im no novice when it comes to diesels, especially the cummins and ive been a really good look at the internals of the 6.0as far as turbo vane repair, stc fittings, head gaskets, egr coolers, oil coolers, glow plugs, the works. but ive got one now that has dumbfounded me, and whats worse is it has so many issues, i dont know where to start. customer states that truck wont take the fuel. i inquire he says the truck runs good until you let off throttle to stop or turn then it wont accelerate. no boost gauge so he wa no help there. its an 04 by the way. says you can cut it off then restart and its fine for a bit. but he says he cant drive it more than 5 or 6 miles before having to stop and restart. i get in the truck and check it with my solus ultra. has codes for crank sensor, which ive never seen on a 6.0 so seems unusual. no other codes. i start truck check my ficm voltages and they are as low as 39v. but they increase when driving and after a few minutes, they creep to 48 and stay. ive driven the truck 5 miles,15 miles, and 20 miles. i have yet to have any issues with low power. cust says when it happens there is no smoke. no codes have returned. it sounds like the turbo ring sticking, but no smoke or codes. no egr codes or smoke, ficm is weak but would that cause one to run poorly, but restart and run fine. ive got some oil leaking at the icp area but ive only had bad icps kill the truck not come and go with low power. any help is appreciated. gonna continue driving to see if it can get something to go on a lil better

 

You found the 2 main probles allready, the FICM should neve go below 46 volts, if it does it will need to be repaired, it can cause all the problems you are talking about and it can also cause a very hard cold start problem. Where is the ICP located? on the back under turbo or on Pass valve cover? The oil can mess up the connector too, that is why the new icp sensors come with a lew pigtail as the original ones get compromised by oil and send false signals. The ICP can cause rough running, dead pedal, stalling out, no start, ect. The main problem you have now is the bad FICM, 39volts at anytime is very bad, the injectors are actually rated for 60volts so 39 isn't going to make it run good at all.

 

Test the batteries too before going for the FICM. Load test them. I've heard of all sorts of really strange issues when driving with weak batteries. Test those, then test the FICM. The FICM is clearly junk or on it's way there. Repair it though, don't replace it or your customer is going to end up in the same boat later on.