[Stripes]

On our 455 in a 69 FB we are getting vapor lock issues when driving in stop and go traffic after running at speed for a while in 50F weather! The engine runs180 degrees. Robmc mechanical pump with return line. The car has long branch manifolds and on the passenger side the fuel line is about 4" away from it. I suspect its heating the fuel in the line. How are others avoiding the heat from these manifolds? I ceramic coated the long branch manifolds, and insulated the lines but the problem continues. Any ideas? Are there any quiet electric fuel pumps I could run as a booster?

[Cliff R]

Describe "vapor lock"?

Fuel boiling in the supply line so the pump isn't pumping fuel at all and carb running empty?

Heat from the engine boiling the fuel in the carb as it's already pretty hot when it enters the fuel bowl?

I work on these vehicles a lot in the hot summer months and to date have never had one experience any issues with fuel boiling in the lines or in the carb.

We get a few complaints to the shop in the summer months from folks with early intakes (like the 1967 Pontiac iron Q-jet manifolds) that has the heat channel to routes some exhaust gasses up under the front of the carburetor. Believe it or not some of these folks are NOT using the correct gaskets and stainless steel shim gasket under the carburetor. We advise them to block the holes at both ends of the heat channel so keep some heat off the carburetor and use the correct gaskets and metal plate under the carb. That usually takes care of any issues they are having......Cliff

[steve25]

What do you mean by "after running at speed " do mean as in right when returning to what should be idle, the motor can't idle?

[78w72]

can you describe the symptoms? hard to diagnose things over the internet, but based on what you describe i dont think you have actual vapor lock... or at least not from the line next to the manifold as you describe. 4" of clearance & a wrapped line will not cause vapor lock, especially at 50*.

i have a 520hp 10.75:1 467 stroker motor with BIG hooker super comp headers & my fuel line runs on the passenger side as well, near the side of the engine where the line runs on top the frame it is about 2" away from 2 of the header pipes, even before i wrapped the line with heat shield it never had any type of fuel boil or vapor lock at all.

this is with a robbmc pump & no vapor return & a q-jet. car runs consistent 180-190 in all temps, even prolonged highway runs, stop & go traffic or idling for 5-10 minutes while tuning etc. lots of summer drag strip runs too.

so i dont think yours is actual vapor lock, might have some other issues causing the problem but we need more info to help.

[shaker455]

Do you have active exhaust crossovers?
Maybe post some underhood layout pics?
Let's see what you got going on

[Formulajones]

X2 on what Cliff has said. I still run stock mechanical pumps on 4 different cars here, with factory lines, and some of those lines are much closer to exhaust, water pumps, lower rad hoses etc... than what you are describing.

I use no insulation, and have never had vapor lock issues as far as lines are concerned and we daily drive a couple of these cars in the AZ heat.

9 times out of 10 it has something to do with the carb, the heat cross over, or both. As long as your engine temps aren't excessively high, and they sound like they are fine, I'd be looking in those areas.

[1968GTO421]

Is it possible he got a tank of bad gas or too much ethanol?

[tom s]

I have a 69 bird with long branch manifolds and have no issues as you describe.I have had at least 4 diff engines in the car.Do you have a after market fuel tank or original?I found the after markets dont have a vent like the factorys and can cause a issue.Tom

[Formulajones]

I was tempted to mention the tank but didn't go there.

Venting is certainly important as well as a clean sock.

["QUICK-SILVER"]

Originally Posted by Formulajones

I was tempted to mention the tank but didn't go there.

Venting is certainly important as well as a clean sock.

Tank vent and sock problems usually show up at speed more than they do in stop and go traffic.

Do need to hear more about what it's doing when it acts up. A carburetor on the verge of loading up (for what ever reason) will run good at speed and then try to load up/flood at idle when everything is heat soaked good.

Need more input
Clay

[Stripes]

Thank you for your responses. I have been chasing this issue for a while and need some help.
Symptoms: short trips no issues. Drive car at highway speed, and stop and go traffic for a couple hours and I can hear a sputter occasionally at steady throttle initially. It continues until the carb is literally running out of fuel. Installed a clear fuel filter and can see the volume of gas decreasing as it gets worse until there is a small trickle feeding engine. Installed air fuel gauge and can see as symptoms increase the motor begins to run very lean as it's running out of fuel. Eventually, the gas petal feels soft and the engine will only idle due to a lack of fuel. Worse in hot weather, issue has continued at least 9 months, although car not driven weekly.
Things I have changed or checked:
1. Replaced factory fuel pump,to new stock, then again to robmc, both with return line.
2. Verified return line open, replaced fuel cap with vented version, verified tank vent also unobstructed (ac car).
3. Replace all fuel lines, rubber between hoses, fuel pickup in factory tank and sock. Tank is spotless inside.
4. Ceramic coated the ra3 long branch manifolds, wrapped exhaust pipes with DEI wrap for the entire lenght.
5. Rerouted the return line to cross under the engine to avoid running it by the passenger side exhaust manifold.
6. Rebuilt the engine, e heads with exhaust crossover threaded and plugged. Factory 72 iron port matched intake on 500hp, 577tq engine.
7. Current 800 4165 spread bore holley, but same issue with the Quadrajet. Tried phenolic insulators, heat sheilds.
8. Replaced the HEI coil, module, wires, cap, rotor, plugs.
9. Installed a robbmc 400 GPU fuel filter, removed others.
10. Installed an electronic fuel pressure guage at the carb.
11. Have tried alcohol free fuel and issue still occurs.
12. Ran entire fuel line thru DEI insulated fire sleeve.
Issue seems to have started after new exhaust were installed, 2.5" stainless mandrel duel with h pipe and magna flow 2.5 straight thru mufflers, running full lenght.

[Stripes]

Photos

[steve25]

You must narrow this down and there's a way to do this, but it's a pain in the Ass, not that running out of fuel in the Carb while driving is any fun either!
You must rig up a fuel supply inside the engine compartment ( maybe 2 gallons will be enough) and both pull fuel from this supply and return it.
If the in doing this the isuue goes away, then you must be pulling air some how in your current feed set up!

[78w72]

Originally Posted by Stripes

Thank you for your responses. I have been chasing this issue for a while and need some help.
Symptoms: short trips no issues. Drive car at highway speed, and stop and go traffic for a couple hours and I can hear a sputter occasionally at steady throttle initially. It continues until the carb is literally running out of fuel. Installed a clear fuel filter and can see the volume of gas decreasing as it gets worse until there is a small trickle feeding engine. Installed air fuel gauge and can see as symptoms increase the motor begins to run very lean as it's running out of fuel. Eventually, the gas petal feels soft and the engine will only idle due to a lack of fuel. Worse in hot weather, issue has continued at least 9 months, although car not driven weekly.
Things I have changed or checked:
1. Replaced factory fuel pump,to new stock, then again to robmc, both with return line.
2. Verified return line open, replaced fuel cap with vented version, verified tank vent also unobstructed (ac car).
3. Replace all fuel lines, rubber between hoses, fuel pickup in factory tank and sock. Tank is spotless inside.
4. Ceramic coated the ra3 long branch manifolds, wrapped exhaust pipes with DEI wrap for the entire lenght.
5. Rerouted the return line to cross under the engine to avoid running it by the passenger side exhaust manifold.
6. Rebuilt the engine, e heads with exhaust crossover threaded and plugged. Factory 72 iron port matched intake on 500hp, 577tq engine.
7. Current 800 4165 spread bore holley, but same issue with the Quadrajet. Tried phenolic insulators, heat sheilds.
8. Replaced the HEI coil, module, wires, cap, rotor, plugs.
9. Installed a robbmc 400 GPU fuel filter, removed others.
10. Installed an electronic fuel pressure guage at the carb.
11. Have tried alcohol free fuel and issue still occurs.
Issue seems to have started after new exhaust were installed, 2.5" stainless mandrel duel with h pipe and magna flow 2.5 straight thru mufflers, running full lenght.

well that covers about everything i can think of...

very strange issue but i doubt its fuel line related as you have wrapped & coated everything related to exhaust heat in the fuel lines. & mine are 2x as close & never had fuel related issues even before i heat wrapped the line & with no vapor return.

also the return on the robbmc is just a vapor bleed, its not a true fuel return line.... but that is supposed to help with or stop actual vapor lock so i think you have something else going on there.

[Formulajones]

Originally Posted by "QUICK-SILVER"

Tank vent and sock problems usually show up at speed more than they do in stop and go traffic.

Do need to hear more about what it's doing when it acts up. A carburetor on the verge of loading up (for what ever reason) will run good at speed and then try to load up/flood at idle when everything is heat soaked good.

Need more input
Clay

I had a clogged sock in my wifes vette that was intermittent. Some days it ran okay and other days it would idle like garbage and part throttle driving was hit and miss, stumble etc... It acted like a carb problem and I chased it for weeks, venting the tank, replacing fuel lines, insulating the carb.

I finally looked down in the tank and it looked like someone had coated the inside with something and it was all coming loose, was wrapped all around the sock. Luckily the tank opening is rather large and dad could fit his hole arm in the tank, reach the sock and pull it off, installed a new one in 5 minutes.

Ran like a raped ape after that.

I'll also add that those things can look pretty clean and still flow like garbage. I started installing the really large socks for a 70's pickup truck I was buying from GM at the time. They are twice the size of the passenger car socks. More surface area.

["QUICK-SILVER"]

Have you tried running it with the gas cap off?

Any of the short rubber hoses have bends or turns that could suck flat?

If not an air leak in the lines some where it sounds like there's something causing a restriction. Restrictions in the supply line causes negative pressure and makes fuel boil easier and/or at a lower than normal temperature. Under vacuum it'll vaporize on it's own. Just gets worse with more heat.

Clay

[72LuxuryLeMansLa.]

Do you have a filter between the tank and the pump? Couldn't that cause this?

[Stripes]

No filter between the pickup sock and the robbmc fuel pump. I suspect a air leak and that is why I put new stainless fuel lines on it theorizing a pinhole in a steel line somewhere.

My plan is:
Drive car and get it heat soaked and the if/when the symptoms appear try removing the gas cap to see if it changes, and then putting the car on a lift and taking heat reading up and down the fuel lines with a IR heat gun. I'll report back and maybe y'all can help me narrow it down. I have considered adding an electric pump, but hate the rat tat tat sound. Also, mounting a vane style pump lower than the bottom of the gas tank is a challenge on a 69 FB, and it can loose prime if you don't.
A in tank pump may be the final solution, but that requires running a larger return line and regulator, i believe. It otherwise runs fantastic, but long drives on the freeway even with the 700R4 overdrive cause the issue to re-appear.

[dataway]

Since you did install a fuel pressure gauge ... what did it say? Does fuel pressure go down close to zero?

[JLMounce]

I feel for you, I struggled with this exact issue in my 1969 Firebird. Did basically everything you have as well.

There is one item you could try that's not on your list. Try a phenolic gasket between the fuel pump and the engine block. There's another source of heat getting into the fuel system right there.

I finally got tired of the car being un-driveable. after about 30 minutes and switched to a FiTech system with an EFI ready fuel tank and in-tank fuel pump. Never a single issue after that.

If you're not keen on going that route, my suggestion would be to put a fuel pump in your tank and run a "corvette" style fuel system with a bypass regulator. There's several retrofit pump systems available, but the best bet would be an EFI tank with a build in sump. Then run a short feed line near the tank to the bypass regulator and the return from there back to the tank. From the bypass regulator, set it at the PSI your carb likes, and run the dead-head fuel line to the carb.

You're pushing fuel here instead of sucking it which is going to help. The pump in the tank is also the quietest deal. You'll only ever hear it when you first turn the key, but haven't yet cranked the motor. After that you won't hear it, even over a very quiet exhaust.

It's a bit of money doing the full EFI conversion route, but to give you an idea on what that did for me. When I was running the carb deal, it was basically impossible for me to put more than a couple hundred miles on the car a year. Since I switched to EFI, I was able to start driving the car thousands of miles per year. In fact, the car is so reliable in every weather condition and every driving condition that if I wanted to, I could daily drive it.