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Message: [QUOTE="Black AOD, post: 1466011, member: 16536"] Listen to sootie ^^^ You need to get a device first that will monitor Rail Pressure or find someone with IDS to monitor it. If you are not getting spikes/drops in RP then it is probably a tune related issue. If you can physically watch RP drop and spike and drop and spike repeatedly while driving (you will probably be experiencing power surges as well as the truck cannot adequately make consistent power from lack of constant fueling) then you have a mechanical or wiring/electrical issue likely. There are several related causes for intermittent RP on a 6.4 Most common is a bad RP sensor in the valve cover, or a clogged fuel filter. Those are easy to fix. Another common item like sootie mentioned is a bad HPFP harness, that's a common issue. The updated ones are better protected. Next would be your HPFP (K16) has a bad PCV or VCV and is probably kicking the bucket. If your truck has 200k+ mileage on an original pump, you should look into this. Injection harness in passenger side valve cover sometimes go bad but not often. Same with the main harness, these are hardly ever the cause but it is possible but very unlikely. Bad Injectors typically don't cause RP issues but it is possible. Last but not least in very very rare cases....Your PCM needs reflashed back to stock and then re-tuned/re-programmed with your custom tunes and sometimes this will fix it. This is somewhat seen here and there. Other times you have to swap a whole new PCM in. This is very rare. Like less than 0.05% of people have ever had to do this. All that being said, sootie is right, you need to get a device and start monitoring your parameters (mainly fuel and rail pressure) first before anything else. On your truck with 200k+ mileage, if RP actually is dropping/spiking and you can diagnose that it is NOT tune related, I would strongly suggest looking into your HPFP (K16) and the harness that is with it in the valley [/QUOTE]