air in IP return line

[funola]

I have been chasing air bubbles in my return line from the IP for quite some time. It's not much, but it's there for sure, and if I rev the engine, the amount of bubbles increase. I am running a 2 tank WVO system where the fuel filters have been re-plumbed so they are under vacuum instead of pressure. Bubbles are only while on diesel, not VO, likely because of the higher viscosity of VO. There are no bubbles in the supply line to the diesel filter. I have clear lines everywhere so I can see exactly where the bubbles are. I have pressure and vaccum tested the diesel and VO filters, all the lines. the switching valves and lastly the IP itself and they do not leak per the tests. So the only item remaining is the lift pump, which I have not tested yet since it's a little more involved.

I am thinking of by-passing the lift pump and gravity feeding the IP with a gallon jug of diesel. If it runs without bubbles then I know the air is coming from the lift pump. Will the IP run with gravity feed? I can alway rig up an electric fuel pump if some pressure is required.

[jt20]

Due to the changes in your fuel routing, it is not reasonable to suspect just one item as the air entry point.

Simply because you do not see the air entering the pump in the supply lines does not mean it is not there. The air is collecting in the pump then being spit it out in visible quantities.

You should really find a way to route the system so that the pressurized regions remain pressurized, and do not experience vacuum.

If you can't, every seal must be serviced, especially those on the filter.

The car may idle without fuel pressure, but why don't you try bypassing your alternate fuel setup and keep the LP in the test setup?

[Fattyman]

Originally Posted by funola

I am running a 2 tank WVO system where the fuel filters have been re-plumbed so they are under vacuum instead of pressure.

Why did you re route the lines to be under vacuum and not pressure? Rigged this way you would never have this problem and leaks would be easy to spot.
It also would have saved you the problem of rigging one just to test for leaks later.

[layback40]

I assume you have gone over every seal looking for the leak. If you drizzle diesel over each seal you will find any vac air leak as the bubbles will reduce.

A diagram of how your system works would help. We may be able to identify a seal or component that doesn't seal well under vac. Does the problem occur with plenty of diesel in the tank & the car facing down hill? This provides positive pressure to the lift pump.

[funola]

Originally Posted by Fattyman

Why did you re route the lines to be under vacuum and not pressure? Rigged this way you would never have this problem and leaks would be easy to spot.
It also would have saved you the problem of rigging one just to test for leaks later.

I chose the vacuum system because it needs only 2 valves. With a pressure system, it would require 3 valves. Also, with a pressure system, if there are any blockages, hoses will blow. With a vacuum system, it will just suck air and stall, a lot less messy. I do agree it is harder to find source of air intrusion with a vacuum system.

[paddo]

Originally Posted by funola

I chose the vacuum system because it needs only 2 valves. With a pressure system, it would require 3 valves. Also, with a pressure system, if there are any blockages, hoses will blow. With a vacuum system, it will just suck air and stall, a lot less messy. I do agree it is harder to find source of air intrusion with a vacuum system.

That's also how my standard greasecar system is. I think they do that because

a) they have to make sure waste oil is filtered to a reasonable extent before it hits the lift pump. This is accomplished with the primary (small inline) filter in the standard diesel set-up.

b) with waste oil, a small inline filter would be likely to block very quickly and be a pain. In the standard setup, the diesel goes from the lift pump to the filter and then back to the injection pump. In the typical waste oil set-up, the oil goes through a big filter (usually heated), into the lift pump then straight to the inlet for the injection pump just beside it. It does create more of a tendency to take in air due to negative pressure from the filter forward, so everything has to be pretty tight and back pressure (partial blockages etc) not too much. so that's why some systems also have an auxillary pump further back but then that may prone to blockage, corrosion and failure. My system also feeds the return fuel straight back into the filter (to keep it hot), so any air leaked in will tend to stay in the circuit longer and tend to build up quickly, another reason why everything under negative pressure (between the filter and the lift pump piston) has to be really air-tight. I have had quite a few problems with air leaks over the years but have developed simple, cheap and quick ways to detect the leaks. Please let me know if you want to know these. I had a frustrating leak recently but it's working fine now although I can still see a few bubbles at times. I have a short length of clear plastic line in the system, which is very handy to monitor what is going on.

[Fattyman]

Greasecar runs pressure with 2 valves. When you select "quick purge" just the feed valve switches over to diesel and the return valve stays on Veg. That way you will get diesel in the veg tank but not the other way around as long as you wait long enough to purge.

[funola]

Originally Posted by Fattyman

Greasecar runs pressure with 2 valves. When you select "quick purge" just the feed valve switches over to diesel and the return valve stays on Veg. That way you will get diesel in the veg tank but not the other way around as long as you wait long enough to purge.

Are you running a Greasecar system on a W123 Mercedes? I have never seen a Greasecar diagram for a Mercedes. And you say they use 2 valves on a pressure system? I'd really like to see a diagram on how they pull that off.

Getting back to my question above. Has anyone run the Bosch IP briefly without the lift pump? Will it run by gravity fuel feed? I may just give it a quick try.

[Fattyman]

I used to run my 300D without the lift pump. I used an electric boost pump though and not gravity feed. I stopped because getting any air out of the system was a PITA even with the boost pump. Here's a schematic of a 2 tank Mercedes system. It does not show the second valve for the inlet side of the IP but you can see where it goes. I do not run a dual tank system on my MBZ's here in LA.

[funola]

I thought we were talking about using the stock lift pump and only 2 valves? Your diagram shows 2 electric pumps.There are very few electric pumps (I'd even say none) that can match the performace of the Mercedes cam driven mechanical piston lift pump.

ps. in case it's not clear, I want to run it briefly without a lift pump only to see if the lift pump is leaking in air, not permanently.

[Fattyman]

True, the Mercedes lift pump works great. Every little bit helps when you're moving thick oil though. Especially on a wagon or any car that has the tank below the motor. You won't have that problem on your 300D. The pumps also act as check valves. I was showing you how to run a 2 valve pressurized system on a MBZ. I thought that's what you wanted. Sorry to muddy the waters.

[funola]

No, I do not want to run a pressure system with the stock lift pump for reasons already mentioned (blowing hoses when there's a restriction).

[Fattyman]

The pumps I've used are rated at 9-12 psi and do not pose any risks of blown hoses etc. The internal bypass pressure of the IP is higher and would simply bypass any unused fuel back to the return circuit anyway so there is no risk of damage there either. Another reason I like pressurized systems.

[funola]

I think those NAPA pumps you listed are solenoid pumps. 9-12 psi is for diesel or gasoline. I doubt you'd get near that with a thicker fuel like VO. The IP spec requires 12 to 19 psi fuel presure @ 3000 RPM. How did it run with those puny elec pumps?

[Fattyman]

Those are the ones. They are constant volume pumps. Variable pressure. My 300D ran fine with just one of those on a single tank system. The spring of the stock lift pump had also been removed so it was running with only the electric boost pump.