Braided return lines a source for air in the lines?

[Rick Blalock]

I have been having some issues with air somehow getting into my veg oil lines. I have already vacuum tested all the lines with my mityvac and hose pinch off clamps and they all hold their vacuum even with pumped up to 20in Hg, nothing gets in through the veg oil fuel lines. I called up the guy at Greasecar and his best guess what the braided return lines from injector to injector and back to the IP. Just wondering, but if there was a leak, wouldn't the fuel go out? not air go in? Not sure if those are under slight vacuum or pressure.

Just wanted to ask some people on here before I buy new lines.
And also, the veg oil filter is brand spankin new and my oil is filtered absolute to 1 micron cold. So the oil filter is not the problem...

Any ideas?
- Rick

 

[Lug_Nut]

Those are under pressure. Not much when the thermal valve is open, a fair bit more until that valve does open. Fuel would come out.
Only lines under vacuum, supply from the fuel tank up to the inlet to the injector pump, can 'suck' in air.
However, with two fuel supplies you have doubled where you need to investigate.

 

[Rick Blalock]

Those are under pressure. Not much when the thermal valve is open, a fair bit more until that valve does open. Fuel would come out.
Only lines under vacuum, supply from the fuel tank up to the inlet to the injector pump, can 'suck' in air.
However, with two fuel supplies you have doubled where you need to investigate.

Good info to know thank you!!

I clamped off the rubber hose where it leaves the HIH, clamped off the return line where it is normally connected to the return barb on the IP, and used my mityvac to vacuum the whole veggie side of the fuel system contained there. And I mean it STAYED at 20in. Hg, there was no air gettin in. Only other place it could be is at the inlet to the IP which I doubt because I have a really good hose clamp that I tightened down pretty tight... I just don't get this! I hope the problem goes away when I get my lift pump.

Thanks again!
Rick

 

[Pat Dolan]

ALL liquids in atomospheric storage have considerable amounts of air dissolved within. All it takes is the simple action of pumping or running around a sharp edge in a fitting to remove them from solution (whereas it can take hundreds of atmospheres of pressure to re-dissolve them in a hurry). My guess is that you are seeing that freed air running down your lines as does every other diesel. This, BTW, is one reason why CRDs are so acurate in timing - they operate above miscibility pressure so have no issues with compressing the little bubbles to mechanically open an injector.

 

[DarrenUK]

If you've got a two tank system the way the fuel system circuit is plumbed will determine weather the braided injector spill/leak lines are under pressure or vacuum.

Lugnut - you sure they are under pressure all the time on a standard system?

I'd kind of expect some vacuum when the thermo valve is looping the returned fuel back into the filter as the transfer pump part of the injection pump will be sucking hard.

Best

Darren

 

[DarrenUK]

I've got some small spring clips that I've fitted onto these pipes.

Like these -
http://trueasset.org.uk/index.php?page=hose-clips

 

[BioDiesel]

"Lugnut - you sure they are under pressure all the time on a standard system?"

Yes. What happens when the engine isn't running?
iirc, someone had a TDI with hard start after sitting three days.
Air was coming in the interinjector lines ( while sitting ). Cured it by clamping the ends of the hoses.