Installed mechanical fuel gauge about a year ago and found that fuel pressure was only operating at 40 psi sometimes less. Installed blue spring mod in the regulator and corrected problem intermittently. Gauge starting leaking in cab few months later so disconnected. I knew truck was needing injectors anyways. This past week I decided to address problem again and decided to replace all 8 injectors but did not want to put them in until fuel pressure issue was solved. Hooked up a new gauge and pressure is consistently operating at 40 idle or WOT. Replaced fuel pump and pressures are good at 60 psi with a little drop at WOT which I understand is normal. I proceeded on to the next challenge of replacing injectors yesterday. Fired up truck, running like new again. Solved all my rough running and cold start issues and I've still got 60 psi! Took truck out to purge the air out with a few WOT's and now the dreaded 40 psi is back.
During the WOT's Transmission shifted hard which I'm used to after and a ecm reset but around 3rd gear truck would not shift and tow haul light started flashing. Scanned with auto enginuity and it listed P0766 and P2703 which I understand are transmission solenoid issues possibly failing.
My question is: Can the truck's ecm limit fuel pressure in any way when fault codes such as these are being detected? Seems that I remember reading here once something about a limp home strategy that the computer will operate in until codes are cleared..... or, do I have another issue you guys can think of that would be causing this fuel pressure drop?
thanks

 

The Tech's here and severa of the articles in the tech folder mention "defueling" at a certain temp or pressure, or when a code is set. That may be what's happening. Unlike a gasser, less fuel means cooler combustion temps in a diesel, and it can sure get your attention when it happens.

 

guess I could clear codes unhook batteries and then see if pressure comes back. I don't know. I have a feeling it's something else like regulator or air in lines. I'll mess with it today and post findings if any.
Thanks

 

The PCM can not limit pressure from the fuel pump. Defueling is achieved through injector pulse width and timing adjustments from the PCM/FICM/IPR to the injectors themselves. The spring at the upper fuel bowl is what controls fuel pressure. I'd replace that with the blue spring and see where you're at.

 

I put the blue spring in it about year or two ago. Guess I can try it again. Can these springs loose tension over time?

 

Good to know the ecm has no control over pump pressure though thanks William