The coolant sensor does tell the PCM when the truck has warmed up enough to start using the EGR system. But I don't think the coolant sensor is at fault seeing as it does not do this while cold.
How is your EGR controlled? Does it have a vacuum line going to it along with a two-wire harness? Or is there only wires going to it and no vacuum line? I believe merlin was say that the WAY the PCM controls the EGR is normally through a duty solenoid that regulates vacuum to the EGR valve.
(ps. Don't tell anyone I said this, because i'm a big advocate of repairing things correctly using the correct parts. But ford has had a problem with their EGR systems, especially on their full-sized pickups.
As the engine wears out, things start to fall out of range, like sensors, intake flow charactaristics because of carbon buildup from the EGR system and the PCV system etc. So they came out with an EGR restrictor plate that goes between the EGR valve and the intake manifold that takes the 1/2" hole down to a 1/4" hole.
My truck had the same problem you describe, and I tried everything because I really didn't want to do a cheezy repair. But in the end I tried it. Runs as smooth as when it was new, no pinging and still passes emissions. Crazy!)
Monday, June 25th, 2007 AT 2:29 AM