Recently I was having some odd fuel issues. I checked every component on the system, and figured out it was a fuel filter clog.

But I should back up. I have two fuel systems on my van. One diesel system with the R&D Stage 1 filter assy and e-pump, and another fuel system for my biofuel. Each system has its own dedicated e-pump (9-11.5psi), dedicated fuel filter head, dedicated fuel lines, and dedicated tanks. There is a selector valve at the IP inlet hose and another selector valve on the return hose, to allow the driver to pick which fuel to use.

The biofuel system has a coolant-heated filter head, HOH heated fuel lines, hotfox heated fuel pickup in the biofuel tank, and a 40-plate FP heat exchanger. So, assuming the coolant is hot, the biofuel is typically around 160-200 deg F, thus it flows effectively like diesel at room temp.

Recently, the IP was starving for fuel whenever I ran it on biofuel. This is odd, because I was running this for years without an issue. I'm still operating on my first duralift e-pump from 5 years ago. I decided to buy a new 9-11.5psi duralift, install new biofuel filters, and checked for any obstructions at every component in the biofuel circuit. Nothing was found. I even called the seller of my brand new duralift pump, and they assured me it wasn't some Chinese knock-off and they said it was very unlikely this fuel pump would fail in 300 miles.

So enough preamble. I installed a fuel pressure gauge to finally find out what is going on.

Diesel Stats:
Turning on the e-pump: 9psi
Idling: 5-6psi
Hwy speeds: 3-4psi

Biofuel Stats:
Turning on the e-pump: 5psi
Idling: 3-4psi
Hwy Speeds: 1-2.5psi

Obviously the biofuel is slightly thicker than diesel, so the pressure is less. Also, the biofuel circuit has a lot more restrictions in the way and a longer path with all the stuff like the FPHE, etc. But getting down to 1-2psi is disconcerting.

I know the IP pulls fuel, and the e-pump is effectively a helper to deliver the fuel passed the fuel filter and help purge out the air (if any bubbles). But how much fuel pressure is too little? As long as it has positive pressure, am I good?

I have never encountered any issues on the diesel side, so I know the IP is good.

 

as long as you have positive pressure, the pump is supplying more fuel than the engine needs.

 

Correct, however...

Differences in inlet pressure will change timing somewhat. Less pressure = more retarded, more = more advanced, to a certain degree. So it's important to keep the pressure /consistent/, more than at a specific pressure.

For reference, my mechanical pump on my 93 starts out at 4psi at cold idle, and drops to 2.5 at WOT - 1.5 PSI of difference throughout the range.

That being said... a couple PSI won't make enough difference to mean anything.

Now, back to genscripter's initial problem: Fuel starvation.
How do you know it was starving for fuel?
What did it do?

If it was just a flow restriction going into the IP, you should find that it idles fine, then as you give it throttle it simply... doesn't have much power. You get no smoke, low EGTs and the engine quiets down a bit under that throttle(retarded timing due to lack of pressure). It wouldn't jerk or surge or sound like a gasser running out of fuel, it would just be... anemic.

If you are getting surging, that's air in the supply, not a supply pressure problem; which also might account for the lower pressure, as air would travel through more quickly, is compressible and would escape into the return quicker as well.

 

Possibly yes, i might have an air leak. But I highly doubt it. Here's why I think it's air tight. With the engine off, I can flip on my e-pump. It will pressurized the fuel lines up to the fuel solenoid. It can even pressurize them up to 5-6psi if you wait long enough. I can leave it on for as long as I want, and if there was a hole or crack somewhere, I'd see some seepage. But there isn't any seepage. Also, when i pressurize it with the engine off, when I kill the switch, it stays pressurized for several minutes. I was kinda impressed that it held pressure for so long. I'd suspect that if there was a hole, it would lose pressure immediately. In fact, my diesel side does not hold any pressure after i kill the diesel switch, but my diesel side runs fine, so I don't really care.

My theory on the surging is that my IP is sucking more than the e-pump is providing. This happens when 2 things occur: the filter is clogged or I'm climbing a mountain. Once the IP pulls more than the e-pump is supplying, the pressure becomes negative, and with any negative pressure on a fluid, it converts some of the fuel into a gas, thus making bubbles, thus the surging. Unlike air bubbles in the fuel lines, this low-pressure fuel bubbles can be easily remedied by laying off the accelerator, which is not always the case for regular air bubbles. Usually if I have regular air, I have to give it lots of accelerator to work them out without killing the engine. But with my fuel bubbles, letting up on the pedal typically lets the fuel pressure return to positive, and lets me keep driving. Obviously I hate this setup and I'm ordering a higher pressure and higher flow duralift (12-15psi) pump to bring up my net fuel-at-the-ip-inlet pressure back up the nominal 4-6psi as desired, but I'm leaving for Albuquerque in 2 days and the darn pump won't be here before I go. So I'm going to limp it at 0-2psi hwy as far as I can on biofuel, and if it gets pissy, then I'll run it on diesel.

I drove it around Los Angeles and Malibu today to test out the new fuel pressure gauge. Even when it dipped to zero psi, it ran fine. I think as long as my filters don't get to obstructive, I should be good to put on a thousand miles without issue. Fingers crossed.

 

Any stock mechanical lift pump i had a guage on would read 1-3 at idle, and zero at WOT. As long as you have positive pressure, youre fine. My epump died on me, i drove my pickup for another 200 miles before it started losing prime. Still would start and run, but would start surging under load. If i understand the system correctly, as long as it is air tight, all is fine and dandy, but once air has entered the system, with no positive pressure, there is no way to purge the air. I assume it was fuel in the tank sloshing, as i was a little under 1/4 tank, and just took a sharp corner when it started. According to Justins, so long as you remain under 6psi everything is fine, but above that, the inlet pressure affects the housing pressure negatively.

That said, the only reason i thought to look at the guage was the tone of the engine changed. It had gotten much quieter, like the timing retarded. Much like the HPCA kickin off, but again. Mileage and power seemed to be the same however.