Fuel tank pressure/suction - Team Camaro Tech
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post #1 of 12 (permalink) Old May 9th, 09, 11:57 AM Thread Starter
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Fuel tank pressure/suction

Hi guys,
Question of the day from me again, haha. I seem to have a fuel issue with my car. It originally had dual fuel lines, but the smaller return line is not connected. After about 5-10 minutes of driving, occasionally the engine will run rough, and if I get out and remove the fuel tank cap, I get a huge rush of air. Seems like pressure, but that seems impossible to me due to the absence of return line, instead seems like it has to be suction. I have had three different vented caps on this car, and nothing makes a difference. I haven't put a fuel pressure gauge on the fuel line simply because I don't have one yet. Just can't figure out the rush of air.

I guess my questions are: anyone else have this issue and solved it? Is there any other place my fuel tank is supposed to be vented? Should I go back to the fuel pump that it originally had so I can hook up the return line? What other suggestions do you guys have?

I run an aftermarket mechanical pump because I like the more stock appearance of mech. Don't really want to go to electric if i can avoid it. Just need to solve the pressure/suction issue. I can even put my ear aagainst the fuel tank and hear what sounds like air bubbles. Obviously the fuel isn't boiling in there.

HELP PLEASE!!

RJ,
and thanks to all in advance
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post #2 of 12 (permalink) Old May 9th, 09, 12:06 PM Thread Starter
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Re: Fuel tank pressure/suction

I meant to add that on hard acceleration from a stop, gas runs out of the tank and I can smell it even. Not obvious though how it gets out. Can't get by or through the cap can it?

RJ
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post #3 of 12 (permalink) Old May 10th, 09, 04:40 PM Thread Starter
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Re: Fuel tank pressure/suction

Hi guys,
Here's another bit of info for diagnosis. I took the car out today. It's in low 90s as far as temp goes. Car runs strong, but after I get on the throttle hard and then back off and go back to regular driving, then the engine starts to miss quite a bit. Like it's either fuel starved or there is an electrical issue. Makes me lean toward fuel issue since I had just run it through the gears hard, and not electrical. Do you think it could be vapor lock? Still don't get the pressure in the fuel tank, seems like that would push more fuel into the fuel line and make it easier to get it to the carb. The little cloth filter in the fuel tank is new, not clogged up. Inline filter is new, aas is the mechanical fuel pump. I need help please. If it is vapor lock, I'll be in trouble, because here in Florida, we're not even into summer heat yet. Does the vapor lock come from the mechanical pump gettting too hot? Would an electric fuel pump be subject to vapor lock as well?

RJ
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post #4 of 12 (permalink) Old May 10th, 09, 05:19 PM
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Re: Fuel tank pressure/suction

Fuel tank is going to build pressure due to the heat. Heat from the road, exhaust heat, and the natural flow of heat from the engine.

The return line you mention is a vapor lock preventative as the filter, OE mounted just above the water pump, to thwart vapor lock. The return line, or what is left of it, may have a check valve for fuel to enter but nothing in the other direction, I'm just thinking out loud.

A roadside quick fix was to put spring loaded clothes pins on the line to radiate the heat. Another is to wrap the line with foil or pipe insulation. Another fix is to replace the steel line from the fuel to carb with hose, hose does not get as hot as steel.

True, the fuel pump may be getting too hot and perculating the fuel, amazing what ethanol does. You might try the OE filter configuration if the retutn line is available, or anything else to cycle the fuel back to the tank for cooler fuel, maybe a cool can. You might the section of fuel line running inside the rail and place it on the outside of the rail to keep it away fromt he header heat.

You can install a Holley red pump and cable it in using 14 AWG from an ignition controlled relay and mount the pump on the trunk floor incline. the pump delivers about 4-6 PSI and works without a regulator.

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post #5 of 12 (permalink) Old May 11th, 09, 06:05 AM Thread Starter
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Re: Fuel tank pressure/suction

Hi Everett,
Appreciate your input as always.
Let me add a few things here. Nothing is changed on my fuel system since last summer here in Florida, and didn't have this issue then. Return line is intact, but not hooked to anything on either end. Don't really thing this is vapor lock, could be a pump isssue. It was a cheap pump I bought on e-bay from Skip White in Tennessee. Think I only paid $25. Thought better of it at the time, but bought it anyway. Think I'll change that first. If you hooked up the return line, how would you get enough pressure to the carb. And isn't 4-6 pounds you mention too low?

Don't know if you noticed, but I mentioned that fuel spilled out the back on hard acceleration from stop, and I can smell it. How is this possible, The cap is only vented to prevent suction if i am correct. It can pull air in through the cap, but not allow it to escape. Where is the fuel coming from that goes all over my bumper? I've changed caps twice.Doesn't seem to be a leak in the filler neck. Are there mechanical pumps that are worth a damn and that still prederve the stock look? If I go to electric pump, what brand and model is "best" for mostly street and occasional drag strip. Should I replace my stock gas lines?
Thanks again.
RJ
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post #6 of 12 (permalink) Old May 11th, 09, 07:35 AM
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Re: Fuel tank pressure/suction

I think its a good approach in changing the pump.

I'll add to change all the hoses in the line as well; they do age and you can't see the inside. As for the steel lines, as long as they aren't leaking and not rusted, don't fix them.

As for the gas cap, you should be able to suck through the middle but not out. Another place for leakage is the outside seal to the filler tube. Is the filler tube smooth and no imperfections in the outer cap sealing area? The internal thread of the tube may be worn to not 'hold' the cap tight enough, the seal of the caps may not be thick enough or large enough diameter, or the backing may have lost its tension. Stant caps work well. After a hard run, immediately check the cap - if moist about the middle of the cap vent valve is leaking, if moist about the circumference of the cap, tube seal is leaking.

Incorporating the return line, the filter has a pressure regulating valve within, kindda. You might try a NAPA pump for a '69 350 as the mech pump supplies volume more than pressure. Pressure is resistance to volume - place thumb over the garden hose coupling and feel the hose expand, same principle.

I offer this for thought, if you are running faster than last year, mech pumps don't suck very well. As you accelerate, the fluid iin the line goes to the rear of the car - momentum thing. The pump suction must overcome this rearward phenomenal, if it can't, then starvation exists. Pumps deliver better than they can suck.

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post #7 of 12 (permalink) Old May 11th, 09, 08:29 AM Thread Starter
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Re: Fuel tank pressure/suction

Thanks for the kind words Everett, and thanks to you and all the other vets here for your service. I bet there are a lot of them here from all eras. Might start a thread here sometime asking vets to mention when they were in and what branch/unit, if you think that is OK. Might be interesting, and never know who you might find.

As for the car issue. the cap and neck appear fine as does the gasket. Could be leaking from the top of the tank maybe, but o-ring is new and seems fine. Maybe from where the return line used to go back into the tank??. Hat e to take it all apart again.

RJ
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post #8 of 12 (permalink) Old May 18th, 09, 02:14 PM
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Re: Fuel tank pressure/suction

I am having the exact same issues!! If you find a solution, please let me know what it is. This is driving me crazy. I have changed fuel pumps several times, put a heat shield and phenolic spacer between my carb and intake, adjusted float levels, changed timing, I just can't find a solution.
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post #9 of 12 (permalink) Old May 18th, 09, 02:18 PM Thread Starter
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Re: Fuel tank pressure/suction

Will keep you posted. My car runs like crap right now though
RJ
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post #10 of 12 (permalink) Old May 18th, 09, 02:27 PM
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Re: Fuel tank pressure/suction

Does the return line seem to be in good condition and at the front of the car, engine compartment?
If it is, why not keep it open ended to allow venting and see if this helps.

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post #11 of 12 (permalink) Old May 18th, 09, 02:31 PM Thread Starter
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Re: Fuel tank pressure/suction

Everett,
the return line is intact, but disconnected at both ends, Hasnt been connected since I bought the car

rj
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post #12 of 12 (permalink) Old May 18th, 09, 02:40 PM
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Re: Fuel tank pressure/suction

Might be worth the time to connect the vent line at the tank just to see if it does prevent fuel starvation.

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