Author Topic: Fuel line question  (Read 2101 times)

Offline nighthawker5

  • Newbie
  • Posts: 22
  • Newbie
Fuel line question
« on: May 01, 2017, 08:54:13 AM »
I'm swapping an lq4 into an 86 k20 and I'm keeping the dual tanks.  I'm using the tanks, switch and sending units out of an 87. My question is there are three lines in/out of the sending unit.  One is the feed for the engine, the other is a return.  Is the third for the egr canister? If I'm not using egr what can i do too get rid of it?

Thanks in advance

Offline GMCMedic

  • Registered Users
  • *
  • Posts: 133
  • Newbie
Re: Fuel line question
« Reply #1 on: May 01, 2017, 09:32:14 AM »
Its for the EGR canister. Plug it

Sent from my SAMSUNG-SM-G920A using Tapatalk


Offline nighthawker5

  • Newbie
  • Posts: 22
  • Newbie
Re: Fuel line question
« Reply #2 on: May 01, 2017, 10:57:16 AM »
Awesome.  Thank you very much.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 5364
Re: Fuel line question
« Reply #3 on: May 01, 2017, 11:34:02 AM »
The third tank line runs to the vapor canister.  It has nothing to do with EGR.  The vapor canister is a passive device that is best left plumbed to the tanks to control noxious fuel odor as well as fuel tank evaporative emissions into the atmosphere.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline ehjorten

  • Senior Member
  • ****
  • Posts: 1004
Re: Fuel line question
« Reply #4 on: May 01, 2017, 01:52:31 PM »
But...how do you do that in our square bodies with an LS conversion?  The LS system has an Evaporative Emissions Purge Valve on the top of the intake, just above the Throttle Body.  Then on the Charcoal Canister there is an Evaporative Emissions Vent Valve that allows air into the system.  These two valves work in conjunction with a fuel tank pressure sensor.  Without the pressure sensor in the tank, how do you make it work?  Having the lines connected to the charcoal canister, but then not connected to the engine is almost pointless.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline GMCMedic

  • Registered Users
  • *
  • Posts: 133
  • Newbie
Re: Fuel line question
« Reply #5 on: May 01, 2017, 02:19:13 PM »


The third tank line runs to the vapor canister.  It has nothing to do with EGR.  The vapor canister is a passive device that is best left plumbed to the tanks to control noxious fuel odor as well as fuel tank evaporative emissions into the atmosphere.

Yes vapor canister. Sorry. Only the word cannister registered.

Sent from my SAMSUNG-SM-G920A using Tapatalk


Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 5364
Re: Fuel line question
« Reply #6 on: May 01, 2017, 03:08:54 PM »
But...how do you do that in our square bodies with an LS conversion?  The LS system has an Evaporative Emissions Purge Valve on the top of the intake, just above the Throttle Body.  Then on the Charcoal Canister there is an Evaporative Emissions Vent Valve that allows air into the system.  These two valves work in conjunction with a fuel tank pressure sensor.  Without the pressure sensor in the tank, how do you make it work?  Having the lines connected to the charcoal canister, but then not connected to the engine is almost pointless.

I've never been sufficiently interested to puzzle it out.  But, blending the two designs into a functional system shouldn't be too difficult with some investigation and a little ingenuity.  Perhaps that's naive.  Nonetheless, the tank needs to be vented and evaporative fumes managed.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline GMCMedic

  • Registered Users
  • *
  • Posts: 133
  • Newbie
Re: Fuel line question
« Reply #7 on: May 01, 2017, 04:40:50 PM »


But...how do you do that in our square bodies with an LS conversion?  The LS system has an Evaporative Emissions Purge Valve on the top of the intake, just above the Throttle Body.  Then on the Charcoal Canister there is an Evaporative Emissions Vent Valve that allows air into the system.  These two valves work in conjunction with a fuel tank pressure sensor.  Without the pressure sensor in the tank, how do you make it work?  Having the lines connected to the charcoal canister, but then not connected to the engine is almost pointless.

  Nonetheless, the tank needs to be vented and evaporative fumes managed.

Is this purely opinion? Other than a few unpleasant fumes and emissions states there is no reason not to let them vent out into the universe. China lets out more than enough pollutants that a few trucks arent going to destroy the world.

Sent from my SAMSUNG-SM-G920A using Tapatalk


Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17400
Re: Fuel line question
« Reply #8 on: May 01, 2017, 07:00:00 PM »


But...how do you do that in our square bodies with an LS conversion?  The LS system has an Evaporative Emissions Purge Valve on the top of the intake, just above the Throttle Body.  Then on the Charcoal Canister there is an Evaporative Emissions Vent Valve that allows air into the system.  These two valves work in conjunction with a fuel tank pressure sensor.  Without the pressure sensor in the tank, how do you make it work?  Having the lines connected to the charcoal canister, but then not connected to the engine is almost pointless.

  Nonetheless, the tank needs to be vented and evaporative fumes managed.

Is this purely opinion? Other than a few unpleasant fumes and emissions states there is no reason not to let them vent out into the universe. China lets out more than enough pollutants that a few trucks arent going to destroy the world.

Sent from my SAMSUNG-SM-G920A using Tapatalk

That's crazy, do you really want to deal with fuel vapors constantly escaping around your truck? It's also a fire hazard and the fuel system needs to be contained. Do you leave the cap off of your gas can and slosh it all around?

Using a TBI system's canister is the simplest solution.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline ehjorten

  • Senior Member
  • ****
  • Posts: 1004
Re: Fuel line question
« Reply #9 on: May 01, 2017, 11:46:16 PM »
The biggest thing is fuel vapors in your garage soaking into your interior.  Yes you can run the lines to the charcoal canister as they are stock, but what do you hook it up to?  On our trucks the canister goes to a ported line.  In the LS the line goes to an EVAP Purge Valve that is below the throttle blade and is controlled by the PCM via a EVAP Vent Valve that is in the stock LS canister that is mounted by the LS fuel tank.  It also uses a fuel tank pressure sensor and uses the fuel level sensor that is like 0-250 ohms, which is different than our fuel level sensors.

If you actually do some research on this you will find that people struggle with figuring this out.  In CA it can actually be an issue because of the emissions cops.

What I think I have figured out is that the EVAP Purge Valve is normally open and only closes when the PCM performs the leak test on the fuel system.  So...it may be entirely acceptable to run a hose from the truck canister to the EVAP Purge Valve.

Just food for thought on this topic.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline nighthawker5

  • Newbie
  • Posts: 22
  • Newbie
Re: Fuel line question
« Reply #10 on: May 02, 2017, 10:26:37 AM »
Ok so plugging it is not the best option.  I can just connect it to the line that's already there. connect the TB to the vapor cannister too and see how it goes. 

Offline roundhouse

  • Senior Member
  • ****
  • Posts: 1441
  • Newbie
Re: Fuel line question
« Reply #11 on: May 27, 2017, 10:51:12 PM »
I'm not sure the 87 switching valve will handle the pressure required for the LS system

The TBI system was 12 psi
You need hoses and a switching valve from a later model truck with high pressure efi unless you're gonna use the low pressure pumps in the tanks to feed the switch valve and then into a high pressure efi pump


Sent from my iPhone using Tapatalk

Offline Dr_Snooz

  • Junior Member
  • ***
  • Posts: 532
Re: Fuel line question
« Reply #12 on: May 27, 2017, 11:55:07 PM »
I had an old Honda with an iffy evap canister. It always smelled up the garage like fresh gas.
1986 GMC C-2500 Crew Cab, 7.4L, TH400 -- RIP Chester

1989 Chevy Suburban V-2500, 5.7L, TH400

1990 Chevy C-3500 Ext. Cab, 7.4L, 3L80

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17400
Re: Fuel line question
« Reply #13 on: May 28, 2017, 08:57:02 AM »
The EFI tank switching valve is rated at 60 psi. No need to over-think it. Use good efi rated fuel hose and clamps though.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline big_al273

  • Registered Users
  • *
  • Posts: 203
  • Newbie
Re: Fuel line question
« Reply #14 on: May 28, 2017, 03:50:34 PM »
The EFI tank switching valve is rated at 60 psi. No need to over-think it. Use good efi rated fuel hose and clamps though.
seconded

only other thing is if the switching valve  is old and/or of unknown condition i would get a new valve personally, failed valves can lead to an overflowing tank
1988 Chevrolet V30 Ex-firetruck
1984 GMC K1500