Need help. Erratic idle. About a month ago, when coming to a complete stop, the engine RPM's jumped to aprox. 1100 causing the truck to learch forward (almost into the back of the car ahead). This would happen occasionally. I replaced the IAC, MAS removed and cleaned the throttlebody, intake runner, EGR, PCV. This problem has not returned. The next problem has existed prior to the erratic idle and has both of the Ford Dealerships puzzled. They want me to drive the truck until something breaks. One of the dealers thinks the converter clutch is not releasing (even though the high rpm happens in PARK). Their are no error codes. When coming to a stop from average driving, the RPM seems very high (850-1100). This causes the truck to continue to pull for a few seconds, then the idle slows to normal (580-600) and all is well. If I put the trans in neutral while the truck is pulling at idle the RPM's jump to about 1100 for a few seconds. This happens more frequently after the engine temp is at normal and the outside temp is above 90. Have noticed when at idle (for 15-25 minutes) in park the engine idle creeps up to between 900-1000rpm's. It will continue to idle high until I drive for a few blocks, then the idle returns to normal. Generally the truck drives and shifts normally, gas mileage is still about 16 in town with air and 23 on the road with cruise. Can not find any vacuum leak, or air leak in the intake system. If I set the cruise control to 30 MPH the truck will not maintain a level speed, it increases and decreases speed to the point where the cruise turns itself off. I can not maintain a steady speed with the accelerator on level ground. If cruise is on and going down a hill the vehicle speed continues to increase to where I have to brake to keep from running over someone. Have had very little trouble with the truck, engine oil changes at 6K, trans flush at 20K. The truck has 62k and always starts and runs all day long. No hesitation, always has power when needed. This is a service vehicle at work and makes 8-10 service calls a day.
Any help work be greatly appreciated.
Andy............
andyldavis@cfl.rr.com

 

Hi Andy and welcome to the forum

Did you replace the IAC with a genuine Ford one?
Did you check very closely for vacuum leaks at the back of the throttle body?
A bad TP sensor can cause a problem like that too. It's easy to replace. Did your dealer monitor all the sensors? Did they run Key On Engine Running tests to check for codes?

 

Hi, Racerguy.
The IAC was replaced at the dealer. Removed TP and cleaned, checked for seal. Took the throttle body off and reattached with new gasket and reseated plumbing in the breather assy. to the air filter.
Next????

 

p.s.-- racerguy,

I did use DVOM to check the resistance of the switch with the butterfly.

Andy.........

 

I think the next thing would be to have the dealer monitor the different sensors and outputs on their WDS while driving the truck to see if the IAC is being commanaded to open and maybe why etc.
Have you tried replacing the PCV?
Anyone else have any ideas?

 

Thanks racerguy, will have checked next service.

Andy..