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Old 11-14-2018, 09:44 AM   #1
RADustin
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Gas vs Diesel Gas Tank Sending Unit

Gents,

What's the difference between a 3 line gas and a 3 line diesel sending unit?

I'm converting my truck to a duramax, so I just want to make sure I choose the right one.

Also on a two line, is it supply and vent, or supply and return?

I'll need a 3 line for the drivers side(supply/vent/return), and a two line for the passenger(supply/vent).

Thanks!
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Old 11-14-2018, 10:43 AM   #2
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Re: Gas vs Diesel Gas Tank Sending Unit

Gas and diesel were pretty much the same til 87 when GM switched over to in-tank fuel pumps for TBI on the gasoline rigs.
Original 78-83ish diesel senders had a fourth line for a tank water drain that was later deleted because it was unnecessary.

The gotcha you may to run into is the the Dirtymax may need a higher total fuel flow GPM than the 80's 6.2L diesel and gasoline engines so the supply and return lines may need to be larger. If this is the case you'll need to modify the sender with larger fuel supply and return lines.

2 line is vent and supply... Doesn't the Dirtymax require a return line? You can't return to the tank that's not in use.
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Last edited by hatzie; 11-14-2018 at 10:58 AM.
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Old 11-14-2018, 10:57 AM   #3
RADustin
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Re: Gas vs Diesel Gas Tank Sending Unit

OK cool.

you're exactly right on the tube size for the dmax feed. I have that figured. I'm going to turn the factory 'feed' into the engine return and remove the pickup filter. I'll add a new feed and return for the lift pump via weld in bungs.

I'm about to update my build thread, I'll explain it more there.

like this-

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Old 11-14-2018, 11:01 AM   #4
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Re: Gas vs Diesel Gas Tank Sending Unit

You do not want to return fuel to the bottom of the tank. It'll aerate the fuel. That'll raise hell with the injection system and wear the fuel pump and injectors out... If she'll run with that much air in the fuel.

Might I suggest that you just use the vapor line on the stock sender and build a new supply and return...

You want to factor in assembly... Those clamps don't look very accessible if you don't have the hand size of an 8 year old to reach thru the sender bung. I think I'd modify the stock sender with larger tubes or put a removable PEM-stud or PEM nut bung next to it with the supply and return lines.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


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And please let us know if and how your repairs were successful.

Last edited by hatzie; 11-14-2018 at 11:10 AM.
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Old 11-14-2018, 11:08 AM   #5
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Re: Gas vs Diesel Gas Tank Sending Unit

Not trying to be a wet blanket. I'm a realist.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
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Old 11-14-2018, 11:56 AM   #6
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Re: Gas vs Diesel Gas Tank Sending Unit

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Originally Posted by hatzie View Post
You do not want to return fuel to the bottom of the tank. It'll aerate the fuel. That'll raise hell with the injection system and wear the fuel pump and injectors out... If she'll run with that much air in the fuel.

Might I suggest that you just use the vapor line on the stock sender and build a new supply and return...

You want to factor in assembly... Those clamps don't look very accessible if you don't have the hand size of an 8 year old to reach thru the sender bung. I think I'd modify the stock sender with larger tubes or put a removable PEM-stud or PEM nut bung next to it with the supply and return lines.
I'm going to check on accessibly tonight. I was fairly sure I can fit my hand through the stock sender location to snap the clips on, if I can't I have other options.

I may lengthen the metal tube from the top bulkhead all the way down into the sump, and not use a rubber line at all. That top metal tube will be welded to the bottom of the ORB 90. If I do this, I'll just have to make a jig to locate the top and bottom holes pretty close to concentric. The bulkheads will be ORB, so there is a jam nut to set fitting rotation and general height, so that's to my advantage.

I'm using an airdog II-4G lift pump, so that should filter the air out. Although I'm still uncertain about returning the fuel above the liquid level or below. I always thought below the liquid level would produce less bubbles. Like filling a bucket of soapy water. If you put the nozzle in the water it doesn't foam the soap as much as if you spray the top. Also returning below in a metal tank would be quieter, less waterfall effect inside the tank.

I can always drill the return tube[factory feed] with holes in the sides to give the fuel a way out before its under the liquid level, and cut it shorter right below the sender. I really don't want to get into trying to modify the sending unit with larger/additional tubes.

I'm not sure what you mean by PEM studs?

Basically I have 5 connections-
1- Lift Pump Feed, 1/2"
2- Lift Pump Return, 3/8"
3- Engine Return, 3/8" [original feed]
4- Vent, 5/16"
5- Jockey Fill, 1/4" [original return]

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Not trying to be a wet blanket. I'm a realist.
no I appreciate the conversation.
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Old 11-14-2018, 12:40 PM   #7
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Re: Gas vs Diesel Gas Tank Sending Unit

https://www.pemnet.com/fastening-pro...fasteners-new/

GM returned fuel to the top of the tank. There's usually a good reason for these kind of design choices.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
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Old 11-14-2018, 01:04 PM   #8
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Re: Gas vs Diesel Gas Tank Sending Unit

I see a few aftermarket sumps that return to the bottom as well.

I'm not sure how the engine will like returning to a small amount of pressure due to fluid height, so I can drill some holes in the tube every 1-2" or so. Then as the tank empties the return will naturally use more of the length of tubing. And I don't have to worry about any weird pressures.

I'll probably do the same thing for the lift pump return.

My biggest concern with top vs bottom return is like I said, noise. Something a late model plastic tank doesn't have to worry about.
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Old 11-14-2018, 02:36 PM   #9
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Re: Gas vs Diesel Gas Tank Sending Unit

if your going this custom of a build why still use the 16 or 20 gal side tanks ?

look in to swapping in a 30 or 40 gal burb rear inframe tank setup .

unless you have something in the way of were it would go like a big custom hitch or air ride components .
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Old 11-14-2018, 02:48 PM   #10
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Re: Gas vs Diesel Gas Tank Sending Unit

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Originally Posted by sweetk30 View Post
if your going this custom of a build why still use the 16 or 20 gal side tanks ?

look in to swapping in a 30 or 40 gal burb rear inframe tank setup .

unless you have something in the way of were it would go like a big custom hitch or air ride components .
or a spare tire.

Im determined to make the saddle tanks work for me.
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Old 11-14-2018, 06:14 PM   #11
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Re: Gas vs Diesel Gas Tank Sending Unit

My LB7 has 1/2" supply fuel line. I used a Wix water separator then the Kennedy fuel pump into a Wix secondary filter right in front of the fuel cooler. Makes 3PSI up to the engine. I did away with the filter at the engine.
So far so good.
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Old 11-14-2018, 06:19 PM   #12
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Re: Gas vs Diesel Gas Tank Sending Unit

I don't know much about newer common rail systems, but I've been around plenty of agricultural mechanical injection diesels. Most returned to the top of the tank. I assume it was simpler, with one less connection to leak.

Smaller Case skid loaders dumped the return half way up the side of the fuel tank. If the tank was more than half full, the return line would leak if disconnected. The large frame Ford tractors, almost from the very get-go in the 60's, used two tanks. The top tank merely gravity drained into the lower one. All fuel line connections were in the lower tank, which meant the return was "under-water" for half of the time. All seemed to work fine.

I don't know how air bubbles would be getting in the return line in the first place. The fuel coming back from the engine should be perfectly clear of air.

The return line doesn't like to be blocked, and will keep an engine from running if it is, but based on the older engines at least, it will build tremendous pressure if blocked. A few times I forgot to remove the vice-grip from the return line and blew the connections apart upon starting an engine.
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Old 11-14-2018, 06:49 PM   #13
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Re: Gas vs Diesel Gas Tank Sending Unit

Depends on how low you're running the fuel.
My folks have a habit of not filling the tank til it's down around 1/8 or so.

I was thinking that if you agitate the fuel enough with the level low you could get the same effect as a blender. Maybe I'm all wet. Food for thought tho. That's why I brought it up. I welcome people poking holes in my theory if you have evidence to the contrary.

Dropping fuel from the top of the tank could be equally bad but obviously isn't... or at least they get away with it so...
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 11-14-2018 at 06:54 PM.
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Old 11-14-2018, 08:00 PM   #14
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Re: Gas vs Diesel Gas Tank Sending Unit

rule #1 with a diesel min of 1/4 tank at all times .

rule # 2 with a diesel refer to rule # 1 .

only time i ever lost prime was fuel filter changes on some even with prefill . and ford power strokes suck . there is no primer pump . the owners manual says just keep cranking .
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Old 11-15-2018, 06:55 AM   #15
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Re: Gas vs Diesel Gas Tank Sending Unit

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Originally Posted by sweetk30 View Post
rule #1 with a diesel min of 1/4 tank at all times .

rule # 2 with a diesel refer to rule # 1 .

only time i ever lost prime was fuel filter changes on some even with prefill . and ford power strokes suck . there is no primer pump . the owners manual says just keep cranking .
Exactly
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Old 11-15-2018, 11:41 AM   #16
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Re: Gas vs Diesel Gas Tank Sending Unit

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Originally Posted by sweetk30 View Post
rule #1 with a diesel min of 1/4 tank at all times .

rule # 2 with a diesel refer to rule # 1 .

only time i ever lost prime was fuel filter changes on some even with prefill . and ford power strokes suck . there is no primer pump . the owners manual says just keep cranking .
Change that to 3/8 of a tank and I'm with you.
I'm getting older so much past 1/2 tank is too long for my bladder.

I'm not my dad... thankfully. There are folks like him out there.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
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Old 11-15-2018, 05:28 PM   #17
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Re: Gas vs Diesel Gas Tank Sending Unit

The flow rate of fuel is not high enough to churn the entire tank contents. This is especially the case with the tank being a general rectangle and not a circle...and the return isn't tangential to the sides.

The airdog lift pump I'm using is 100GPH or about the 16 gallon tank contents every 9.5+minutes. It's not anything crazy. The 3/8" return will flow at most ~4.8 ft/s(about 3mph).

Figure a power steering system is 10-15GPM or about 6-9 times greater than the fuel flow and comes out of a tiny reservoir. That's why power steering reservoirs need to be baffled(or some how dampened) on the return because they can easily churn the contents and cause cavitation.

Either way, my lift pump will fix any bubbles or prime issues. My concerns are noise and the pickup running dry.

I am going to change things around and get rid of the internal rubber hose and just use a one piece tube from the top all the way into the sump. I'll also drill the return in a few places to ensure the return is never against pressure.

I appreciate everyone's conversation.
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Old 11-15-2018, 08:06 PM   #18
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Re: Gas vs Diesel Gas Tank Sending Unit

no problem . keep posting up . look forward to the results .
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Old 11-20-2018, 07:44 PM   #19
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Re: Gas vs Diesel Gas Tank Sending Unit

Quote:
Originally Posted by RADustin View Post
...The airdog lift pump I'm using is 100GPH or about the 16 gallon tank contents every 9.5+minutes. It's not anything crazy. The 3/8" return will flow at most ~4.8 ft/s(about 3mph)...
The rule of thumb that I've heard most often for maximum design flow rate in a pipe is 5 ft/sec. This was assuming water, but light fuels are similar enough. That's merely a rule of thumb. I'm sure it can be surpassed easily, but 5 ft/sec would give you minimal pressure drop, no erosion of pipe surfaces from turbulent flow, and would make efficient use of the pump.
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Old 12-05-2018, 05:00 PM   #20
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Re: Gas vs Diesel Gas Tank Sending Unit

back to my original question, it appears the diesel sender units do have a 1/2" feed. this is useful for many LS swap guys and diesel guys alike.
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Old 12-05-2018, 10:12 PM   #21
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Re: Gas vs Diesel Gas Tank Sending Unit

I used a stock lb7 sending unit (modified) into a custom tank for my project. Very similar to yours. I have read your build, excellent work. You can make the retaining ring/ seat & weld on to tank. I can post some pics in a day or two. As a side note never return fuel from above it draws air in with it. Stock sending units for Duramax return to bottom.
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Old 12-05-2018, 10:26 PM   #22
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Re: Gas vs Diesel Gas Tank Sending Unit

even my tbi senders on my 87 the return line goes to the bottom .
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Old 12-06-2018, 08:29 PM   #23
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Re: Gas vs Diesel Gas Tank Sending Unit

As you can see its just a bolt down retaining ring. There is an index tab that needs to be clocked as you want it (about 1 o'clock in tank pic and facing on sending unit.)
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The sending unit base is spring loaded on the 3 rods. They simply need shortened as the saddle tank depth is less than it can compress to.
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Old 12-07-2018, 12:45 AM   #24
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Re: Gas vs Diesel Gas Tank Sending Unit

I wouldn’t be concerned with returning fuel below fuel level as long as the return isn’t pointing at the pickup and is some distance away. Same with returning at the top. The issue there would be a fluid jet entraining air when it hits the fuel surface, like mentioned in the bucket of soap example or bubbles in the toilet after a pee break. Back pressure on the return is the same top or bottom, save for the slight additional restriction of a longer bottom return line.
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Old 12-07-2018, 04:58 PM   #25
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Re: Gas vs Diesel Gas Tank Sending Unit

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...
that's pretty cool man!
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