Fuel Evaporating or Draining Back?
 

My 69 Lemans has a factory metal fuel line with no return. New rubber lines, new Spectra pick up in tank, new vented fuel cap, a rebuilt Carter 6907 fuel pump, late 70s Buick QJet built by Cliff that I used on bird years back - marine epoxy used at well plugs. When the car sits even for just an hour or two, I have to prime the carburetor to get it started. I don’t notice much of a pump shot and extended cranking doesn’t help. Once I manually prime it and get it started,! Runs well and will start up as long as it’s not left sitting. Fuel is 87 octane, 10% ethanol (MD). Any ideas on what the issue is?

What temperature does your engine typically run? Could be fuel percolating in the fuel lines. Try one of these one way valves placed at the fuel pump inlet hose. https://www.amazon.com/Engraved-Dire...43209887&psc=1

I have a similar issue I've been trying to get sorted. Mine takes a day or so before I have to give it a few cranks to get fuel to the carb. Some folks say it's the fuel evaporating, but I didn't have this problem prior to my rebuild (when the carb was also rebuilt, too).

I was debating installing an aftermarket check valve, but based on old threads I read most mechanical fuel pumps already have anti-drainback valves by design. The fuel pump in my car is at least 10-12 years old so I'm going to try two things - a fuel filter with an anti drainback valve and a fresh fuel pump.

A fuel pump cannot function without exactly the same kind of check valve(s) as has been suggested.

Remove the fuel pump inlet hose, connect a vacuum pump to the fuel pump. Find out if the check valve leaks, and if so, how much/how bad is it?

Would be worthwhile to determine if the intake manifold is being over-heated by an exhaust restriction on one side leading to too-much exhaust blowing across the intake manifold exhaust crossover. A seized-shut (or partially-shut) heat riser (EFE) valve, for example, or the inner wall of a double-wall exhaust pipe having collapsed. Crimped exhaust tubes, etc.

The engine runs at 180 exactly, which is the thermostat rating. It hasn’t been driven on road, just idle time.

This engine has RA manifolds and the correct intake gasket for a 1972 intake on a 1969 engine. When I rebuilt the fuel pump using a Then and Now Automotive kit, I performed the blowing and suction test on the diaphragms and they operated as designed. Won’t hurt to check again now that the fuel pump is on a running engine.

I have considered an in-line electric pusher pump to prime the fuel line but wanted to check here first for any testing I may have overlooked.

I'm far from an expert but, I do have a mechanical pump and a Quadrajet. No other external check valve other than what the pump (and qjet filter??) may have. After running the motor, it can sit for up to 72 hours before requiring cranking/priming the bowl. After that it does take a few seconds of cranking. If it was a daily driver, it would fire right up every time. So that tells me there is something up with your system. I would try to find and fix that before adding a pusher pump.

I dont think mine is quite as bad as an hour or two, but if the Q jet on my Olds 455 in my wagon sits overnight you have to crank it for a good 7-10 seconds to get fuel into the bowls and get it started. As soon as it gets fuel it fires right up.

I recently rebuilt the carb that is on there and it made a huge difference. But I didn't do anything to the well plugs. So Ive been wondering if maybe they are leaking. It also probably has a 40+ year old fuel pump so it wouldn't surprise me if it had seal issues.

That said, even my Firebird which has an electric pump and fuel injection, it has to crank for a second after it has sat. Not nearly as long as the wagon with the carb, but more than if it had been running an hour or two before. As has been mentioned here before the ethanol blend fuel does evaporate quickly.

To check for leaks, pull the carburetor, set it up on a raised stand. (I use four 2 inch long bolts, one in each corner with nuts, for legs.)

Place the raised carburetor on a clean paper towel. Fill the float bowl with fuel and let it sit. If it's leaking you should see it. I try to do this test with a hot or at least warm carburetor as heat can help expose cracks.

Good advice. 100% honestly, if I pull the thing off Im probably just going to do the little kit to it that Cliff sells. Might as try and generate some content out of it.

But yes Ive read that unless they are severe they often don't leak until good and heat soaked. Which makes sense.

Does not having a return line to the tank affect this at all? It’s as designed by PMD.

Check your fuel line clamps & accelerator pump shot

I have a big Holley and a Robb Mc fuel pump. If mine sits for a while, I have extended cranking time also.


This modern pump piss has a high evap rate.

Using the same carbs from the 70s and 80s. They didn't do this back then, but they do now. Its the junk gas.

Rubber lines get old and you can't always tell they leak. They may not drip, but under suction they may leak. If they're old, replace them. If you suspect leaking, retighten the hose clamps for starters.

I refuse to use mech pumps anymore, even the ones that were 'good' back then, aren't anymore.

I've argued with folks over the RobbMC ones too, even the 1100 ones, they just don't work as good as electric.

Most people don't even realize they are leaving power on the table until they switch.

You do a 'proper' bypass and electric pump setup and there will be no question. If you have evap or drainback issues, with an electric, gone.

If an EFi conversion tank is available for your model even better. And you don't have to run EFi.

I've seen people throw all kinds of time & money at a mech fuel system only later to finally cutover. Just cut to the chase, do it right, do it once.

Even back in the day folks knew the factory systems didn't cut it. They were borderline right off the dealer lots. At best.


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All the rubber lines are new and the fuel pump rebuilt with an American made kit from a reputable recommend company. I’m starting to think the carb plugs are leaking even if sealed with marine epoxy.

The new fuel evaporates like crazy. It's designed to. For spills on ground, evaporate before it soaks in. I put a Holley Blue on my 470cid. Turn key, hear pump fill carb. When pump sound changes a little, she full, ready to go

It's possible. Leaking fuel leaves a witness mark, should be able to tell.

Maybe the epoxy cracked, it happens. I know JB Weld doesn't do well with heat cycles and gas, not sure what was used.


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My issue may be two -fold, evaporating fuel is one thing but cranking excessively doesn't seem to restore prime. Only after manually adding fuel to the bowl will it start up. Although I checked and didn't see any evidence of a fuel leak in the long hard line, the car sat outside for about 20 years. It still had fuel in the line but smelled like varnish when I flushed it. Is it common for steel lines to develop pin holes? This line has what appears to be undercoating on it.

Do a pump test into a bucket and time it, might be a pump issue.

Or maybe a fuel sock, or crap in the tank.



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General replacement tanks for that car are only like $175.



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My '79 T/A with the factory carb does the same thing that most of the people in the thread are reporting. Starts just about immediately if I crank it up within 14 or so hours of the last start, but if it sits for a day or more, It'll crank for 3-6 seconds after one pump press on the gas pedal before starting up.

I always assumed it was a probably loss of cranking compression because of my aftermarket cam, but apparently this is a very common thing with QJets even when they were new because of their shallow fuel bowls. Lars Grimswud is a QJet disciple over in the Corvette space and he remembers when these cars were for sale and brand new, any cars that were sitting on dealer lots for days on end wouldn't start until after several seconds or more of cranking.

But then you see videos of people starting QJet cars cold at literally the flick of a key. I dunno what to think.

The OP's issue does sound odd, though. Maybe invest in a return line? The check valve in the fuel filters were originally meant to prevent fuel drain back into the rest of the system in the event of a rollover crash, but they also help retain some of the fuel in the carb for starting. Do you also have a thick insulating gasket to also help with preventing fuel from boiling in the carb?

Gas these days in general, even the non-ethanol gas is formulated different from the gas of yesteryear. It's a lot more volatile across the board, and evaporates more readily. It doesn't take much evaporation in combination with the QJets shallow fuel bowl design to compromise the fuel needed for an immediate cold start.

I do - it's nearly 1/4" thick

Good idea for the pump test - the pick up assembly is new and the tank appeared surprisingly clean inside

Eventually I will install the EFI tank I bought from you - but need to resolve this issue to buy myself time

Are you positive your gas cap is venting?

I suppose not - but it is new from Ames and once the car is started it runs well and will start up well within the hour.

Fuel disappearing
 

I've had this problem before and assumed that the only way this could be happening was for the fuel to be evaporating out of the float bowl. I'm not an expert, but fuel fills the float bowl from above and can't "drain-back".

If evaporation is the problem, is there a way to reduce it?

LeMans gas caps are supposed to be closed and venting thru a vent line from the filler tube.

I have found fuel "drain-back" thru fuel inlet is a myth.
Fuel evaporation is not, as the carb is 100% vented to atmosphere.
It was always the case then and more so now with todays fuels aimed for closed fuel systems and fuel injection engines.

FWIW

So, when enough fuel evaporates or leaks from the float bowl, the needle valves open and some fuel may come in until the fuel pressure goes down. Then it seems like fuel would continue to evaporate potentially all the way to the fuel pump??

Then when we go to start the engine, fuel has to fill the fuel line and the float bowl(s) before the engine can start? I’m not sure how cars with the return line would act differently? Are there other considerations I’m missing?

Alcohol in fuel makes this problem more pronounced.

I ended up putting an electric fuel pump just in front of the gas tank on both of my Pontiac's. Quadrajet on one Tri-Power on the other. Turn the ignition on, wait for the change in sound from the pump. Pump the gas to set the choke, and they start right away. It is either that or buy race gas.

Is there really a need for a car to start fast? Define a slow start.

Cranking for 3-5 seconds after it's sat for a week does not sound unacceptable to me. Certainly not excessive by any means. In fact that's pretty normal. I think people have gotten spoiled with EFI

Now if you're sitting for just an hour and your cranking for several seconds then you likely have an issue to address and a reason for question. Putting a pump at the back of the car is just a bandaid for the root cause. It'll mask the problem but it's not "fixing" it.

It's likely draining out of the carb somehow, or you have an excessive heat problem. There is no way what so ever all the fuel in the float bowl will just evaporated into the atmosphere on its own in just an hour. Not gonna happen.
Now if it's hotter than it should be percolating or something then I'd dive into that area.
The fuel in the float bowl is leaving some other way.

Think about it this way. You can have dry lines, a mechanical fuel pump that's dry or bled off. None of that matters if there is fuel in the float bowl. The engine will start and run with just fuel in the bowl and actually idle for a good 30 seconds or more. It doesn't even need fuel pressure. That's more than enough time for the pump to grab fuel and fill things up.

I wouldn't listen to that guy if I wanted to learn anything!

For example, anyone who has worked with a Holley knows that the needle valves are at the top of the float housings and that a Holley has countless places where fuel can leak.
In addition, all mechanical fuel pumps have a check valve that prevents fuel from "flowing back into the tank".

FWIW

I have been forced (reluctantly) to install electric fuel pumps on ALL my older vehicles because of the panther-pxxx currently being sold as gasoline.

Instead of guessing, test. After the engine starts and you know there is fuel in the carburetor, fill the carburetor through the bowl vent until it runs out, measuring the amount of fuel added. Let the engine sit for 24 hours after being turned off when hot, and repeat the test. Compare the amounts of fuel.

The twin 4-barrels on my shop truck will be bone-dry after 30 MINUTES! Since they are AFB's there is no way for them to leak down, the fuel evaporates! With a conventional fuel pump, had to crank sufficiently long to fill 4 fuel bowls; seemingly forever.

One other item to consider, is that if the bowl(s) have evaporated dry; the accelerator pump may NOT function on the first stroke even after filling the carburetor bowl, as the accelerator pump well is dry!

Jon

This is how my line is connected - from filler neck to fitting- no line on the other nipple and no idea wher it would lead to/connect

This is probably worth little to sold the OP’s problem but for discussions sake, after having a similar circumstance I ended up tapping and plugging the well plugs which seemed to improve but not solve my issue. I also made adjustments to the choke and ultimately came to the conclusion that there must be some evaporation.

With less than a few days sitting I get very fast startups after one accelerator press. More than that or all the way to sitting over the off season for a few months I usually crank for 2-3 seconds to bring fuel to the bowl, then press the accelerator once to set the choke and squirt in a pump shot followed by another 2-3 seconds of cranking before it fires.

Here is a pic from the 1969 Service Manual:

Similar design on my 70 LeMans.

grivera has the U-shaped pipe in place and the diagrams show nothing connected to the open end of the pipe, like his. Assuming the grommet is out of place intentionally, that's the vent design and it should work as intended.

Why they used the u-shape pipe, I don't know. Probably something to do with managing liquid gas... maybe regulatory? Seems weird because, at least on mine, the tank has another vent on the front side that is basically a vertical tube. It has a filter stuffed in it, but no u-shaped turn. ????

Mike

Mine has that vent at front of tank too- I bought/installed the hose with the foam on end of it from Ames

grivera - does a 69 have a return line from the pump to the tank?

My 70 does, even though it is basically a trickle - more of a controlled vent back to the tank than actually moving fuel to reduce load on the pump.

The tank connections are kind of busy, at least on my 70.

Here's a 70 drawing that might help and some pictures of the U-tube I snagged when replacing the tank.

I'm a believer in evaporation being the dominant source of loss and significant enough to be a PITA after only sitting a day or two.

I also know the mechanical pumps have a built-in check valve.

IIRC, the stock filter at the carb inlet also has a check valve, correct?

Regardless, I think I've asked before but what specific electric pumps are used for this? I suspect I will want one.

Mike

Some pumps have two check valves--one inlet, one outlet.

Some pumps have six check valves--multiple inlet, multiple outlet for higher flow volume. No doubt over the course of automotive production, there's pumps with other numbers of check-valves.

It's entirely possible for them to not seal completely. Even slight leakage of an inlet and outlet will drain everything from the needle valve to the level of the fuel in the tank, given enough time.

Thus my suggestion to connect a vacuum pump to the inlet of the fuel pump, to see if--or how badly--the inlet check valves leak.

Mine does not have a return line. Also, I have a vented gas cap which someone mentioned shouldn’t be. Not sure if that has any effect on my situation. I’ve been working on the 66 so didn’t get time to mess with this today other than getting that picture.

I know some caps have vented printed on them and they don't vent for chit.

I know many have converted on earlier cars to an external vent/separator, and have reported improvements, but can't give specifics on that. Most I've seen make those remarks don't provide data.

The Rick's tank I got came with a neck that has the late model type baffle and cap that ratchets when tightening, that I am fairly certain isn't vented. I run an OE type separator/vent so don't really care. The way I have it setup it has like 3 5/16 vents that are open to atmosphere, and I have no smell or vent issues.

I've tested free-flow on the last 2 tanks trying to troubleshoot a delivery problem, at the end of that I know I have zero delivery/vent problems.

My issue was a fluttering relay that was going bad, and figure that out by fluke. Friggin cost me a couple tanks, multiple pumps, and a handful of filter elements. The Rick's tank pump hat comes with a twin pump harness, and when I installed the tank I went ahead and upgraded my wiring to two 10g feed wires and a pair of 70a relays. I just happened to swap the relays around and my issue went away. Scratching my head, I tried switching the relays back and the problem came back. Pitched the one relay and all is good.

Granted, it started with gas I got at a station that had a ground pump brown out, and it pumped tons of metal particles into everyone's tank for a couple weeks. They eventually shut that station down, my GF told me they shut the place down and corded it all off. I should have submitted a claim, could have at least got some compensation. The one shop I get inspections at actually mentioned they had a rash of folks with filter & pump service calls.

Now I only go to stations that are certified as 'Top Tier' providers, you can look them up here:

https://www.toptiergas.com/gasoline-brands/

Strongly suggest not buying gas at places not listed. They have inspection standard and test ground tank fuel quality/contamination on regular basis, and will force a station to correct issues or they won't fill the tanks.

As another suggestion, if you see a tanker filling a station, go somewhere else, and wait a couple days before you go back. Filling kicks all kinds of crap up in the tanks and you don't want to run that chit.

Live and learn.


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Good point. I have never torn a pump apart but every online drawing I find suggests the valves are simple - like a disc or flap that may not have a backing spring to hold it closed. If this is the case, and since there is no significant back-pressure or residual pressure other than the weight of the fuel, I can see how these valves could leak a little.

I think these valves are primarily designed to seal against the internal pressure as the diaphragm moves so maybe not reliable for preventing slow drain-back? They don't have to seal very well to allow the diaphragm to push fuel through the outlet.

So how to know how much they leak?

Would be interesting to know the "specs" on reverse flow rate for these valves.

If, for example, there were 3 oz of fuel in the bowl and line, then a leak would have to be 3 oz/24 hr to drain it in a day. This is over 3.5 cc/hr, which I guess is feasible but sounds like a lot....maybe not?

A 1/2cc/hr leak would drain 3 oz in about a week. That sounds feasible to me, but just guessing.....


A vacuum pump might apply enough differential pressure to close a valve that leaks a little under just the weight of fuel above. IMO, this could fail to detect a small leak but if the outlet check valve leaked a lot, this would detect it.

Mike

The fuel filter check valve used from 1975, as an vehicle accident rollover security valve so fuel could not escape from fuel tank thru the carb onto the ground, has NOTHING to do with drainback from carb TO fuel tank.

This is described in the 1975 Pontiac Service Manual.

FWIW

This assumes that the inlet valve in the carburetor is at the bottom. I haven't seen any carburetors like that in the 40 that I have serviced carburetors.

The Carter 6907 has springs for the fuel inlet/outlet valves. The rebuild kit included new springs, pins, diaphragm, and sealing discs.

Just a guess, but seems unlikely to leak a lot. Maybe others are different?

QUOTE "Regardless, I think I've asked before but what specific electric pumps are used for this? I suspect I will want one.

Mike"

END QUOTE

Mike - we were distributors for Carter for years prior to and a few years after their sale to Federal Mogul. Naturally, we looked at Carter products first. I used both the Carter race pump P4594 (7 psi) and P4070 (5 psi) on my own personal performance street vehicles. The P4594 was probably overkill, but I have never experienced any problems with either. I did install the high-buoyancy floats in my dual AFB non-Pontiac set-up using the P4594.

I have the P4070 on my 1974 GTO 350 with the 850 CFM TQ.

Jon