Engine/Transmission (1998.5 - 2002) Siphon effect.......

[Big White Beast]

I have been watching my fuel pressure gauge rather closely since installing a new lift pump and pusher pump. Due to job location I ran the tank fairly low before I got a chance to fill up. When I left the station I had a solid pound more of pressure. Now this could be a fluke but it seems where the pump is mounted on the frame a full tank of gas would create a siphon effect making it easier all around on the pumps. The pumps are new so I'm going to run a few tanks down and look for similar results.



If this is the case with a pusher added I hate to think what the stock lift pump has to overcome.



Gary if you read this the Purolator fuel filter did not change the pressure at all. When I blew through it there was absolutely no restriction. Can't imagine how it filters. Well I was planning on yanking it at 5K miles just to see how restricted it gets. I'm sure your model is the same as it's similar in size.



Garrett



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[Gary - K7GLD]

Fine Garrett - I have posted the final results of my own effort (includint the Comp, minus the pump wire) both in the 24 valve and RV-towing group...



Dunno about the filter - and you hafta sort of cross yer fingers as to their compatibility with diesel fuel - I know *I* sure have pressure galore after my changes - the Purolator filter doesn't really HAVE to be super fine - it's more to protect the pusher pump from possible larger chunks that might damage either of the 2 pumps...



So, what is YOUR final idle/cruise/WOT fuel pressure?

 

[Diesel Freak]

not a fluke... . if the pump is mounted below the level in the tank, the pressure on the pump suction will be higher (available pump suction head). I see almost 1psi difference between an almost empty tank and a full one.

 

[Big White Beast]

Gary...

18 at idle and most cruising. I have sucked it down maybe 2 lbs at WOT. Had to keep my eyes on the road. These are post filter readings. The pre filter readings are way to high for the gauge. I contacted Stewart Warner and asked them about the resistance of the senders. I then added some resistance to try and compensate a few pounds. I tried several different ones in 13 ohm increments and didn't like the results. It did lower the overall reading but not as linearly as I would have liked.



Called Dakota digital today as they have a 0-30 sender and their prices are reasonable. $80 for the gauge and $25 per sender. They are back ordered on the gauge. I'll find out tomorrow when they might ship. I'm not a big fan of digital gauges but for the fuel pressure it would be fine. It's mounted in a cup down low so I won't see it that much anyway.



Garret

 

[Bigsaint]

The last time I was changing a dead Mallory pump, I, too, discovered the siphon effect. I guess luck ( or the lack thereof, depending on how you look at it) had me changing the past failed pumps with more than 1/2 a tank of fuel. This last time I was just under 1/2 and found that the siphon flow had stopped when I disconnected my compression fitting from the inbound hardline. In the past, I would normally get a good diesel bath from the neverending flow, this time it just dribbled out and then stopped while I was doing this . Truck was parked level as usual.



While I see no difference between a full tank and 1/2 a tank with the aftermarket pump mounted near the supply, I could easily see how you would lose some psi's when a single pump is mounted on the block and sucking the fuel all that distance. I wouldn't sweat +/- 1 psi, alot of factors including fuel temp. seem to play a role in what you see on the trusty FP gauge.



Scott W.

 

[Steve Campbell]

Remember tose posts complaining of...

5-8 gal of fuel left in the tank when the guage said empty? I think that was DC's way of "getting by" with a faulty set up!!!

 

[Gary - K7GLD]

"5-8 gal of fuel left in the tank when the guage said empty? I think that was DC's way of "getting by" with a faulty set up!!!"



NOT really!



Believe me, you'll APPRECIATE that cushion if yer ever unfortunate enough to run yer truck outta fuel - and THEN hafta do the drill to purge lines and get it started!



ON the other hand, with the fuel heating caused by the constant recycling of fuel thru the VP449 for cooling purposes, as well as other pumping actions that all add heat to the fuel, it's probably a good idea to keep the tank running as nearly full as possible to keep that "coolent reservoir" as cool as possible to provide maximum benefit downstream.



Later model Camaros like my '98 SS were noted for failed in-tank mounted fuel pumps when fellas either constantly ran the fuel too low, or ran outta gas - the pumps relied upon the gas for pump cooling, and when fuel got too low or ran out, the pumps quickly failed - and a BIG job to drop the tank for a replacement!

 

[TomGolden]

Ok, I have a question for you guys who have either added a pusher pump, or moved any sort of pump back by the tank. For this siphoning/diesel bath when you cut into the hardline near the tank, is it better to have the tank mostly empty, or mostly full - to avoid getting bathed in #2 when you cut the hard line. I am planning to install my pusher pump back by the tank tomorrow, and right now the needle is on 1/8th of a tank, so I want to know whether I should leave it or fill the tank first.



Thanks,

Tom

 

[Gary - K7GLD]

TOM - by ALL MEANS, do it when the tank is as EMPTY as possible!



Have a foot or so of 3/8 rubber tubing ready, and as soon as you sever the fuel line, quickly slip the tubing over the line from the tank and blow back the fuel to stop the siphoning action - good idea to remove the tank cap to keep vapor build up from heat, etc. , from providing enough pressure to again force fuel to flow...



Be prepared - yer gonna get a bath anyway!



You will get a small amount of flow back from the engine side, so have some rags or a small container ready to catch that - it won't be much



Good luck - keep us posted on results!

 

[TomGolden]

Thanks Gary



Rather than get a mouthful of #2, I was thinking of using my air compressor to blow the fuel back into the tank :-laf



By the way, I have what appears to be the same setup (minus the filter) that Garret shows in his picture.



Am going on a trip up to Reno tomorrow afternoon, so if I get this done in the morning, I'll see how the pressure does on the trip through the mountains.

 

[WadePatton]

siphon

this is why I almost always refill at 1/2 tank, and will continue to do so until my gauge is installed.

 

[Gary - K7GLD]

"By the way, I have what appears to be the same setup (minus the filter) that Garret shows in his picture. "



YUP - so is mine, except that I attached my filter directly to the pump with a close nipple... Less hose and clamps that way...

 

[Big White Beast]

I got lucky......

My driveway slopes down and I always back in. So when I cut the fuel line I hardly had any come out. Tank was around a 1/4 at the time.



Tom, it took me longer to wire it than it did to phiscally install it. I got anal and soldered the connection to the existing LP wire. It wasn't too bad from under the truck. I also finally installed my Painless Wiring fuses from Genos at the same time so I couldn't tell you how long that portion took.



My truck is noisy so I don't notice the pump that much. I used a solid 1/4" thick piece of rubber between it and the frame. I would highly recommend something to cushion it.



Garrett

 

[Gary - K7GLD]

Garrett, I'd sure like to see HOW you managed to solder directly to the stock LP wire!



Mine was so short, there was NO WAY I could get at it properly from above OR below the truck - other than MAYBE removing the starter! As it was, I simply took my time, and slooooowly and carefully whittled a little insulation from both wires to the pump, and when I had bared enough wire, carefully tinned a tiny spot on each wire with solder - then carefully and tightly wrapped the pre-tinned ends of my connecting wires around them, and applied the iron and a bit more solder, praying I got a good electrical connection!



Then, I ran the new wire to the relay I added to do the actual 12 volt switching to the new pump - now I don't have to put up with the racket from the new pump, or the excess presure that caused hard starting.



I'm not comfortable that the existing control circuitry for the stock pump is heavy enough to operate BOTH pumps at the same time, either... Time will tell...

 

[TomGolden]

Garrett, I absolutely plan on using something thick (like rubber) to insulate the pump bracket from the frame to eliminate vibrations and sound



Gary, I'll bet the way Garrett was able to solder into the existing LP wires is the same way I'm going to do it - cheat!! :-laf :-laf I went to Cummins West in Sacramento and bought the $8 pigtail wiring harness that is intended for the early model 24 valvers, that is like an 8" extension. That way, you can solder into it sitting on your work bench, then easily plug it into the lift pump up by the VP44



Tom

 

[Big White Beast]

The LP wire is only a relay trigger.....

There is no way I'd run both pumps off of one circuit. Wish I would have know about the pigtail. Could have kept the stock wires undamaged. Oh well.



Gary once you pull that flex tubing back and unwrap the two wires I was able to expose a good 3 inches. The just taped them back up and taped a new tube for the new wire together. It actually looks pretty factory except for it's newness.



Garrett

 

[Gary - K7GLD]

"That way, you can solder into it sitting on your work bench, then easily plug it into the lift pump up by the VP44 "





@#$%^&@&%@$... .



NOW ya tell me!





SO, do ya have a SIMILAR trick I can use to avoid damage to the VP44 pump wire when I connect my Edge Comp to it?

 

[RADdodge]

Siphon effect or head pressure? Either way, the lift pump is above the fuel tank. It sucks!



As far as installing a pusher pump, I marked the fuel line for cutting and bending and then dropped the tank. Dropping the tank seems daunting, but it proved to be fairly simple and easy. I siphoned as much of the fuel out of the tank (I used a couple of 5 gallon cans) and dropped the tank. Then I then removed the fuel line and cut, bent and reinstalled it. I wouldn’t have lost any fuel if I had kept my eye on the siphoning job! It was also easier to cut and bend (that is, if you’re going to bend it). I installed a shut-off valve before continuing the fuel line into the Racor filter. So I bent a 90 as soon as it cleared the tank. I couldn’t have done that with the line still in the truck.



I too, was concerned about the pusher pump noise, so I ordered the isolation kit. I have a couple pictures with descriptions of the setup in Reader’s Rigs. I also included a circuit diagram. Even if you don’t use my setup, it might give you some different ideas. It also has the Dodge wire color codes, sizes and connector pins. Click the link below if you want to see the pictures and diagram.



Honestly, dropping the tank was easy. I resisted but was almost forced to do it due to circumstance. Knowing what I know now, I’d just drop the tank. Some have done it by raising the bed, but I’d have to drop my Transfer-Flow tank to do that I believe. Anyway, there is always more than one way to skin a cat as they say (sorry cat lovers). If you have any questions about the setup I used, drop me a private message or email.

 

[TXRam]

The higher pressure with higher fuel level is due to the "Net Positive Suction Head Available". This would be true for a tank 10 feet above or below the tank also (i. e. , if you were pumping directly from an in-bed tank, your fuel pressure would be about 2 psig higher than with the stock tank fuel). Diesel is almost as dense as water, so the rule of thumb would be ~2ft of liquid heightwould be equal to 1 psi. This is also true when measuring the pressure - if you're checking it with a mechanical gauge, move the gauge up and down a few feet. You should see the pressure get higher as you lower the gauge.



When I installed my new lift pump on the frame, I disconnected the stock pump first and used my son's CO2 tank (for paintball gun) with some hose to push all the fuel back into the tank, BUT remember to take off the filler cap first. Other wise the little vent on top of the tank will not vent fast enough and the tank will pressure up some, and when you remove the CO2 tubing from the fuel line, diesel will blow out everywhere (don't ask me how I know this ).

 

[dmurdock]

guys, pull the fuel line at the rubber line near the fuel filter (pump side), feed compressed air to it for a while to empty the line. You can use compressed air at the filler nozzle (stuff an air hose thru a tennis ball. . works good) to re-pressurize the line.



ps, when i said stuff, i just meant drill/cut/etc a hole on each side of a tennis ball, then you can stick the end of an air nipple or whatever that small air attachment is called thru one hole, and air will be forced out the other side (tennis ball makes the seal, of course). I came up with this rigged up contraption when helping a neighbor change a fuel filter (then found out he had a dead lift pump).