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desertjoe 
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Joined: 23 Sep 2013
Location: New mexico
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 Topic: Does an engine really need,,,,
Posted: 13 Dec 2020 at 11:07am |
OK, I been contemplating something,,,why do some 70's to 80's trks have a return line from mechanical fuel pump back to the tank,,,and some don't? I worked on a guy's trk some time back that had a 350 with no return line and yet another trk recently that DID have a return line? I was going to have to do some repairs to the return line on my 82 Silverado,,SOoooo, I go to Oreillys and ask for a fuel pump with ONE line FROM the tank and no return line and the dude tells me,,,ALL fuel pumps have to have a return line back to tank,,AND,,some fuel pumps have 3 lines on them,,,!! WTH,,?? He told me there are no pumps available with only one line TO/FROM the tank for the BBC's,,,?? As a fact of the matter,,the 406 SBC that I built for ole red has a Holley pump with one line FROM tank and ONE line to the carbs. Ain't NO return line,,,!! SOOO,,what gives,,??
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DougG
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Joined: 20 Sep 2009
Location: Mo
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Posted: 13 Dec 2020 at 11:43am |
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Ive wondered that too , but where does the excess gas/ fuel go ? Im thinking they have to return
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Michael V (NM)
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Location: NM
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Posted: 13 Dec 2020 at 11:47am |
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emissions maybe?? regulate fuel pressure?? I had a 72 chev pick up that had that 3 line setup,, never understood why. I don't know..hopefully there is some old time chevy folks out there that know...
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john(MI)
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Posted: 13 Dec 2020 at 12:21pm |
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You've probably been all over that engine and know, but didn't some chevys have a return line from the carb?
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D14, D17, 5020, 612H, CASE 446
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PaulB
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Posted: 13 Dec 2020 at 12:28pm |
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Those return lines help to cut down on vapor locking.
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If it was fun to pull in LOW gear, I could have a John Deere.
Real pullers don't have speed limits.
If you can't make it GO... make it SHINY
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SLee(IA)
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Joined: 11 Sep 2009
Location: Polk City, Iowa
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Posted: 13 Dec 2020 at 12:48pm |
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Ray54
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Posted: 13 Dec 2020 at 3:38pm |
Part of the emissions control things. Vapor recovery system. Dad had a new 75 Chevy to pull his new 20 foot GN stock trailer. Guy got him to haul 8 bulls from Tucumcari New Mexico to California. He supplied a 100 gal gas tank in the bed. My dad really like you could move cows all day at home and never have to think about gas.
So in 81 replaced trucks on the trailer. He would not think about the standard 2 tank set up you could get from the factory. Put his big tank in back, turn that tank on and it filled the regular gas tank with the by passed fuel from the vapor recovery canister. Over fill the regular tank and you flooded the vaper recovery canister which flooded carburetor and kill the engine. Let it sit a few hours and it would run again. The local dealer was not known to have the best shop and they never figured it out. But once it was figured out and second manual valve to direct the by passed fuel all was well but took way over a year to figure out. Cannot say what year it was put in but 75 did not have it and 81 did. May have been different years in different states as California has lots more rules than other places.
So Joe I think you will need to keep the by pass line open to get rid of extra gas they pump to carry the vapor away.  But then again a guy as clever as you if you don't need smog test on it.  |
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Coke-in-MN
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Joined: 12 Sep 2009
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Posted: 13 Dec 2020 at 8:38pm |
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Most units without air conditioning used a 1 line system but with air they used a return line - a 77 Suburban i had used 2 lines , my 77 Pickup had 1 line , a 82 chev pickup had 2 lines and also a 2 tank system so the gas lines had a motorized valve to switch between tanks where my 77 had a solonoid that switched tanks but when key was shut off defaulted to 1 tank -
So it depended if or how truck was equipped - and YES it was to prevent vapor lock of system .
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Life lesson: If you’re being chased by a lion, you’re on a horse, to the left of you is a giraffe and on the right is a unicorn, what do you do? You stop drinking and get off the carousel.
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desertjoe
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Posted: 13 Dec 2020 at 8:50pm |
Well,,it prolly is to do with vapor lock ,,,SOOO,,here is another thing...I remember years ago several guys installed a manual "switching valve" under the drivers side seat where you could reach down with left hand and switch from tank to tank while driving. If I remember right, they were like a "T" but didn't have provision for any return line,,just two inlets and one outlet. Anybody remember them?
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Dakota Dave
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Posted: 13 Dec 2020 at 9:56pm |
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We have them on two old grain trucks. On the 66 there is no return line and it works fine. The 77 you have to make sure the main is empty befor switching the return goes back to the original tank and if you run from the aux with the main full it runs over. They are just plumbing two way valves.
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Coke-in-MN
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Posted: 13 Dec 2020 at 11:46pm |
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My 86 Diesel also used a return line so the motorized tank valve was kind of a real plumbing octopus as return line , supply line both needed to be routed correctly - but the fuel pump was single line which went to filter and filter fed the injection pump with a return line from injectors going back to proper tank
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Life lesson: If you’re being chased by a lion, you’re on a horse, to the left of you is a giraffe and on the right is a unicorn, what do you do? You stop drinking and get off the carousel.
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GSTROM99
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Joined: 12 Dec 2020
Location: Greene, IA
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Posted: 14 Dec 2020 at 12:59pm |
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My '78 Suburban with 454 big block had a fuel smell for a long time. I never saw any drips. Turns out the 1/4" return line had rusted and that was the culprit. Replaced the whole line (tank to pump) and no more issues.
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desertjoe
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Posted: 14 Dec 2020 at 8:16pm |
Well, M99,,now that you mention that,,I have also been thinkin I smell gasoline every time I go by the Silverado. I did some re-routing of the return line with some new rubber fuel line hose and I crawled under the trk twice now thinkin about a possible leak ,,,but NO LEAK!! The trk was parked on the dirt when I was re-routing the lines so hopefully thats what it is,,,CHIT,,I lost a 1/2 tank before I got it stopped,,,,
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