I'm in the process of converting my 97 Suburban to run on WVO. I've installed a fuel pressure gauge just before the IP and some of my tubing is Teflon so I can see my fuel now. I haven't run the WVO yet. When running on diesel I noticed that I have very small bubbles in my return line under the hood going back to the fuel tank. As the truck warms up they get bigger and after running it for about 15 minutes they aren't going away. Also, there aren't any bubles going into the IP. Could this be caused by what I believe is the original injectors with about 177K on them? I know that they can leak diesel when they start to get old, but can they leak compressed air also? I also noticed when reving my engine that my fuel pressure gauge is taking a dive to zero. It seems like the truck is running rough when it does this. I pulled the fuel line that feeds the IP and did a volume test and got about 46 oz/min. When the truck is idling I have about 5 psi on my new gauge. Going to road test it next to see how it feels with a load. Should be interesting as it looks like I'm going to be troubleshooting before runnning the WVO. Fingers crossed.... Who knows, maybe it will run better on the WVO system. I want to make sure it runs right on the diesel first.

I don't know if it's possible for a pressurized system to take on air externally or through the injectors. It seems like it would leak to the ground if it was leaking although I've read the injectors can leak internally they will knock loudly.

Run the lift pump with the engine off and see if you still get bubbles in the return line. Maybe bleed the fuel filter --- if you put a tube on the cap, you'll see if there are bubbles coming out of it. What about the water seperator?

Not knowing for sure, but it seems logical that the clyinder pressures could be forcing air back through the return line off the injector.

When I put a couple of clear return lines on my truck I noticed the same thing, but I have not found anything since then because, I blew a head gasket.

yes, injector can in rare instances casue air to get in system
Even so bad as to stall the engine, if the injector sticks open, I have seen air push back into pump. not good on it at all.
brian.

Wonder if the injection pump could cause it? Guess the thing to do is put some clear tubing on the injector return line to see if I see bubbles. Do each side.

I'd make sure my lift pump and OPS circuit was in good shape. The "drops to zero" part is what would drive my fix.

It has a Carter lift pump and I measured about 46 oz/min of fuel after my fuel filter and 3 way fuel selector valve. Yes the zero pressure thing bothers me, I don't want to kill me injection pump. I drove the truck the other day and it performs like it did before I touched it so these bubbles and fuel pressure think may have been like this all along. Hopefully a year from now I'll have troublefree WVO mileage under my belt, and not a major failure which leaves me wondering was it the WVO that caused it or damage that happened before the WVO (ie. bubbles)
I do have a low mileage IP, a Snap On MT480 injection timer, and new Delphi injectors. Neither look easy to install.

I noticed that the end of the tube for my fuel bleeder is still wet. Think it should have dried up by now. Going to plug it and see if that makes any difference. Maybe the valve seal is tired.