3 Line mech. fuel pump?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

383drvr · 10-13-2003, 07:18 PM

My '83 has dual tanks, Q-jet carb, and has a 3 Line mechanical fuel pump on the engine. Why in the heck is there a third line? The only thing I can come up with is that its a return line for an optional TBI system.... Any ideas? I'm just trying to simplify everything for when I do my rebuild, and would like to eliminate some of this nonsense....

Also, on a different subject, Is it ok for me to plug the vapor canister line that goes to the tanks? When I removed the canister and plugged that line last time, the tank selector valve split and spewed gas... Its possible that that was a coincidence, because the valve didnt work and was damaged to begin with, Its just that I dont want to go through having to replace it again.... (junkyards suck). Does plugging that line create pressure in the tanks that can crack the valve???

Thanks for listening

*1FaastC10* · 10-13-2003, 08:01 PM

the third line on your put is a return line. it is not for an "optional" TBI setup, as TBI wasnt available until 1987. the vent line to the canister cannot be plugged, the tanks rely on it to relieve pressure from the tank, jsut as you suspected. if you can find a vented gas cap that will fit your truck, then you would be able to plug that line.

low 84 · 10-13-2003, 08:44 PM

i went through the same thing a few months ago. like jeremy said the third line is the return. the big one coming from the tank is the feed and the small one is the return. i removed the return line and plugged it back at the tank and switched to a holley 110 g/ph mech pump. i also removed the vent line that was going to the engine compartment and got a vented gas cap. haven't had any problems with it. now i only have one line coming from the tanks instead of 3.

383drvr · 10-13-2003, 09:01 PM

Thanks guys! I do have vented caps, Thats why it boggled my mind when the tank valve blew... I have since checked the caps to see if the rubber vents were stuck, they seem to be fine as far as I can tell... I guess it was just time for the selector valve to give up...

Thanks again

Mike76251 · 10-13-2003, 11:33 PM

The third line is for fuel return as stated but what was not stated was; why is there a third line?
To prevent vapor-lock by constantly cycling fuel through the fuel pump to cool it.

Destructo · 10-14-2003, 12:12 AM

Well, mine is plugged at the fuel pump, somebody put in a 79 engine out of a truck and it must have had only 2 lines like usual so I guess instead of putting the right pump on it they just plugged it off

they did a lot of dumb things on my truck, I wish I could have a chat with them lol.

crackajacka · 10-14-2003, 12:29 AM

o so thats what the screw in that hose is for on mine tho its over on the drivers side where the canister is. no wonder it relived a lot of pressure when i went to fill up lol

383drvr · 10-14-2003, 04:46 PM

LOL.

Thanks Mike for the explanation. Now I can make an edumacated decision on what I should do with the lines.

low 84 · 10-14-2003, 06:37 PM

you don't need the return line. try finding an aftermarket mechanical pump with a return line, carter, holley, edelbrock? you won't find one cause you don't need it. just a vented cap and your good to go. sounds like you are eliminating most of the useless emmsions stuff anyways. might as well lose that too

fanman · 10-14-2003, 07:01 PM

Ok, now here's a question. When I replaced the engine in my 77 3/4 4wheel drive, I had to use my stock pump. The one on the new engine only had two lines (mine has 3). But the new engine came out of a 77 3/4 two wheel drive. I know it's the factory style pump (my grandpa owned the truck since new). So why are there two differn't pumps for the same year?

*mike reeh* · 10-14-2003, 08:14 PM

the 2 wheel drive truck didnt have AC did it? 3 line pumps were only on heavy duty cooling system trucks with AC (in 77)

mike

crackajacka · 10-14-2003, 11:56 PM

just wondering how would u go about gettin rid of the third line?

10-13-2003, 07:18 PM	#1
383drvr Registered User Join Date: Oct 2003 Location: Dallas, Tx. Posts: 308	3 Line mech. fuel pump?? My '83 has dual tanks, Q-jet carb, and has a 3 Line mechanical fuel pump on the engine. Why in the heck is there a third line? The only thing I can come up with is that its a return line for an optional TBI system.... Any ideas? I'm just trying to simplify everything for when I do my rebuild, and would like to eliminate some of this nonsense.... Also, on a different subject, Is it ok for me to plug the vapor canister line that goes to the tanks? When I removed the canister and plugged that line last time, the tank selector valve split and spewed gas... Its possible that that was a coincidence, because the valve didnt work and was damaged to begin with, Its just that I dont want to go through having to replace it again.... (junkyards suck). Does plugging that line create pressure in the tanks that can crack the valve??? Thanks for listening

10-13-2003, 08:44 PM	#3
low 84 Registered User Join Date: Jan 2002 Location: Houston, TX Posts: 2,144	i went through the same thing a few months ago. like jeremy said the third line is the return. the big one coming from the tank is the feed and the small one is the return. i removed the return line and plugged it back at the tank and switched to a holley 110 g/ph mech pump. i also removed the vent line that was going to the engine compartment and got a vented gas cap. haven't had any problems with it. now i only have one line coming from the tanks instead of 3. __________________ '51 F100, backhalf with a narrowed 9 and coil overs, 18.5" mickeys, monte front clip, +400" sbc in the works '05 1500 Crew Cab RIP '84 swb 5/7 drop, solid cammed 408 w/ fully ported dart heads, th350 w/ 10 inch hughes, 12 bolt w/ 3.73 richmonds and a locker '80 swb 4x4 in progress: 7" lift, 350 th350/np205, d44 and 1.5" ORD tie rod, 14BFF w/discs , armored diff covers, 40" MTR's

10-14-2003, 12:29 AM	#7
crackajacka Stylin Join Date: Sep 2001 Location: Tuttle, OK Posts: 620	o so thats what the screw in that hose is for on mine tho its over on the drivers side where the canister is. no wonder it relived a lot of pressure when i went to fill up lol __________________ Ive been traveling for some time, with my fishin pole and my bottle of shyne. Most Girls like finished products and just dont understand what a good addiction does for you http://www.paulwall.us/

10-14-2003, 06:37 PM	#9
low 84 Registered User Join Date: Jan 2002 Location: Houston, TX Posts: 2,144	you don't need the return line. try finding an aftermarket mechanical pump with a return line, carter, holley, edelbrock? you won't find one cause you don't need it. just a vented cap and your good to go. sounds like you are eliminating most of the useless emmsions stuff anyways. might as well lose that too __________________ '51 F100, backhalf with a narrowed 9 and coil overs, 18.5" mickeys, monte front clip, +400" sbc in the works '05 1500 Crew Cab RIP '84 swb 5/7 drop, solid cammed 408 w/ fully ported dart heads, th350 w/ 10 inch hughes, 12 bolt w/ 3.73 richmonds and a locker '80 swb 4x4 in progress: 7" lift, 350 th350/np205, d44 and 1.5" ORD tie rod, 14BFF w/discs , armored diff covers, 40" MTR's

10-14-2003, 07:01 PM	#10
fanman Registered User Join Date: Aug 2002 Location: Auburn, MI Posts: 156	Ok, now here's a question. When I replaced the engine in my 77 3/4 4wheel drive, I had to use my stock pump. The one on the new engine only had two lines (mine has 3). But the new engine came out of a 77 3/4 two wheel drive. I know it's the factory style pump (my grandpa owned the truck since new). So why are there two differn't pumps for the same year? __________________ "When I start my car in the morning I want the neighbors to think the world is comming to an end." Homer Simpson I'm not stuck, I've still got some gas left"

10-13-2003, 08:01 PM	#2
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	the third line on your put is a return line. it is not for an "optional" TBI setup, as TBI wasnt available until 1987. the vent line to the canister cannot be plugged, the tanks rely on it to relieve pressure from the tank, jsut as you suspected. if you can find a vented gas cap that will fit your truck, then you would be able to plug that line.

10-13-2003, 09:01 PM	#4
383drvr Registered User Join Date: Oct 2003 Location: Dallas, Tx. Posts: 308	Thanks guys! I do have vented caps, Thats why it boggled my mind when the tank valve blew... I have since checked the caps to see if the rubber vents were stuck, they seem to be fine as far as I can tell... I guess it was just time for the selector valve to give up... Thanks again

10-13-2003, 11:33 PM	#5
Mike76251 Senior Member Join Date: Mar 2003 Posts: 2,499	The third line is for fuel return as stated but what was not stated was; why is there a third line? To prevent vapor-lock by constantly cycling fuel through the fuel pump to cool it.

10-14-2003, 12:12 AM	#6
Destructo Registered User Join Date: Sep 2003 Location: Lewisville, NC Posts: 1,836	Well, mine is plugged at the fuel pump, somebody put in a 79 engine out of a truck and it must have had only 2 lines like usual so I guess instead of putting the right pump on it they just plugged it off they did a lot of dumb things on my truck, I wish I could have a chat with them lol.

10-14-2003, 04:46 PM	#8
383drvr Registered User Join Date: Oct 2003 Location: Dallas, Tx. Posts: 308	LOL. Thanks Mike for the explanation. Now I can make an edumacated decision on what I should do with the lines.

10-14-2003, 08:14 PM	#11
mike reeh 5 day ban, learn to behave. Join Date: Nov 2002 Location: san diego, ca Posts: 683	the 2 wheel drive truck didnt have AC did it? 3 line pumps were only on heavy duty cooling system trucks with AC (in 77) mike

10-14-2003, 11:56 PM	#12
crackajacka Stylin Join Date: Sep 2001 Location: Tuttle, OK Posts: 620	just wondering how would u go about gettin rid of the third line? __________________ Ive been traveling for some time, with my fishin pole and my bottle of shyne. Most Girls like finished products and just dont understand what a good addiction does for you http://www.paulwall.us/

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