New owner and new forum member. I just bought a 2005 250 6.0l 6-speed as the title says, it has 78,000 miles. Mods are leveling kit, MRBP SS duals with mufflers and a hyperchip tuner. I have a scangauge hooked up and the previous owner either just replaced the oil cooler or kept the coolant changed on regular intervals the deltas are within ten degrees at all times EOT rarely gets about 200 and if it does just for a few minutes as it cools down to below 200. I actually think I have a stuck thermostat because the ECT fluctuates from about 182 to 192 frequently. I have put about a thousand miles on the truck and so far it has run well. I run both archoil fuel and oil treatments which seemed to help the truck alot on starts and a slight shutter I had at quarter throttle. Just getting to know the truck I have noticed only at temperature between shifts the RPMS hang for a second or are slow to return down to idle speeds. It makes for clunky shift engagements. When the truck is cold or below 180-190* it drops quicker for smoother engagement of the next shift. It doesnt matter how you lift off the throttle the revs just hang at that certain rpm for a second or so. Any ideas would be greatly appreciated. Thanks Reilly.

 

Have you tried removing the tuner and see if it still does it. Your tuner is more of a "fooler" that tricks the PCM into adding more fuel and timing. I'd suggest removing it completely, and repeating your experiment.

 

I can do that for sure, that did cross my mind recently but i havent tried it yet. I also watch boost and dont think its a vane issue as boost subsides quickly after its made. Ill check it back to stock and let you know thanks.