Fuel Return Routing

[misterdoyle]

Hello, I have recently become the proud owner of an

[rhill2901]

I would be concerned about the liftpump putting too much negative pressure on the return line. The return line should be around 1-2 psi... If you pipe it the way you have it configured this will drop to below -6psi.

I would use the 3 port solenoid valve in the return line as well; oil returned to the oil tank and diesel returned to the diesel tank... Of course, Contamination will happen when you switch between tanks...

[misterdoyle]

That's a big jump from 1-2 psi to -6 psi. Will such a change have a negative effect on the Lift or Injection pumps? Could the IP be adjusted to compensate for this change in presure?

[rhill2901]

any variation in the return line pressure will affect

[rhill2901]

any variation in the return line pressure will affect the timing of the IP. Lowering the return pressure will retard timing. In this case -6psi would give severe results.

[britannic]

Originally posted by rhill2901:
I would be concerned about the liftpump putting too much negative pressure on the return line. The return line should be around 1-2 psi... If you pipe it the way you have it configured this will drop to below -6psi.

I would use the 3 port solenoid valve in the return line as well; oil returned to the oil tank and diesel returned to the diesel tank... Of course, Contamination will happen when you switch between tanks... I've scoured my library for information on return line pressure and can only find info for the IP housing itself, so I'm very interested in knowing where you found out about the return line pressure being 1-2psi after the IP. Thanks in advance!

The IP uses a check valve in the return line to control housing pressure (around 12psi with HPCA off). YMMV but when testing, I found no variance in timing with the return line running straight into a fuel can.

As already stated, restricting or applying a negative pressure to the return line will definitely affect timing and produce odd running behavior. In fact, if your engine runs away or won't shutoff, the best way to stop it is to pinch the return line hose at the IP.

[rhill2901]

My return line pressure readings under various engine speeds was obtained by monitoring the pressure at the IP. I couldn't find any documentation to indicate ideal. The 1-2 psi seems reasonable since the return line dumps fuel back into the tank at the top of the filler spout.

[misterdoyle]

Thanks for the insight everyone. Still not sure how I'm going to do this. I'll post a note with links to pic's when I get it going. Of course now that the temp has droped and the snow is flying it may take me a bit longer. Plus I think I may wait untill a friend of mine lets me help him install a similar system into his Mercedes 300D. Nothin' like a guinea pig...

[big swifty]

Shmoyle Doyle

There's theory and then there's reality. I have been running My SVO system plumbed according to the attached schematic. I have never noticed a difference in the way the vehicle drives whether returning fuel to the diesel tank or recirculated into the supply line. My EGT's aren't affected at all either.




http://www.imagestation.com/picture/sraid90/pf7c24547c373ae3e3aef44d339987045/fa76ed42.jpg


EDIT:
Looking at the pic that I posted, I just wanted to clarify some things that might be confusing...
1) Supply and return arrows are reversed on the inj pump. not a big deal.
2) from the 6-port solenoid the chosen fuel is first routed to the lift pump THEN to the IP.
3) I also have a manually switched auxilliary 12V fuel pump in line after the 6-port solenoid which comes in handy during filter changes etc. As a final tweak, I added a 3-way valve to purge any air during filter changes/modifications to the system. This valve saves a lot of headache and starter cycling during purging.

[ 11-26-2003, 04:20 AM: Message edited by: big swifty ]

[misterdoyle]

Big Swifty,
If I am reading this correctly your diagram also has the SVO supply and diesel return reversed. Or am I missing something?

I've been giving this system alot of thought and I wonder if routing the return line back to the front of the lift pump through some sort of one way valve that opens when the presure on it is greater than the 'recomended' presure (<2 psi) would be a reasonable solution.

[big swifty]

Sam

Perhaps its a bit confusing...
the green diesel return line is also labled "flush line" Before shutting down, you flip the switch and purge the SVO line of grease; therein lies the purpose of the 3 port solenoid and "modified return loop". The SVO return while running on grease is looped back into the supply side.

Clearer???

[misterdoyle]

Yes, thanks.