You need to clean and rebuilt it, it will work properly.

I know this post is goes back a few months but i would like to comment a little on this system and parts cleanliness. As most know, the lpg regulator on these trucks do not operate like the systems found in earlier Nissans equipped with model J Impco regulators. Unlike the older trucks, these are not dependent on direct vacuum from mixer as these are without. These have constant fuel vapor pressure to injector holder. The injector holder has 2 ports which feeds into the throttle bore. Also said holder has internal relief valve that will purge or dump excess pressure from injector holder into throttle bore via a small port which is located 90 degrees clockwise from main injector port. This is to relieve pressure from the flexible fuel supply hose from regulator to injector holder due to heat and gas expansion. Even with engine not running, dumping will continue to occur until pressure is no longer enough to open dump valve. This replaces the "weeping" hose as found on the older models. It is very important to drain tar from these regulators. I occasionally remove the lpg assist injector from time to time and inspect or clean as these can on occasion get gummed up if improper fuel other than whats recommended is used or the vaporizor is not drained as required. When replacing any component mounted on or including injector holder or ETC, i take great pains to make sure components are free of any dirt, dust, rodents or their nests. With a contaminated injector holder any debris can cause sticking main injector or damage, damage to lpg assist injector or its seating or worse a possibility of preventing purge valve not to seat or stick when dumping. After all it does have a final chance screen located in the injector holder itself and isn't there for looks. There is also filter mounted in lock-off assy. The purge valve being internal to injector holder itself is not accessible. I rather take a few more minutes and make sure regulator is drained every 200 hrs and replacement injector holder parts are squeaky clean when replacing or removing for diagnosis. Sure saves time and trouble down the road. Just my few cents worth.

the small valve on front

I am having same problems, with no books. What is the boost valve?

Had 5 psi at regulator. Found boost valve gummed up. Just cleaned it with parts cleaner and reinstalled. Presto, it fired up. Thanks for your help.

Jim;
Install a pressure guage in the 1/8 BSP tap port of the regulator with the coolant at operating temperature and see that the pressure holds at 4.6 - 4.8 psig for 1 min. While this is being done also, see what the PC says the pressure is. You may have one of the two reliefs in the throttle body gummed up and not sealing.

I have drained it every PM. I replaced the vaporizer and still have the same problem. Any other suggestions?

Jim;
You have a truck which hasn't been maintained per mfg. spec..
The solution is to replace the vaporizer - regulator at approx. + 800.00 USD or take it to an authorized dealer to have it cleaned and rebuilt for a third of that. Once this is done and the regulator is reinstalled and readjusted. The engine will need to be relearned. At 200 hour intervals the tar drainning is required.