I tried this on another forum a while back. Must have stumped everyone. About 18 months back I ran out of fuel after owning the truck for 3 months. Then I found out about the fuel sending unit. The 1688 code showed up.( Don't know if it was there prior ) I replaced the lift pump, and then would start the truck, clear the code and it would drive fine with NO code till I shut down. Immediately if I checked the code it would be there. I spoke with Blue Chip regarding this issue. At that time he had not heard of a sequence as I described. He said " as long as drive ability is not affected drive it". We've put almost 20,000 miles on it without a hick up. I recognize this is a grey area, but $1200 to $1500 to replace it is spooky. Particularly if there is little experience with these symptoms. Everything I've read points to a failure. That seems to be an excepted fact anyway. I've ordered an AD FRRP and will put that on, to avoid aggravating the pump further. Past that I'm stumped.

 

Clear the fault code, turn the ignition key OFF for at least 60 seconds, turn the key ON. Without starting or cranking the engine check if P1688 resets. If P1688 resets at key ON, replace the injection pump. If the fault only resets with the ENGINE RUNNING, look for a loose power or ground connection to the injection pump, or a bad fuel pump relay.

 

Just got back with a load of hay, truck running good. ( cold ) Did the tests again, and got code only after key is rotated to off position. I did wait 60 + seconds. Code does not reset while running. ,,Dam!! Recommendations/suggestions on good sources of pumps. I plan on keeping the truck another 15-20 years.

 

Blue Chip is supposed to have an excellent pump but it's pretty high. There's another one out there that is around $950. My local Cummins shop sells them for about $900 with all current mods that are supposed to make this p.o.s better. You might check online and local sources.

 

Had good luck with industrial injections vp44s too

 

I'll be researching I P's tomorrow. Gonna have a drink and try to wrap my wallet around the situation. Darn thing runs really well!!