibly not THE culprit...because that wouldn't have any affect on the problem I encounter around the 1500 rpm/ 55 mph mark would it?

And I apologize for the double post, I did not see the other post.

 

Woops, didn't notice the 1500RPM thing... COP? (coil over plug?)

 

Yeah, I realize that could be a problem but since replaced ALL 10 in february, the prospect of a faulty one is nausiating. But then again, I had this problem prior to replacing them. Could this studder be possibly related to the overdrive/transmission, and if so how can I test it?

 

Okay so I took off the IAC today.. (what a pain in the ***...seriously, big hand in a small space sucks) the IAC actually looked pretty clear, i shot it with some carbo clean for good measure and reinstalled it in the plenum with a new gasket. In the egr though i did find a nice carbon "booger" but nothing shocking...any other ideas guys? (I still don't have it all back together yet because my truck's height forces me to work outside only and its rainin in texas!!!! but i doubt the IAC or EGR were the culprit)