'84 300SD vegetable oil conversion - PeachParts Mercedes-Benz Forum

gbrebes · #1 01-20-2005, 06:10 PM

I apologize in advance for a lengthy post. I have been setting up a two tank vegetable oil conversion on my 300SD and its all hooked up, but I am having a problem. The vegetable oil in a tank in the trunk is pumped by a Facet electric fuel pump rated at 4-7 psi. Through coolant heated hose on hose fuel line to the engine bay where there is a seperate heated filter for the vegetable oil that goes into a Pollack motorized 6 port fuel selctor valve. I ahve a boost gauge installed right before the IP inlet to monitor fuel pressure. When it is running on diesel the gauge stays around 14 psi. But when I switch to veg the pressure rises and eventually goes past 20 psi, the highest reading on the gauge. Here is what I posted on the Infopop sight.

I have made a little more progress in troubleshooting my situation. I temporarily put in PEX tubing to monitor what was going on. It seems that when I switch to veg, the looped return line which initially still has diesel in it, overpowers the pressure of the Facet pump and diesel starts to creep backwards towards the veg tank and I'm assuming through the veg filter. With this configuration I still get high (more than 20 psi.) readings on my boost gauge (probe now installed in a tee right before the fuel enters the injector pump). So I disconnected the looped return and had it pour into a coffee can (16 oz.) while I ran the car on veg. I saw through the clear lines that veg was going through the 6 port valve to the injector pump, but I ran the car at idle at the curb for 10 minutes or so and veg oil never started to return out of the banjo bolt fitting to the 6 port valve. I got about 24 oz. of diesel out of the disconnected return line before I gave up. While I was running the veg return line into the coffee can I noticed that the boost gauge behaved exactly as it should. On diesel it stayed around 14-15 psi, on veg it stayed around 4 psi. So it seems that the diesel returning out of the banjo fitting has a higher pressure than the veg oil going towards the loop tee, which is right before the veg filter inlet, and its forcing its way backwards through the veg line. I guess its also making the psi rise so high. My question is, how long does it normally take the veg oil to run through the IP and the engine until it starts to show up in the return when the car is idling? I also revved the car a few times to get more fuel moving through the system. Also, I got rid of the manual brass valve with the veg return looping directly before the veg filter and the diesel return going directly to the tank. I got rid of the "green" line. If anyone has any ideas I would appreciate it.

Again, sorry for the novel like quality of this post if any of you diesel guys have any ideas, I would really like to hear them, I even stumped Dana Linscott.

Gilbert Brebes

gbrebes · #2 01-20-2005, 06:15 PM

Here's the photo of the routing of the fuel and return lines through the six port Pollack valve.

rwthomas1 · #3 01-20-2005, 10:55 PM

It seems that when I switch to veg, the looped return line which initially still has diesel in it, overpowers the pressure of the Facet pump and diesel starts to creep backwards towards the veg tank and I'm assuming through the veg filter.

I have only studied WVO setups but it would seem that if this was happening you have the hoses hooked up incorrectly. Another posibility is the fuel pump on the IP is still pumping diesel and since it can't pump into the IP its backflowing into the return system? Did you replace the hollow bolt in the fuel filter to block the return? The WVO has to follow the same path as the diesel. I would run the WVO and diesel feeds through the 6port valve and then into the prefilter. This will increase your purge time but will guarantee the IP pump and filters are working as intended. RT

gbrebes · #4 01-20-2005, 11:11 PM

Thanks for the reply,

Yes, I have replaced the banjo bolt with a solid bolt. However, it seems that my original bolt is 25 MM long and I replaced it with a bolt that is 20 MM long as recommended at the Infopop sight. Could this present any problem? When you suggest that I run the diesel and WVO into the 6 port valve and then to the prefilter, do you mean the plastic filter before the lift pump? I would then be sending veg oil through the diesel filter? I'm having a hard time understanding. Right now, the diesel and WVO go to the six port valve and the fuel outlet of the valve goes directly to the IP inlet.

Thanks for the interest and help,

Gilbert

JamesStein · #5 01-21-2005, 09:45 AM

I'm looking at your diagram and I'm either confused.. or your trying to push veg oil backwards through your diesel filter return.

On the diagram what lines go together?

The way I am reading it yellow and blue are switched off the orange return line.. red and purple are switched off the brown line.

Personally I've never understood why everyone seems to go with a looped return. Why not just return the oil to the oil tank and the diesel to the diesel tank and forget messing with the banjo bolt.

Old300D · #6 01-21-2005, 11:25 AM

Quote:

Originally Posted by JamesStein

I'm looking at your diagram and I'm either confused.. or your trying to push veg oil backwards through your diesel filter return.

On the diagram what lines go together?

The way I am reading it yellow and blue are switched off the orange return line.. red and purple are switched off the brown line.

Personally I've never understood why everyone seems to go with a looped return. Why not just return the oil to the oil tank and the diesel to the diesel tank and forget messing with the banjo bolt.

I mentioned that possibility on the infopop forum. I just am not comfortable with looping the fuel return with a fuel pump providing pressure. Then again, your 6 port could be leaking internally as well.

rwthomas1 · #7 01-21-2005, 04:09 PM

The way you have the WVO plumbed it does not go through the lift pump, correct? This is a mechanical lift pump. It will continue to try and pump diesel unless you give it somewhere to go. Yes I am saying you need to run the WVO through the stock filters. Unless the 6port valve can be installed after the lift pump and when the WVO side is active the diesel side is a loop so the lift pump just pumps diesel in a happy loop and the Facet pump handles the WVO. The way you have your system plumbed is quite confusing but clearly the diesel is being forced back into the WVO system. The only way this can happen is the lift pump is backfeeding diesel to it. This stuff is the #1 reason I don't like looped returns. RT

gbrebes · #8 01-21-2005, 06:55 PM

Thanks Old300D and R.W. Thomas, I am always slightly amazed when someone answers a post of mine, I feel like I know so much less than you guys and I am not quite worthy of the friendly help.

I have included a picture of my Pollack motorized 6 port valve with the lines colored according to my set-up and my awful squiggly-lined drawing. So yellow (WVO) and purple (diesel) go to brown (IP inlet) and orange (return from banjo fitting on top of fuel filter) splits to Red (diesel return) and Blue (veg return).

As to the wisdom of looping the return instead of returning to the veg tank I am beginning to have my doubts, but it seems that so many people have done this already. RW Thomas, I have read on other sites that the mechanical lift pump on an 84 300SD operates on a cam that lifts off and disengages if the diesel outlet is plugged and their is no demand for fuel. I am sure I have not used the correct terminology for that last sentence, but is this incorrect? Does the mechanical lift pump always pump away even if the fuel has nowhere to go? If I had known this, I probably would not have set up teh conversion in this manner.

Thanks again,

Gilbert

engatwork · #9 01-21-2005, 07:43 PM

You got to be cautious when tieing two pumps together in series. The
"stronger" pump will overcome the "weaker" unit. My suggestion is to put a check valve in there somewhere or more closes match the head capabilty of the pumps. The "best" pumps we run at work are rated for 5400 gpm at 200' feet of head

.

rwthomas1 · #10 01-22-2005, 12:04 AM

Gilbert,
I have no idea how the lift pump works or if it disengages with excess head, etc. I really don't know all that much about WVO plumbing. My diagnosis was based on the info you provided. Since the fuel pressure was building up and diesel was being forced down the WVO line something is obviously doing it. The only logical answer is the lift pump and after looking at your diagram it seemed possible. I do know the Facet type pump is not really that strong and sort of self regulating. Consider running a return line. You are in CA and unless you do a whole lot of driving in cold (freezing) areas a simple uninsulated return line should work great for your climate. RT

gbrebes · #11 01-25-2005, 06:44 PM

I think I have finally solved my problem. With Tony's help of course. I actually had the hoses hooked up to the 6 port valve correctly. And by coloring the diesel as Tony suggested, it was easy to see through the PEX hose and tell that teh black diesel was not getting into the fuel input line when on veg. But while running on veg I never did get the return fuel to turn from black (diesel) to gold (veg oil). So I set up my hoses like Tony's system where he is testing how much veg oil gets returned from the injector return lines. Instead of the IP return going to the banjo fitting on the fuel filter, it goes to the return line in on the 6 port valve. The return from the diesel port of the valve then goes to the barb on the banjo fitting that previously had the IP return hooked up to it. I started the car up and switched to veg and the boost gauge showed a smooth and steady 4 psi. I am using the solid bolt on the banjo fitting on the fuel filter, but diesel was still geting into the return line from the filter while on veg.

Thank you so much everyone for caring about my problem and helping with all the suggestions.

I still have a few questions though. Should I still use the solid bolt on the banjo fitting with this configuration? Is there really no alternative to PEX for see through hose? It is extremely helpful to be able to see the fuel in the lines. And lastly, I have a 5 inch long air bubble in my veg line that I haven't been able to get out. The high spot of the veg line is between the coolant heat exchanger on the way to the heated veg filter. Even after running the eclectric pump with the veg line disconnected from the 6 port valve and getting out all the air, the bubble won't move from this high spot. The car seems to be running fine without any hiccups or hesitation even with air bubble, but is there any way to get rid of it?
Thanks,
Gilbert