Fuel Drain Back

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Fuel Drain Back

Postby SLY327 » Sun Jan 25, 2015 1:49 am

Hi , have a 327 in 75 Monza, just fitted new mechanical fuel pump, because I thought it was the problem, but no. It has a fuel return line back to tank that has been disconnected (was 4 cylinder originally), The pump works fine but the inline filter never gets full and it is always hard to start and I think the fuel is draining back to tank, Q. does the return line tube at the sender need to be blocked off or open ? And does the filler cap need to be venting or non venting ? Cheers Simon
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1975 Chevrolet Monza 2+2


Re: Fuel Drain Back

Postby SLY327 » Sun Mar 01, 2015 5:51 am

If anyone is interested or has the same problem, I found the problem, rubber hose from fuel sender to main fuel line perished and letting in air, all fixed :th:
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Re: Fuel Drain Back

Postby cjbiagi » Sun Mar 01, 2015 7:19 am

Good catch on that problem. Yes, there are two short rubber hoses that connect the tank to the main fuel lines. Very easy to never think of those and overlook them. After 40 years I would bet that all of them need to be replaced. I know I replaced them when I refinished my fuel tank a number of years ago, good thing for everyone to check. Oh, by the way, originally that second line was a vapor line connected to the emission canister, not a return line. Although it can be used as one if need be.
Clyde.........75 Monza 2+2
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Re: Fuel Drain Back

Postby Monza Harry » Sun Mar 01, 2015 3:25 pm

SLY To add to Clyde's post that vapour line can be used as a return line but I would alter it inside the tank! In the as-is state it could cause a lot of aeration as the baffle in there would cause the returning fuel to fall in a spray and drag air in with it. I would remove the little baffle (a small "C" shaped cover and add a tube to reach down to near the bottom of the tank and preferably away from the pick-up) I know it sounds easy and it will offer some challenge as space and routing without interference to the sender and the pump (we know you are using a mechanical one [still?] is tight) Did you remove the electric one or are you going to stage them in series? I am not sure if you will need a vented cap or not as the factory set-up is vented with the vapour canister set-up that Clyde had mentioned. Let us know if you find a vented cap that fits our cars as I am sure a few of the team will want/need this info sooner or later, therefore I would GUESS that a vented one will be necessary, if the canister line is blocked off at the rear and I wouldn't leave it open in the engine compartment myself! If you block off the line with a vacuum cap make sure you use a clamp on it, or better yet mechanically cap/remove it (weld/braze etc. off the tank of course) Harry
Last edited by Monza Harry on Sun Mar 01, 2015 3:33 pm, edited 1 time in total.
I'm not a hoarder I'm a preservationist 78 Monza Spyder (Soon To Be 2+2 with Spoilers)
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Re: Fuel Drain Back

Postby zzip » Sun Mar 01, 2015 3:32 pm

Hey SLY,

last year a had the same problem :

viewtopic.php?f=20&t=40859

Good luck

Peter from Antwerp
1988 Monza L4 2.5L
1989 Monza Spyder L4 2.5L
1990 Monza Spyder V6 3.2L
1991 Monza Spyder V6 3.8L
2013 Monza V6 3.8L
2014 Starfire V6 3.8L
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Re: Fuel Drain Back

Postby SLY327 » Tue Mar 03, 2015 7:39 am

Thanks guys, I am not using the vapour line, it is just running from engine bay to tank (not connected), I have capped the tube on the sender unit so no air can be drawn in that way, is the electric pump in the tank? as there is no electric pump mounted externally, previous owner may have got rid of it, engine came with mechanical pump on engine. The fuel filler is a metal threaded unit, after market job, looks like it is vented, cheers Simon
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1975 Chevrolet Monza 2+2


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