I'm new to burning SVO, and have about 6k miles on veg. oil in my 1991 F250. I'm trying to figure out if the looped return i built is workable, or if only a return-to-tank system is suitable for the 7.3 IDI.

I've noticed bubbles coming from the injector return lines (clear tubing), and going back to the SVO fuel filter in the looped return. Lacking an air vent on that filter head, the bubbles eventually end up in the feed to the IP (about 10 min. at 2k RPM). The bubbles happen both on SVO and diesel, but of course on diesel the return goes back to the tank so any bubbles are purged by the tank vent.

I didn't have any fuel leaks at the injectors, but i replaced the injector caps and o-rings, thinking maybe they were admitting air. I see no reduction in return-line bubbles after doing that work. It usually takes about 5 min. at idle before they appear.

Does anyone know if small bubbles in the injector return lines are just normal for this engine? I'd like to know if i mucked up the injector cap replacement or am just totally wrong about bubbles. I have just un-done the looped return and run line back to the tank, because i need to use the truck. The return-to-tank setup works fine.

Thanks for any insights!

 

you may have a air leak on suction side of your fuel system. it could be between the IP and your filter head, as the transfer pump in the IP can pull a vacuum on the filter, if your supply pump (lift pump) is having trouble pushing fuel thru the filter, however the IDI was intended to have a return to the tank from filter head as foam or tank slosh can accumulate air in the top of filter assembly, the return from the filter has in most years a check valve and small orifice (to maintain fuel pressure ) and in having constant return from filter will carry away heat and air back to tank. if you have the original filter head there is a heater mounted in the top, there are two o rings that can leak...

 

Speedwrench, thanks for the insights.

What really didn't help is that my VO low pressure pump was downstream of the VO filter head. This meant there were at least 20 or 30 places an air leak could have been happening and getting sucked into the line, all made worse by having the VO filter under suction. Because of that, I couldn't even plumb in an air purge line as some have done (off the filter housing with a needle valve to restrict it). Even tiny bubbles would gradually accumulate until there were too many.

As it turns out though, this is a kinda old thread and that truck is no longer with me. The VO system worked awesomely once I changed away from the looped return, but unfortunately everything /else/ on the truck went to hell (brake lines, tranny, radiator, frame rust), so I pulled the VO system and sold the truck. Used some of the parts to convert a 7.3 Powerstroke, which of course has a totally different fuel system so all these questions went away. Nowadays all I worry about is keeping the VO hot enough, since everything is down on the frame rail now.