Stumbling after 20 miles of driving

[Test Pilot]

So I've got a problem with my truck that takes about 20 miles of highway driving to appear, once it's happens it will re-occur about every 10 miles or more frequent. It stumbles and won't pull, it rarely ever dies, but I have to pull over, if I shut it off and sit for 2-3 mins I can start it up and keep going.
Also when it does this the fuel filter is empty or near empty and no visible sign of fuel is flowing into the filter unless I shut it off as I mentioned.

Things I've done so far are;
1. Disassembled and cleaned inside of carburetor.
2. Replaced fuel filter 2-3 times just to be sure. The most recent one is a clear plastic one so I can watch for fuel flow or debris. No debris at this point
3. Pulled fuel line away from all heat sources to eliminate chances of vapor lock
4. Replaced fuel cap with new cap that is vented, or at least thats what they told me at the part store. It does have the vent valve and it does "rattle"
5. Replaced fuel pump, and all rubber line from the pump to the carb. I didn't replace the line betwee the hard line and the pump, but it's not that old and is fuel injection rated line.
I think the issue is the rubber line between the hard line coming from the cab and the steel line that runs along the frame. This look as though it could be the original line, it does have spring clamps on it that are not holding very tight as I can twist the rubber line without removing the clamps.
The line feels flimbsy.

My question is what else might I be missing? I'm going to get some fuel line tomorrow and replace it just because it's questionable.

Any ideas?
Thanks

[crushermechanic]

when it acts up again pull your cap off the tank and listen for a vacuum. sounds like a venting problem. i had a similar problem on mine till i moved the tank.

[Test Pilot]

Originally Posted by crushermechanic

when it acts up again pull your cap off the tank and listen for a vacuum. sounds like a venting problem. i had a similar problem on mine till i moved the tank.

I forgot to mention I have done this, there is actually pressure, I can feel it escape pass the cap when I take it off.
And while I'm remembering things I forgot to put in my original post, I've pulled the cap while driving and it didn't get better. So I did the whole stop and wait for 2 mins routine. Left the cap off and took off again with the same results.

[CG]

Mine does the same thing, but wont go as far as 20 miles before it stumbles. If I let off for a few seconds it "fixes itself" for a little bit, but stumbles again. Never does this just driving around at 50 or less.

Ill be watching this thread...Ive checked all the stuff you have.

[tnblazerk5]

have you pulled the sending unit in the tank and looked at the fuel pickup sock ?

[Test Pilot]

Originally Posted by tnblazerk5

have you pulled the sending unit in the tank and looked at the fuel pickup sock ?

Not yet, but the tank and sending unit are fairly new less than 3 years old.
I'll add it to my next thing to check.

[tnblazerk5]

Originally Posted by Test Pilot

Not yet, but the tank and sending unit are fairly new less than 3 years old.
I'll add it to my next thing to check.

sumthing else i just thought of , make sure all your clamps are tight and not letting air in the line , i had that problem on a olds i had . i know it sounds kinda weird but it didnt leak any gas but was letting air in the line

and yes change out the rubber line from the pickup unit to the frame hard line as you were talking bout , you have changed everything else so it wont hurt to go ahead and change that also

[Test Pilot]

Originally Posted by tnblazerk5

sumthing else i just thought of , make sure all your clamps are tight and not letting air in the line , i had that problem on a olds i had . i know it sounds kinda weird but it didnt leak any gas but was letting air in the line

and yes change out the rubber line from the pickup unit to the frame hard line as you were talking bout , you have changed everything else so it wont hurt to go ahead and change that also

I know what you mean about the air think being wierd, the though crossed my mind while laying underneath the truck earlier tonight. When I rotated the hose a trace amount of fuel ran up the line, just enough to darken the hard line. So it's just a tad on the loose side. The line I suspect to have this problem is the one I plan to replace. I'll post an update when I get it fixed and test drove again.

[71swb4x4]

What year is your truck? Are you saying your tank is pulling a vacuum, or that it is pressurized?

[Test Pilot]

Originally Posted by 71swb4x4

What year is your truck? Are you saying your tank is pulling a vacuum, or that it is pressurized?

It's a 68 and the tank is pressurized. It seems worse/higher pressure when closer to full, which would make sense due to a smaller air pocket in the tank to compress.
My 71 Bronco is the same way in terms of pressure, but not the stumbling problem even on a 2 hour treck to the off road park.

[westofb]

What is the longest length of rubber line you have in the fuel lines? Long runs of rubber lines have been known to collapse from the suction of the fuel pump, use hard lines and small prices of hose to connect them together.

[cdowns]

Originally Posted by westofb

What is the longest length of rubber line you have in the fuel lines? Long runs of rubber lines have been known to collapse from the suction of the fuel pump, use hard lines and small prices of hose to connect them together.

and the long lengths of rubber lines are great fire starters too,have seen many vehicles burn to the ground due to the lazymans fix for running gas lines

[Test Pilot]

Originally Posted by westofb View Post
What is the longest length of rubber line you have in the fuel lines? Long runs of rubber lines have been known to collapse from the suction of the fuel pump, use hard lines and small prices of hose to connect them together.
and the long lengths of rubber lines are great fire starters too,have seen many vehicles burn to the ground due to the lazymans fix for running gas lines

The longest length is less than 18" after the pump and less than 12" before the pump. Just areas where flex has to occur.

Does it have HEI?Sounds like the module is giving up the ghost

It has HEI, but it's running out of fuel, in some cases it will actually run the filter as dry as it absolutely can and no more fuel is coming in. So I need to fix my fuel flow before I can condem the HEI module just yet.

[70cst]

This is what's great about this site....one can be Sitting here alone, climbing up the walls trying to solve a problem....then you come to this sight and get help from those who have had similiar experiences. And bingo problen is solved!!!

[special-K]

Does it have HEI?Sounds like the module is giving up the ghost.

[DT1]

Originally Posted by special-K

Does it have HEI?Sounds like the module is giving up the ghost.

Mine did the same thing a while back and I traced it to my new Pertronix II module. I changed back to points and the problem was solved. Later I replaced the Distributor with a Delco HEI. I like the HEI but I've had to replace the module twice in two years. Pretty sure it has something to do with shutting the engine down with the air conditioning still on.

[Test Pilot]

Originally Posted by DT1

Mine did the same thing a while back and I traced it to my new Pertronix II module. I changed back to points and the problem was solved. Later I replaced the Distributor with a Delco HEI. I like the HEI but I've had to replace the module twice in two years. Pretty sure it has something to do with shutting the engine down with the air conditioning still on.

Mine is certainly running out of fuel, if you see my above posts you'll note I stated I can see in the fuel filter and it's empty or near enough that no fuel can get to the carburetor.

[dznucks]

the old original line that you didnt replace yet may be collapsing on itself. It will shut off fuel flow at higher rpms, ie. higher fuel pump vacuum.

That loose line is esentially a vacuum leak in your fuel system. it will aerate your incoming fuel and drop the pumps vacuum, thus not pulling fuel.

i imagine that your fuel is flowing fine at low rpms and idle, but its having a hard time keeping up at higher speeds.

When you switched to HEI did you remove the entire resistor wire going to your dizzy?

[Test Pilot]

Originally Posted by dznucks

the old original line that you didnt replace yet may be collapsing on itself. It will shut off fuel flow at higher rpms, ie. higher fuel pump vacuum.

That loose line is esentially a vacuum leak in your fuel system. it will aerate your incoming fuel and drop the pumps vacuum, thus not pulling fuel.

i imagine that your fuel is flowing fine at low rpms and idle, but its having a hard time keeping up at higher speeds.

When you switched to HEI did you remove the entire resistor wire going to your dizzy?

I think it is that line, I'm going to get some line today and replace it. The loose line and the only line not yet replaced are the same line.

There are situations where fuel won't flow at low rpms, indicating to me once it's collapsed, it doen't take much of a vacuum to keep it that way.

The truck had HEI before I bought it, so I can't answer that question, but again, without fuel it doesn't matter if I'm getting spark.

[mr48chev]

What is happening is that you are pulling a vacuum on the tank with the fuel pump.

think about it for a minute. You only have one line going to the tank and that is to the fuel pump which acts as a vacuum pump to pull fuel from the tank. The cap isn't venting correctly and is allowing a vacuum to build up in the tank. When you open the cap, especially when the tank is below a quarter you can hear the sides of the tank pop back out when the air goes in.

I went through three new out of the box caps that were supposed to be vented before I got one that actually was. I even walked to town once to get gas as I thought I was out of gas and then started carrying gas with me and that's when I figured it out.

[Test Pilot]

Originally Posted by mr48chev

What is happening is that you are pulling a vacuum on the tank with the fuel pump.

think about it for a minute. You only have one line going to the tank and that is to the fuel pump which acts as a vacuum pump to pull fuel from the tank. The cap isn't venting correctly and is allowing a vacuum to build up in the tank. When you open the cap, especially when the tank is below a quarter you can hear the sides of the tank pop back out when the air goes in.

I went through three new out of the box caps that were supposed to be vented before I got one that actually was. I even walked to town once to get gas as I thought I was out of gas and then started carrying gas with me and that's when I figured it out.

That is not the case in MY situation. I opened the cap and there is pressure. Gas bouncing around inside the tank will generate pressure, happens in my Jersey Cans on my Bronco when bouncing out enough on the trails to have them swell. I have gas in my right eye from having my face too close to the cap when I took it loose to check for it.

Also if you read above, I can drive with the cap off.
No cap on at all meaning I have a 1.5" hole to balance out with the atmosphere

[mr48chev]

You are wrong in this case, if you had pressure in the tank it would push the fuel to the pump. As I said before, think about it. The pump pulls the fuel out of the tank and it takes air going into the tank to take up the space. When no air goes into the tank it creates a vacuum which eventually stalls the engine. When you open the cap it is the air rushing in pushing some of the fuel vapors out that you see as pressure escaping
And yes I full and well know that a gas can bouncing around in the back of the truck will have pressure in it just as one sitting out in the sun will but that can does not have a fuel pump pulling against the sealed can. If it did, it would collapse the can..

[Test Pilot]

Originally Posted by mr48chev

You are wrong in this case, if you had pressure in the tank it would push the fuel to the pump. As I said before, think about it. The pump pulls the fuel out of the tank and it takes air going into the tank to take up the space. When no air goes into the tank it creates a vacuum which eventually stalls the engine. When you open the cap it is the air rushing in pushing some of the fuel vapors out that you see as pressure escaping
And yes I full and well know that a gas can bouncing around in the back of the truck will have pressure in it just as one sitting out in the sun will but that can does not have a fuel pump pulling against the sealed can. If it did, it would collapse the can..

If I had a non vented cap I would agree with you, but the cap is vented. I won't go to the trouble of putting my manometer on it to prove there is pressure.
AGAIN the same problem happens when the cap is on or off the truck!!!!
When there is a 1.5" hole in the top of the gas tank, how can it be pulling a vacuum?!?!?!?!?!

[westofb]

Originally Posted by Test Pilot

If I had a non vented cap I would agree with you, but the cap is vented. I won't go to the trouble of putting my manometer on it to prove there is pressure.
AGAIN the same problem happens when the cap is on or off the truck!!!!
When there is a 1.5" hole in the top of the gas tank, how can it be pulling a vacuum?!?!?!?!?!

he is right, from the pump to the tank, the pump will pull a vacuum on the tank, this is how it sucks fuel from the tank to be used in the carb....when i was talking about long lengths of rubber line, i actually meant between the pump and the tank. This is where the line will suck shut and stall the engine...from the pump to the carb the pump is producing pressure, rubber lines not that big of an issue on the pressure side, except for possible leaks and fire hazards! I would limit lengths of rubber lines in the fuel system to 2-3 inches, just to make connections between lengths of hard line or filters.

[mr48chev]

There is no possible way that you can have a pressure build up in that tank except when it is sitting in the sun and heat. You have one line out and no lines in.

If it still stumbles when you leave the cap off the sock on the pickup tube is probably full of dirt or trash.
And I don't care if it is a brand new out of the box vented cap, unless you have tested it it is still suspect. As I stated earlier, I went through three brand name caps that were even marked vented but none of them would vent. I finally got a winner with the fourth one off the same parts shelf with the same parts number.

One other thing it could be is that the inside of the hose between the line and the tank is collapsed inside and sucks in and blocks off the fuel.