Isn't an F550 limited to 75mph or so?
I had one of these awhile back, (albeit an F-350) guy stated it would just 'slow down' after driving it for long periods of time.
Same deal, P1000, and the one time it came in it had an overspeed code.
After extensive road testing, inconclusive.
I eventually got the truck to set a P040D for the EGT sensor. From what I have been told, when this sensor goes out of range, or over temp, the engine de-rates. There is no indication of this on the message center, or anywhere else. The truck will just slow until the EGT comes back into range, and then power will be restored. I guess this sensor was just on the edge of being fucked to the point where it could set a code, but would still de-rate the engine. After seeing the issue a bunch of times, there was a code set for the sensor, but the MIL did not come on, ever.
Just something to look at. I was not given a spec by the hotline regarding what temperatures this occurs at though, sorry I can't help out with you there.
Why is he driving a LOADED F-550 at 75mph, anyway?