Vapor lock. Different gas?

[Makinchips208]

Have a couple of these old trucks, carburetor with mechanical fuel pump, seems like anymore they all want to vapor lock in the summer heat, worse if going slow or climbing hills, which is everywhere here.

So I have tried electric fuel pumps back close to the tank with some success, but this one has the pump on the side of the block.

Is the gas different and causes this now days? Or what’s the deal? Never used to have this problem in years past.

 

[Redmanss]

Old vehicles hate new(er) ethanol. Find pure gas and give that a couple tanks before you throw more money at it.

This site isn't as accurate as it used to be, but here's a start to find 100% gasoline in your area. https://www.pure-gas.org/

 

[BScore]

 

[BScore]

 

[DarnYankeeUSMC]

How tight is the gas cap? Does it seal the tank and let off pressure if you open it?
Your tank vent could be clogged or the cap is sealing off the tank. As you use gas it's could be causing a vacuum in the tank. Try running the truck without the gas cap.

 

[BScore]

See either video’s descriptions for the parts listing.

 

[Makinchips208]

Yea have tried removing the cap. Not difference.
I was thinking of possibly a phenolic spacer? Didn’t try yet.
The return line idea looks like a great idea! I’ll try that.

 

[DarnYankeeUSMC]

How tight is the gas cap? Does it seal the tank and let off pressure if you open it?
Your tank vent could be clogged or the cap is sealing off the tank. As you use gas it's could be causing a vacuum in the tank. Try running the truck without the gas cap.

It could be as simple as a gas cap with a vent to fix the problem.
If your current cap has a vent it could be clogged.

 

[DarnYankeeUSMC]

Yea have tried removing the cap. Not difference.
I was thinking of possibly a phenolic spacer? Didn’t try yet.
The return line idea looks like a great idea! I’ll try that.

Are all the gas line connections sealed? Any rubber lines dry rotten? Any air getting sucked into the fuel line could cause your problem.
Next check would be removing the tank and pulling the pickup from the tank to see what condition the sock filter is in. If it's covered in rust you may not be getting a good supply to the pump. Mech or electric.

 

[Makinchips208]

Are all the gas line connections sealed? Any rubber lines dry rotten? Any air getting sucked into the fuel line could cause your problem.
Next check would be removing the tank and pulling the pickup from the tank to see what condition the sock filter is in. If it's covered in rust you may not be getting a good supply to the pump. Mech or electric.

On the 79 I pulled the tank, cleaned, it was pretty clean already, replaced the sock.
Replaced the line all the way to the tank, and installed an electric pump. Problems still existed, although slightly better.
I relocated the pump and line to the outside of the frame rail, mostly fixed it. I can hear the pump rattle if it is 100 degrees or so and I’m sitting idling.

The tan one in the pic, I have done none of that yet, since the 79 still isn’t quite good yet.

 

[Makinchips208]

And on the 79 I bypassed the whole tank selector solenoid as well to isolate problems. So currently using only one tank on that truck.

 

[DarnYankeeUSMC]

On the 79 I pulled the tank, cleaned, it was pretty clean already, replaced the sock.
Replaced the line all the way to the tank, and installed an electric pump. Problems still existed, although slightly better.
I relocated the pump and line to the outside of the frame rail, mostly fixed it. I can hear the pump rattle if it is 100 degrees or so and I’m sitting idling.

The tan one in the pic, I have done none of that yet, since the 79 still isn’t quite good yet.

How is the float level in the carb? Is it too low? What condition is the float? Cracked float could also be the issue.

 

[DarnYankeeUSMC]

Have you rebuilt the carburetor and cleaned all the passages? What brand carburetor is on the truck?

 

[Makinchips208]

Both have edelbrock. Maybe that’s the problem? Lol
One is older and one is new.

Come to think of it Quadrajets were the oem.

 

[Makinchips208]

And I never considered the float because it works perfectly on a cool day or all winter long. Only acts up in the heat.

 

[James40004]

Edelbrock or holley, can't remember which, makes a base gasket that is 1/4" thick. Solved problem on my edelbrock

 

[DarnYankeeUSMC]

Do you have an engine temperature gauge? Are the trucks running hotter on the high temperature days? Fuel line being close to the block, heads or intake and a few degrees more could cause it.
After that I am out of ideas.

 

[memyselfi]

Both have edelbrock. Maybe that’s the problem? Lol
One is older and one is new.

Come to think of it Quadrajets were the oem.

Normally Edlebrock are bad for restart when its hot or sitting for a day or more. If you trucks sits for a day is it a PITA to start ?

I have the spacer on mine, which helped but did not cure the issue.


https://www.bing.com/search?q=Edelbrock+vapor+lock&FORM=AWRE

 

[912173]

Edelbrock or holley, can't remember which, makes a base gasket that is 1/4" thick. Solved problem on my edelbrock

I put a 4 hole carb spacer under my holley that was getting really toasty sitting on top of the manifold with a exhaust heat crossover making it even hotter. It made a huge difference.

 

[Makinchips208]

Do you have an engine temperature gauge? Are the trucks running hotter on the high temperature days? Fuel line being close to the block, heads or intake and a few degrees more could cause it.
After that I am out of ideas.

Only the factory temp gauge which isn’t precise enjoy to show a small variation.
I really appreciate the responses and ideas! You gave me some things to check in to.

 

[Makinchips208]

Normally Edlebrock are bad for restart when its hot or sitting for a day or more. If you trucks sits for a day is it a PITA to start ?

I have the spacer on mine, which helped but did not cure the issue.


https://www.bing.com/search?q=Edelbrock+vapor+lock&FORM=AWRE

Usually it bump starts.
Sometimes it cranks a couple seconds and needs a little throttle to start when hot.

 

[n2ishun]

Well this is gonna ruffle EVERYONES feathers.

There is no such thing as "vapor lock".

Sure fuel can boil out of the bowl or line......how does this lock anything ?
The fuel pump will push gas regardless, it will self prime and 10 seconds of cranking will push all air out of the line and clear any *bubbles* that may exist.

The problem will be either way too rich, way too lean, or something in the ignition system (like the module in the HEI) that expands and stops working at a certain temp.
Edelbrock carbs suck, they are an exact copy of the 1950's era Carter AFB (aluminum four barrel).
It was shit in 1950, it's shit now.
The OEM Quadrajet is 10 times the carb, every day, all day, and next week too.
You just have to know how to work on it......and most pros haven't a fucking clue.

At the tank, there is the 3/8" feed (to pump) line, there is a 5/16" vent to charcoal canister line, there is a 1/4" vent to "roll over" valve.
The tank won't be sucking it's self into vacuum any time soon, but it can have pressure if the roll over valve is stuck (semi common).

Vapor lock was a 1940-1950 era rumor that spread way too far.
But by all means put those clothes pins on the rubber portion of the line....should fix you right up.
/sarcasm

 

[Ison]

This guy is awesome, I’ve learned a lot from him.

 

[Makinchips208]

Well this is gonna ruffle EVERYONES feathers.

There is no such thing as "vapor lock".

Sure fuel can boil out of the bowl or line......how does this lock anything ?
The fuel pump will push gas regardless, it will self prime and 10 seconds of cranking will push all air out of the line and clear any *bubbles* that may exist.

The problem will be either way too rich, way too lean, or something in the ignition system (like the module in the HEI) that expands and stops working at a certain temp.
Edelbrock carbs suck, they are an exact copy of the 1950's era Carter AFB (aluminum four barrel).
It was shit in 1950, it's shit now.
The OEM Quadrajet is 10 times the carb, every day, all day, and next week too.
You just have to know how to work on it......and most pros haven't a fucking clue.

At the tank, there is the 3/8" feed (to pump) line, there is a 5/16" vent to charcoal canister line, there is a 1/4" vent to "roll over" valve.
The tank won't be sucking it's self into vacuum any time soon, but it can have pressure if the roll over valve is stuck (semi common).

Vapor lock was a 1940-1950 era rumor that spread way too far.
But by all means put those clothes pins on the rubber portion of the line....should fix you right up.
/sarcasm

the edelbrocks run pretty smooth, jetted right they give good power right off idle. Works for me since I’m not into racing old squarebody chevys around the hills.
Rebuilt and tuned a few Quadrajets, they are real nice as well, had an old friend teach me the way of the Quadrajets. R.I.P.
I like em but when they are wore out I don’t feel like redoing the bushings and such.
And I don’t have any canisters on here either. They “fell” out somewhere.

 

[n2ishun]

the edelbrocks run pretty smooth

Then why the thread ?

 

[sirhrmechanic]

Adding about 5 percent diesel to your tank ie. A gallon of diesel in a 20 gallon tank…

Can definitely help with vapor lock on an older engine.

Cheers, Sirhr

 

[Makinchips208]

Adding about 5 percent diesel to your tank ie. A gallon of diesel in a 20 gallon tank…

Can definitely help with vapor lock on an older engine.

Cheers, Sirhr

Really? I will try that. Thank you!

 

[James40004]

Holley part # 108-58 is gasket. Fel pro carries it too. Should be for a 1982 f600 4v with 370 or 429

 

[chevy_man]

Well this is gonna ruffle EVERYONES feathers.

There is no such thing as "vapor lock".

Sure fuel can boil out of the bowl or line......how does this lock anything ?
The fuel pump will push gas regardless, it will self prime and 10 seconds of cranking will push all air out of the line and clear any *bubbles* that may exist.

The problem will be either way too rich, way too lean, or something in the ignition system (like the module in the HEI) that expands and stops working at a certain temp.
Edelbrock carbs suck, they are an exact copy of the 1950's era Carter AFB (aluminum four barrel).
It was shit in 1950, it's shit now.
The OEM Quadrajet is 10 times the carb, every day, all day, and next week too.
You just have to know how to work on it......and most pros haven't a fucking clue.

At the tank, there is the 3/8" feed (to pump) line, there is a 5/16" vent to charcoal canister line, there is a 1/4" vent to "roll over" valve.
The tank won't be sucking it's self into vacuum any time soon, but it can have pressure if the roll over valve is stuck (semi common).

Vapor lock was a 1940-1950 era rumor that spread way too far.
But by all means put those clothes pins on the rubber portion of the line....should fix you right up.
/sarcasm

It definitely was a thing. Fuel line coming from the tank and running right next to exhaust up to the mechanical pumps. Pump creates vacuum in the line to draw fuel, fuel boils at a lower pressure, fuel pump doesn't pump vapor effectively.


On the pressure side it never locks. The boiling point is raised and the fuel stays liquid. It just can't get pushed into the bowl because the weight of the fuel isn't being overcome by pressure from the pump.

It is possible that the fuel is boiling in the bowls and is creating a pressure higher than the fuel pump. So there's fuel vapor bring sucked out, but not enough to run on and it creates a lean condition extreme enough to starve the motor for fuel and cause runnability issues.


Either way, if you have a carbd vehicle that restarts and runs fine after a 15 minute cool down it's a heat issue, and lowering that heat will fix the issue.

 

[Makinchips208]

It definitely was a thing. Fuel line coming from the tank and running right next to exhaust up to the mechanical pumps. Pump creates vacuum in the line to draw fuel, fuel boils at a lower pressure, fuel pump doesn't pump vapor effectively.


On the pressure side it never locks. The boiling point is raised and the fuel stays liquid. It just can't get pushed into the bowl because the weight of the fuel isn't being overcome by pressure from the pump.

It is possible that the fuel is boiling in the bowls and is creating a pressure higher than the fuel pump. So there's fuel vapor bring sucked out, but not enough to run on and it creates a lean condition extreme enough to starve the motor for fuel and cause runnability issues.


Either way, if you have a carbd vehicle that restarts and runs fine after a 15 minute cool down it's a heat issue, and lowering that heat will fix the issue.

@DarnYankeeUSMC mentioned temperature as well. Would a lower thermostat in the summer be a possible help?

 

[DarnYankeeUSMC]

@DarnYankeeUSMC mentioned temperature as well. Would a lower thermostat in the summer be a possible help?

Probably not. The thermostat helps with the engine getting up to temp sooner and keeping it there in cold temps. What is the condition of the radiator inside? If you take the cap off the radiator first thing in the morning before starting the level should be low enough to see if there's scale building up causing poor circulation in the radiator. Dirt built up in the outside clogging the fins and not allowing air through it. Is the fan shroud still there? Some cars and trucks of that era had a small plastic dam attached to the bottom of the core support. It forced more air up through the radiator instead of going under the vehicle. If it had one, is it still there? Does the fan blade have a clutch and is it working? What is the condition of the antifreeze? If you are only running water you should drain it and use a 50-50 mix of antifreeze

 

[DarnYankeeUSMC]

@DarnYankeeUSMC mentioned temperature as well. Would a lower thermostat in the summer be a possible help?

I kinda skipped over the t-stat a bit.
It could be the wrong temperature stat or not opening at the correct temperature. So changing it to the correct one or even changing to ensure that it is opening at the right temp is not a bad idea.
Engine timing will also cause a high running temperature if it's not correct.
Changing the t-stat and setting the timing are fairly cheap and easy to do. Probably a good place to start along with checking the radiator condition when you change the t-stat. You will have to drain some of the fluid to change the t-stat so if it is several years old you may as well drain all of it and put new antifreeze in it.

 

[fpgt72]

The booze in the gas will "burn off" quicker then the "gas" will. The fuel pumps to a carbed car will just not push the volume to keep this from happening.

If you have it laying around you can try to insulate the gas line, bit of a pain in the ass, or you can setup a return line back to the tank, that round trip will keep the pump always pumping and the fuel will be cooler coming out of the tank and not have a chance to "boil".

You can't pump "air", so until things cool off you are going to be stuck. On a mechanical fuel pump this may take a bit of cranking to have that pump get the gas where it needs to go, depends on how far from the pump to the carb and size of filters inbetween. I generally ran a filter before and after the pump.

"real" gas is LIKELY to solve the issue, but here that is about $1 more a gal, and I don't want to spend that if I don't have to.

My triumph GT6 had this issue, it also has "soft" valve seats so it gets an lead additive, not sure they sell it anymore, I bought a case and it does not take much, I need to look again. I setup a return line and it solved the issue, the odd thing was it only happened on the back carb, it was a real head scratcher, those carbs are not like "normal" carbs.

All of this is just in my view, and what I did to take care of my issues on my older things. Personally I think some of the suggestions are just so far off in left field, but that is just me. A SS hard line back to your tank is a pretty easy thing to do. I did have to drop the tank to do mine, and I suggest sticking a cheapo electric fuel pump in there after you think you got everything drained. If you need to do metal work on the tank, let it air out for DAYS before you do anything, you don't want to be johnny the living torch.

 

[GreenGO Juan]

it works perfectly on a cool day or all winter long. Only acts up in the heat.

Do you have access to a wide band O2, 5 wire sensor with a digital display. I used to loan mine to an old school "mEh CaRb iS BeTtEr tHaN Fi type"

It sounds like a fueling issue. O2 will tell you immediately.

MTX-L PLUS: Advanced Digital Wideband Air/Fuel Ratio Gauge Kit, 8 ft. Sensor Cable

MTX-L PLUS AFR 8ft

www.innovatemotorsports.com

Definitely sounds like a heat related fueling issue. Wether a vapor lock or air density/fuel mixture. The wide band is the key to knowing. You can also properly tune for 10% E.

 

[Makinchips208]

Do you have access to a wide band O2, 5 wire sensor with a digital display. I used to loan mine to an old school "mEh CaRb iS BeTtEr tHaN Fi type"

It sounds like a fueling issue. O2 will tell you immediately.

MTX-L PLUS: Advanced Digital Wideband Air/Fuel Ratio Gauge Kit, 8 ft. Sensor Cable

MTX-L PLUS AFR 8ft

www.innovatemotorsports.com

Definitely sounds like a heat related fueling issue. Wether a vapor lock or air density/fuel mixture. The wide band is the key to knowing. You can also properly tune for 10% E.

I do not have one of those. The auto shop probably does, maybe I’ll hook up with him and check that.

It runs fine all day every day, until it chugs out on a higher heat day if I’m usually going slower like hauling a load. When it chugs out it acts just like it runs out of gas, the clear fuel filter under the hood will be empty in this condition, I have also unhooked the line and let it pump air until it catches fresh fuel from the tank and begins pumping liquid again.
Guess I should have said all that I’m the first post, but didn’t want to make it too long.

 

[37L1]

Before you laugh:

Put a wooden clothes pin on the fuel line going to the carb.

 

[Makinchips208]

Probably not. The thermostat helps with the engine getting up to temp sooner and keeping it there in cold temps. What is the condition of the radiator inside? If you take the cap off the radiator first thing in the morning before starting the level should be low enough to see if there's scale building up causing poor circulation in the radiator. Dirt built up in the outside clogging the fins and not allowing air through it. Is the fan shroud still there? Some cars and trucks of that era had a small plastic dam attached to the bottom of the core support. It forced more air up through the radiator instead of going under the vehicle. If it had one, is it still there? Does the fan blade have a clutch and is it working? What is the condition of the antifreeze? If you are only running water you should drain it and use a 50-50 mix of antifreeze

Fan shroud is good.
Has a clutch fan that fits well and moves a lot of air.
Radiator is clean on the outside.
Radiator is clean on the inside.
Coolant is old, have no proof if it is close to the proper ratio. So I need to just change it completely.
Thermostat is a number of years old, appears to work ok going by the factory gauge on the dash. I can pull it and test it and/or replace it same time as coolant change.
Oh, just realized, on the 79 I do have a custom steel bumper, and a winch, the winch is obviously right in front of the radiator. So air flow is possibly reduced by that, but that fan seems to move a lot of air regardless.

 

[DarnYankeeUSMC]

I do not have one of those. The auto shop probably does, maybe I’ll hook up with him and check that.

It runs fine all day every day, until it chugs out on a higher heat day if I’m usually going slower like hauling a load. When it chugs out it acts just like it runs out of gas, the clear fuel filter under the hood will be empty in this condition, I have also unhooked the line and let it pump air until it catches fresh fuel from the tank and begins pumping liquid again.
Guess I should have said all that I’m the first post, but didn’t want to make it too long.

You say that it's usually when you are hauling a load. Are you heavy on the pedal when it happens?
Do you know if the CFM and jetting on the new carb is even close to what the old one was ?
What is the condition of the fuel line? Original? Have you taken it loose and blown compressed air through it? Even if you did it could be partially restricted starving the pump when you are hauling. I've seen some old pickups that were full of shellac from old gas. Did you clean the pickup when you had it out? (Since you changed the sock. I'm assuming that it is the original pickup and it's depth is correct)
If running a return line fixes the issue. I wouldn't be happy until I figured out why it is happening. Millions of vehicles with carbs operated in high temperature areas and were fine. Something is causing the problem. Using a work around is not fixing the problem.

 

[Makinchips208]

You say that it's usually when you are hauling a load. Are you heavy on the pedal when it happens?
Do you know if the CFM and jetting on the new carb is even close to what the old one was ?
What is the condition of the fuel line? Original? Have you taken it loose and blown compressed air through it? Even if you did it could be partially restricted starving the pump when you are hauling. I've seen some old pickups that were full of shellac from old gas. Did you clean the pickup when you had it out? (Since you changed the sock. I'm assuming that it is the original pickup and it's depth is correct)
If running a return line fixes the issue. I wouldn't be happy until I figured out why it is happening. Millions of vehicles with carbs operated in high temperature areas and were fine. Something is causing the problem. Using a work around is not fixing the problem.

600 or 650 cfm can’t remember of top of head.
I’m sure the oem Quadrajet was at least 750 at wot.
It is a basically stock small block Chevy 350.
As far as testing restrictions, I blew air back up the line and it was churning and bubbling in the tank pretty good. Although not a precise test.
And that’s what gets me is I know I there are millions of carbureted vehicles working fine every day.
The carb is empty of fuel when this happens. If I can cool it down and get fuel pumping again it will take off fine. By cool down I mean the temp under the truck and in the engine bay, I sprayed water all under on all the fuel lines and pump all the way back to the tank at one point and it seemed to be an immediate fix, but only until it heated right back up.

-I’ll start with coolant and thermostat to make sure it’s gtg.
-check ignition timing
-I’ll check float level and operation, and maybe measure the o2 if I can get a tool.
-Then I think I will replace the pickup tube in the tank, and go new line from there to the engine.
-thick gasket under the carb to help insulate it.
-diesel in the fuel
-return line I’m sure would help, and if it fixes it then great, but it bugs me until I know the “why”

I have tried ethanol free gas also. With no change.
The biggest difference is the electric fuel pump back close to the tank on the 79. That took it from almost undriveable to driveable.
There must be some leak or restriction between the pump and the pickup in the tank? Either air getting in or enough negative pressure that in the heat the gas just turns to ‘gas’?
Looks like the tank needs to come back out?

 

[Makinchips208]

And it happens most if I drive up to work or head for the mountains, elevation goes from 1,240’ up to 3,330’ in six miles.

 

[Bantam1]

Do you have hard line running for most of the distance between your pump and the carb? Do you have an inline fuel filter in there somewhere? What is the GPH or LPH of the fuel pump you have? A stock mechanical pump is probably around 40-60 GPH. It is possible the pump isn't adequate to deliver enough fuel, or could be getting hot and not working so well because it's struggling to keep up. It could also be too much delivery and that causes other issues. Most modern vehicles run them in the tank to help keep them cool. The pump could be cavitating as well.

Do you have a fuel pressure gauge? This may help diagnose what's happening. A simple inline one would be fine and usually pretty cheap. Pressure should probably be around 6-8psi if I remember correctly. It's been a while since I played with carb fed stuff.

If your tank is not venting pressure and you have all rubber hose, the line could be partially collapsing. I only used fuel injection hose even if it was for carbs. I liked the reinforced design of the hose and they were much harder to collapse.

The evap system should be working to eliminate pressure but it that isn't working correctly, and your gas cap has no vent (most modern vehicles do not have a vent on the cap) it could explain the issue. The elevation change you have isn't enough to explain the problems you are having.

You can get heat insulation wrap for the lines near the engine. That can help with some of the heat issue. I would make sure the floats aren't sticking on the carb not letting fuel into bowls.

 

[GUNNER75]

Well this is gonna ruffle EVERYONES feathers.

There is no such thing as "vapor lock".

Sure fuel can boil out of the bowl or line......how does this lock anything ?
The fuel pump will push gas regardless, it will self prime and 10 seconds of cranking will push all air out of the line and clear any *bubbles* that may exist.

The problem will be either way too rich, way too lean, or something in the ignition system (like the module in the HEI) that expands and stops working at a certain temp.
Edelbrock carbs suck, they are an exact copy of the 1950's era Carter AFB (aluminum four barrel).
It was shit in 1950, it's shit now.
The OEM Quadrajet is 10 times the carb, every day, all day, and next week too.
You just have to know how to work on it......and most pros haven't a fucking clue.

At the tank, there is the 3/8" feed (to pump) line, there is a 5/16" vent to charcoal canister line, there is a 1/4" vent to "roll over" valve.
The tank won't be sucking it's self into vacuum any time soon, but it can have pressure if the roll over valve is stuck (semi common).

Vapor lock was a 1940-1950 era rumor that spread way too far.
But by all means put those clothes pins on the rubber portion of the line....should fix you right up.
/sarcasm

^^^^ This and would like to emphasize "Edelbrock Carbs Suck"!

 

[DarnYankeeUSMC]

And it happens most if I drive up to work or head for the mountains, elevation goes from 1,240’ up to 3,330’ in six miles.

As suggested by @Bantam1 . Your mechanical pump may be giving up. Which would explain why it's working fine until you have a heavy load and pulling up hill.
A small block mechanical pump is cheap and easy to replace. For that matter, it would only be a few dollars more to run a new line from the tank to the pump. I'd do that before pulling the tank and pickup again.

 

[Ison]

Sounds like vapor lock is occurring in the mechanical fuel pump as the engine heats up with the gains in elevation (climbing the hills). Watch the video I posted above for ways to prevent this from happening.

 

[BCP]

I had the same problem with a family member's honda accord. It has to do with the summer blend gasoline. IIRC we fixed it with a new fuel rail that had a line connecting the two rails together or some shit to prevent vapor lock.

Sooo, it's fixable but you might need a new fuel system (carb, lines, pump etc).

 

[Makinchips208]

Thanks guys! You have given me plenty of things to check and some to fix. I’ll update when I find the problem.