Jump to content
Forums Gone... but not forgotten!
Pontiac of the Month

58Bonne's 1958 Bonneville

2019 June
of the Month

Recommended Posts

1970 Pontiac GTO. 455, Edelbrock Performer RPM, stock fuel lines, in line filter, stock fuel pump. stock fuel tank. Holley Ultra Avenger 750 CFM carb electric choke and vacuum secondaries.

So this weekend, I took @50 mile round trip and put the car back in the garage, I shut off the motor, and heard what sounded like percolating from the gas tank. I remove the gas cap and and its under high pressure and the percolating stops after a huge "woosh" out of the filler neck. I put the cap back on and the pressure builds again. I slide under the rear of the car and put my finger over the breather hose on the tank and feel pressure escaping from the hose. I have 2 other hoses leaving from the tank to the front of the car, one on the drivers side leading to the fuel pump and another leading down the passenger side to who knows where? I see where there is a crushed section in the steel line on the passenger side near the front of the car but that has been that way for many years. The tank bottom was warm but not near enough to boil fuel, the lines were a bit warmer than the tank but not by much. Now in front of the engine block between the fuel pump and carb is the steel factory line that leads to the fuel filter and the carb. THAT line was hot enough to have you removing your hand within a few seconds of touching it. The closest ANY fuel lines get to the exhaust is maybe 6 inches when the exhaust goes over the rear axle but the lines didn't feel that hot. After about 5-10 minutes of resting in the garage the pressure seems to dissapate from the tank. What do I have going on here? and better how do I fix it? What am I looking for?

  • Confused 1

Share this post


Link to post
Share on other sites

Tired of these Ads? Register Today!

First question - does your goat have factory air conditioning?

Share this post


Link to post
Share on other sites

Breather on the tank has to be partially plugged, as you said it's leaking pressure.   Can't think of any reason for the system to be HOT.  DR side is supply, Pass side lines are return.  

Share this post


Link to post
Share on other sites

No air conditioning. Steve where is the return line going to? I don’t have anything going to the carb from the return line? Where do I look for where the return line is going? 

Share this post


Link to post
Share on other sites

360 - since you don't have factory AC, check to see if you have an evaporation charcoal canister. On Lucy, it is located in the passenger side close to the radiator support. The return line should be from the evap canister back to the fuel tank. If the hard return line is dented or bent, then it's allowing excess pressure to build in the tank. Think of Boyle's Law.

Share this post


Link to post
Share on other sites
Posted (edited)
1 hour ago, JUSTA6 said:

Breather on the tank has to be partially plugged, as you said it's leaking pressure.   Can't think of any reason for the system to be HOT.  DR side is supply, Pass side lines are return.  

 

51 minutes ago, 360Rocket said:

No air conditioning. Steve where is the return line going to? I don’t have anything going to the carb from the return line? Where do I look for where the return line is going? 

Well, bare with me here, I don’t think you have pressure! I think you have a bad gas cap! That system I believe, vents most of its vacuum/pressure through the gas cap check valve. When the fuel pump draws fuel it must replace it with air, if it doesn’t a vacuum is created. I had this problem with the Z more than once. A quick check is leave the gas cap loose only to the first catch. 

No charcoal canister on your car right? If there is then the bent line is relevant.

Edited by Last Indian

Share this post


Link to post
Share on other sites

No canister on the car. The tank isn’t creating a vacuum, it’s creating pressure. 

The return line in the passenger side is going to the bottom of the fuel pump. I’ll go and uncrush that return line and see if that fixes it. 

Share this post


Link to post
Share on other sites
1 hour ago, 360Rocket said:

No canister on the car. The tank isn’t creating a vacuum, it’s creating pressure. 

The return line in the passenger side is going to the bottom of the fuel pump. I’ll go and uncrush that return line and see if that fixes it. 

In either case, without a canister, the cap is the problem, I would suspect. Certain fuel pumps returned a portion of gas back to the tank. By your second explanation of the second line that means it’s retuning pressurized fuel to the tank and the cap isn’t releasing the pressure. That said that doesn’t sound typical for an OEM system, because of emissions. While only you can judge the condition, I have seen vacuum act just like it was pressure and literally blow the cap of a tank and into a guys face! 

A local parts store could check the cap for you.

Either way be careful buddy!

Share this post


Link to post
Share on other sites

 

Ok I’ll definitely look in to the gas cap as well. I have the old one laying around here somewhere. Thanks!  

Share this post


Link to post
Share on other sites

Randy....sorry haven't checked back in.  Been busy.  Looks like most info has been covered for you N stuff to check.  I don't believe the cap is vented (could be totally WRONG!) BUT... Esp since you said you checked the vent/breather hose off the tank.  Should go up and behind the bumper? Or at least higher than the filler neck.  You said it was blowin air.   This should be a wide open vent and not allow any buildup in the tank unless partially plugged.  That said... a vented cap couldn't hurt .   Think of it like a gas can, when trying to pour with the vent cap closed.  This is the vacume effect Last Indian is talking about.  Flow will slow to a stop and then it will inhale air to relieve the vacume and start the flow again. Open the vent and it will flow smooth.  I'm a lil more concerned about the heat buildup in the fuel system.  Was hoping someone would have some thoughts on that.  It has me stumped.  Being hot on the engine is one thing...tank should be cold.

  • Like 1

Share this post


Link to post
Share on other sites

Drove the car back and forth to work yesterday with zero issue. baffling.

Share this post


Link to post
Share on other sites
22 hours ago, JUSTA6 said:

Randy....sorry haven't checked back in.  Been busy.  Looks like most info has been covered for you N stuff to check.  I don't believe the cap is vented (could be totally WRONG!) BUT... Esp since you said you checked the vent/breather hose off the tank.  Should go up and behind the bumper? Or at least higher than the filler neck.  You said it was blowin air.   This should be a wide open vent and not allow any buildup in the tank unless partially plugged.  That said... a vented cap couldn't hurt .   Think of it like a gas can, when trying to pour with the vent cap closed.  This is the vacume effect Last Indian is talking about.  Flow will slow to a stop and then it will inhale air to relieve the vacume and start the flow again. Open the vent and it will flow smooth.  I'm a lil more concerned about the heat buildup in the fuel system.  Was hoping someone would have some thoughts on that.  It has me stumped.  Being hot on the engine is one thing...tank should be cold.

Agreed JustA, with heat issue, but if the the second line from the fuel pump is returning fuel to the tank, and I don’t know that for a fact, that would explain it. Randy, can you check and see if that is what that line is doing? 

Share this post


Link to post
Share on other sites

I have a theory and it is just a theory. The return line is coming from the fuel pump. As we all know, it is Pontiac's solution to over pressurization and vapor lock. Therefore it returns excess gas and gas vapor back to the fuel tank.

The pinched lined that 360 mentions narrows the flow of this gas/gas vapor back to the fuel tank, slowing its flow back to the tank. This constriction allows the slowed fuel to pick up significantly more ambient heat that it normally would.

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, Frosty said:

I have a theory and it is just a theory. The return line is coming from the fuel pump. As we all know, it is Pontiac's solution to over pressurization and vapor lock. Therefore it returns excess gas and gas vapor back to the fuel tank.

The pinched lined that 360 mentions narrows the flow of this gas/gas vapor back to the fuel tank, slowing its flow back to the tank. This constriction allows the slowed fuel to pick up significantly more ambient heat that it normally would.

Good possibility Frosty. That why I suggest if 360 can drop that line out of the way and put a temporary one in. Then take that to a container/gas can and see what comes out, or see if there is suction, cause we don’t know for sure at this point.

Looking at that system JustA is right, no vented cap for that year tank. Not that it would hurt. Still something is going on. Pressure, heat and or vacuum needs to be resolved.

Share this post


Link to post
Share on other sites
On 3/7/2019 at 8:00 PM, Last Indian said:

Agreed JustA, with heat issue, but if the the second line from the fuel pump is returning fuel to the tank, and I don’t know that for a fact, that would explain it. Randy, can you check and see if that is what that line is doing? 

LOL< To be honest with you I slid under the car, (just barely, lol) and I'm running my hand along this supposed return line that runs along the frame and I get to the top of rear shock support and I can't follow the line anymore. I say this because I don't want to cut out that pinch unless I know 100% that I'm not cutting a brake line. At some point I'll have to find a lift.

Share this post


Link to post
Share on other sites

So I'v once again looked under the car at the line on the passenger side and traced the line from the tank, along the frame, thru the K member, and over to one of the connections on the bottom of the fuel pump. I could NOT locate a canister anywhere along that line. it goes from a rubber fuel line at the tank, to a solid metal line along the frame and then comes out the K member as once again a rubber line. I'm going to hopefully soon cut out the smashed part of the line and probably replace it with rubber unless they make some kind of splice for the metal line?

Share this post


Link to post
Share on other sites

Hope that solves your problems bro.

  • Like 1

Share this post


Link to post
Share on other sites
14 hours ago, 360Rocket said:

So I'v once again looked under the car at the line on the passenger side and traced the line from the tank, along the frame, thru the K member, and over to one of the connections on the bottom of the fuel pump. I could NOT locate a canister anywhere along that line. it goes from a rubber fuel line at the tank, to a solid metal line along the frame and then comes out the K member as once again a rubber line. I'm going to hopefully soon cut out the smashed part of the line and probably replace it with rubber unless they make some kind of splice for the metal line?

You can order an exact reproduction line from Classic or Inline Tube, if that's important to you.

Share this post


Link to post
Share on other sites
Posted (edited)

It is and then It isn’t. Might make this a temp fix and order it down the line. Still not sure that it is the fix for the tank but worth a try. 

Edited by 360Rocket

Share this post


Link to post
Share on other sites
9 hours ago, Last Indian said:

Did you or can you disconnect that line at the hose connection of the fuel pump and see what happens? Does it pump out gas, does it suck, is it vapor? It might help diagnose that’s going on!

I'll see if I can get to it tomorrow.

Share this post


Link to post
Share on other sites

make sure the Patch piece is up to ethanol rated gas line. 70 didnt have the canister as far as i know. The crushed piece is the issue i bet.

  • Like 1

Share this post


Link to post
Share on other sites
On 3/13/2019 at 10:19 AM, Last Indian said:

Did you or can you disconnect that line at the hose connection of the fuel pump and see what happens? Does it pump out gas, does it suck, is it vapor? It might help diagnose that’s going on!

So I disconnected the hose from the passenger side line and its definitely a fuel return line. Zero pressure on the system but the line was under pressure. I bet the pinch in the line is causing back pressure on the fuel pump from the return line and heating up the fuel? Probably rough on the fuel pump as well.

 

18 hours ago, indymanjoe said:

make sure the Patch piece is up to ethanol rated gas line. 70 didnt have the canister as far as i know. The crushed piece is the issue i bet.

Probably use bulk fuel injector line from Auto Zone. Eventually get the expensive replacement line from "an undisclosed repop parts dealer".

Share this post


Link to post
Share on other sites

I'm still back at the tank, even with the pinch, as to why it's not vented properly.  It should have to be vented with fuel flowing back to the tank.  Fix the pinch, then on to the vent.  Good luck, hope it's all simple.

Share this post


Link to post
Share on other sites
2 minutes ago, JUSTA6 said:

I'm still back at the tank, even with the pinch, as to why it's not vented properly.  It should have to be vented with fuel flowing back to the tank.  Fix the pinch, then on to the vent.  Good luck, hope it's all simple.

image.png.c3dd234b2d4fbf5887a39d9978a84a9f.png

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


Tired of these Ads? Purchase Enhanced Membership today to remove them!
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.