First drive in almost a year ... fuel leak and vapor Lock at 88 degrees outside!
 

Just got my car out of the body shop having had a new trunk floor installed and fixing some 30+ year old body work that finally started showing some rust flowers. I wound having the rear quarters, trunk opening and filler panel stripped, metal patched and the car painted from the doors back and restriped. Car looks good once again.

The drive (65 miles total) was uneventful until just after filling up. I started smelling fuel and pulled off. It was leaking off the corner of the gas tank onto the ground. I knew the tank had been removed and I thought, a hose clamp might have been missed but knew it was all in steel lines in that area. I needed an oil change anyway so I pulled into a shop and looked under the car. I found the unused vent on the corner of the tank missing the rubber plug I had put on. Made sense since it started to leak just after filling up. I figured the cold gas expanded in the tank, blew off the plug (which I hadn't clamped on) and ran out like an overflow. I plugged it with a vacuum cap and hose clamp, got the oil changed and off I went.

About 15 minutes later, the car began to vapor lock, bucking like a bronco like turning a switch on and off but not stalling. I switched on the my Holley red "pusher-pump" momentarily and it went back to normal as traffic began moving so I switched it off w. no further hiccups the rest of the way. My engine temp was 200 in stop and go and 190 on the highway and my pickup/sock/supply and return lines/pump probably have about 500 miles on them, but it definitely tried to vapor lock.

Another good reason to have that inline electric fuel pump.

Next on the agenda is the Sniper EFI install.

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Car looks good!

Vapor lock was the number 1 issue that drove me to my FiTech. Not a hint of it ever again, even in triple digit heat.

Todays pump gas isn't going to like 200 degree engine temps with 6 lbs. of fuel pressure for a carb.

We've talked about this in depth before when the subject comes up.

No sense in worrying about it now if going EFI. 60 PSI fuel pressure solves those problems.

Yep. I couldn't believe the struggle by brother had with vapor lock in his 461. He started to hate driving his GTO because every time he turned the car off when hot, it didn't want to start again for 15-20 minutes. He then began using ethanol-free gasoline and boom, problem solved.

Yep, just a brief annoyance.

I had goosed it at 40mph under a bridge to hear the exhaust note and with the kickdown to low, the car sidestepped like I was on dancing with the stars. It was running so good I was actually thinking "why am I going to EFI?".

A few miles later, the vapor lock smacked me with a dose of a reality and I thought "Yep, that's why.

I'm betting you'll like the EFI ;)

You can not plug the tank vent and expect the fuel to flow properly.
Fix the tank ventilation factory style and add an electric fuel pump (Carter P4594) on floor pan above rearend. Remove the mechanical fuel pump and cover the hole with a CBB plate.
Job done.

FWIW

So let me confirm this with you, you say you have a mechanical pump drawing through a electric one that you only turn on when you need it?

I kept the stock mechanical pump, with the Carter pusher 6 ir 8 psi i forget. There is a vapo-lock behavior these days when fully heated up running hot with the electric cooling far off.

You suppose i can remove the pusher pump and still run low 12s?

I installed the 1970 (pre-emissions) vent and filter system which uses 2 vent lines ('70 tank had only 2 vents) and capped the 3rd, unused vent off. '71's used all 3 vents but had a closed vapor system with charcoal filter, vapor return line and an accumulator all tied into the vacuum system.

The '70 vent filter is mounted high on the frame and just vents to atmosphere. My '71 frame still had the provisions and pre-drilled holes to mount the filter as a carryover from '70. The '70 - 2 vent system is just as capable a system for venting the tank.

As-mentioned, I already have a good electric fuel pump wired to a switch for intermittent use mounted ahead of the tank with the inlet below bottom of the tank. What you suggest is just the same style pump as I have but you suggest mounting in the wrong location. Rotary vane pumps, like the Carter and my Holley are made for gravity feed and not intended to lift fuel up from below. The inlet should always be below the level of the fuel to work efficiently and not cavitate.

While removing the mechanical pump would eliminate any tendency for vapor lock, the constant noise of the electric pump is not for me. That is why I installed it on a switch and use only when needed. When I felt the onset of vapor lock I turn it on. In cooler weather it is not needed, the car runs the same with or without the electric pump on. The mechanical pump is more than adequate without the whine.

Do you realize that when the electric pump is not running that the pump is then a great restriction and that if the fuel is hot enough at that point that the difference in pressure can make for vapor lock taking place right there at the pump?

I wouldn't think a mechanical pump would like any restriction at all on the suction side. I think some electric pumps have a by-pass, not sure how much restriction they create.

If you're going to run a pusher pump, that will be off some of the time, you need to run a bypass with a check valve.

That's an old wive's tale. I tested mine before installing. Free flow is not restricted. When the pump is off, the rotors will freewheel and rotate to the point where the fuel flows around the vanes.. Fuel flow moves the vanes away from the inlet and outlet ports and flows through the unrestricted path. It is no different than a surge tank with a plenum inside. It naturally assumes this position when fuel is flowing and the electric pump is not running. The suction from the mechanical pump is sufficient to make the fuel flow through the pump on a dry start too.

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Just because it is able to pull fuel through it doesn't mean that it's not a restriction. Doesn't take much pressure drop to make high vapor pressure fluid boil.

^^^ THIS ^^^

Carter makes an electric fuel pumps that is designed with the bypass (#4602RV). I think it is used mainly on motorhomes.

ANY other electric pump is a restriction, and it does NOT take much restriction with the crappy fuels currently offered to get the fuel to flash to vapor.

If I had this problem, and were using the later-style tank with multiple vents, I'd plumb all the vents together and keep them open. Think about it a different way - your tank built up enough PRESSURE to push off the cap you had installed over that extra vent nipple. That means it would also build up VACUUM when the tank is being emptied. It might need that vent to be open to atmosphere to function correctly.

It will take you about ten minutes to install a foot of fuel line with some kind of filter/sponge in the end of it and tie it up above the tank and go for a test-drive.

Good luck!

I get what you're saying, but the 3rd vent is not needed unless the evaporative emission system is installed. Early '71's came with a 2 vent tank and no emission system. Later production added the third vent as part of the emissions control. '68-'70 and early production '71 all used the same 2 vent tank. Both were positioned on the passenger side. Securely capping the 3rd vent (driver's side) does no harm and functions the same as '68-'70/ Early '71.

All replacement tanks for '71- '72 have the 3rd vent to eliminate early/late production confusion on ordering. If you have a two vent breather, you plug the unused vent on the driver side vent to match the early configuration ... or plumb it into the larger line. I just failed to secure the cap, my bad.

Does your floor pan have the cutout for the fuel separator?

I ask because once you go to EFI, and you want (and should) to use the EFI tank with in-tank pump, you will be limited to 2 vents on the tank, and one would be needed for a vent/separator return.

What I do is use the 2 vent tubes on the right of the standpipe for vent/return, and let it vent to atmosphere. There's no to little smell this way, and if you wanted to, you could even route one to the charcoal can.

It works good.

If the cap came off the one capped vent on the tank the way you had it, that means psi built up, and you may not have sufficient venting. Just a thought.

EDIT: The EFI tanks technically have a return which enters via the pump assembly to tank fitting. There are 2, small vents on the tank itself.


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Carter says their rotary vane pumps may be located up to 24" above fuel tank top.
I guess you didn´t read the instructions?
Mine has been running "dead head" for 25+ years at the location i mentioned.
I´m sure it would work for you too.

https://static.summitracing.com/glob.../crt-p4070.pdf

FWIW

It's your pump not mine and I could care less. I assumed by the shape it was also a rotary vane type. You must not have read those instructions too closely because it clearly states it is a diaphragm style pump, not a rotary vane type ... so it can lift. If you understood the instructions, you would recognize mounting as close to the bottom of the tank as possible is the preferred method as it places less strain on the diaphragm. Good on you that it's still working but it wasn't intended as a basement sump pump.