Vapor lock, I think

[RC92234]

I'm fairly sure I'm getting vapor lock: just running along normally, sometimes while moving sometimes stopped at a light, it's as if suddenly there is NO gas and the engine dies. One time only while in motion I was able to slip into neutral and restart, but only that once. Turns over real nice, but never fires. A no-start situation for roughly the next two hours. Then, voila! As if nothing ever happened.

As I understand, some say it's just air, some say it's gasoline vapors, but either way a bubble of vapor gets in the fuel line and prevents any gasoline getting to the engine. and it's usually (always?) because of the fuel line overheating.

One solution is to get an electric fuel pump mounted at the tank so that the fuel line is always under pressure, instead of the fuel being sucked from a long line starting at the tank by the engine-mounted fuel pump. Another solution is to insulate the fuel line at the point it's picking up the extra heat.

I'm having trouble finding that spot. I had a new tank installed, and I've had a new full exhaust system installed. It sort of looks like the fuel line may be too close to the exhaust shortly after it exits the tank in the photo below, but that's about a two inch distance from pipe to pipe where my finger is, and it's a rubber hose (which should? provide some insulation itself?) Past that point the solid fuel line goes up and over the rear wheel assembly then forward to the engine. I can't see any other point where the fuel line comes at all close to the exhaust.

Thoughts/advise/witty remarks? Thanks in advance.

 

[Don C]

Just being close at that one spot is common to all of our cars, and I never had vapor lock, even when I lived in Las Vegas. I believe it would take a longer section of exposure to heat, than that.

A couple of other possibilities. If the sock (filter) in the tank is partially plugged it will create a high vacuum in the fuel line, causing bubbles to form. Even though the tank is new it may have had crud in it, or you may have put in some dirty fuel.

A tiny crack, hole, or loose fitting will pull in air into the line. This crack can also be in the line in the tank in the pickup tube.

Does this occur no matter what the fuel level is in the tank?

Another one just occurred to me, and that is a hole in the fuel pump diaphragm, or a leaking (air) connection on the inlet side of the fuel pump.

 

[tony-muscle]

Although I insulated that section of hose in my car, I don't see it heating up that much to the point of being an issue. Depending on fuel mixture, pressure, it will have to be around 140F to create major issues. I think it will have more chances on getting near that temperature in the engine compartment, specially if in direct contact with the block or coolant hose.

 

[Bentworker]

What does your fuel delivery plumbing look like from the pump to the carburetor?

 

[RC92234]

Just being close at that one spot is common to all of our cars, and I never had vapor lock, even when I lived in Las Vegas. I believe it would take a longer section of exposure to heat, than that.

A couple of other possibilities. If the sock (filter) in the tank is partially plugged it will create a high vacuum in the fuel line, causing bubbles to form. Even though the tank is new it may have had crud in it, or you may have put in some dirty fuel.

A tiny crack, hole, or loose fitting will pull in air into the line. This crack can also be in the line in the tank in the pickup tube.

Does this occur no matter what the fuel level is in the tank?

Another one just occurred to me, and that is a hole in the fuel pump diaphragm, or a leaking (air) connection on the inlet side of the fuel pump.

And I never had vapor lock in my '71 and yet I put 43,000 miles on that car in one year. I don't think I've ever had it until now, if vapor lock is the problem. Besides "dies like it is out of gas, but let it set two hours and it's like nothing ever happened", how else to I determine what's the issue? Many of the things you mentioned seem as if they would endure after a engine quit. A dirty sock isn't going to just get clean, a hole in the diaphragm would be a constant issue, etc.

It has happened four times total; 3 times left me stranded (one time it did restart while underway). It seems to occur at full engine operating temperature and maybe it's coincidence, on nice days (high 70s/low 80s). Example: run around, make a few stops, maybe drive 10-15 miles total. After the last engine off stop, maybe out of the car 5 minutes, about 1 mile down the road it dies.

As to fuel level, I'm always between 1/4 and 3/4 full. I have a lot, lot, lot of trouble filling the car due to (I think) California pump emission controls; I have to force the seal of the nozzle to the gas outlet, and hold it wedged in place at a perhaps 1/4 the usual flow. And I still have never dared to "fill 'er up".

Tony: I'm going to retrace the fuel line as I hadn't considered coolant hoses, but under the hood, well, the fuel comes to the fuel pump from the wheelhousing near no hoses; maybe the problem occurs from the pump to the carb?

Always an adventure

 

[RC92234]

What does your fuel delivery plumbing look like from the pump to the carburetor?

I just took a look to be sure: original metal tube from the pump squirreling it's way up to right before the rubber hose that attaches to the fuel filter; fuel filter attaches to the carb. The metal goes up behind the power steering so it's not easy to trace but I ran my finger along the tube and it's not in contact with anything until it reaches the rubber hose.

 

[Don C]

Yeah, California gas nozzles can be a real PITA. We had a motorhome with a 50-gallon gas tank and the fill tube set at an angle in a recess, had to pack a lunch when I wanted to completely fill it.

 

[Bentworker]

2 hours? What flavor ignition do you have?

 

[midlife]

Have you checked for fuel in the carb when the engine won't start? Work the accelerator linkage and look for a squirt of fuel into the carb. The symptoms you describe also depict a bad coil.

 

[Blayne9067]

My 71 429cj experienced vapor lock several times many years ago after I removed the Carter return-to-tank pump, re-installed return to tank pump and problem solved.
I still have the car and recently did some engine upgrades and ended up installing another non-return to tank pump, problem came back. I could actually see my fuel pressure gauge at the carb start dropping after engine was at full operating temp. Ended up rebuilding my return to tank pump and problem solved again.
I should also mention I live in CA and I can fill tank without any issue, pump handle wide open until tank is full or whatever I want to put in it. I would take a look at your tank vent for an obstruction.
Good luck, vapor lock is no fun

 

[Heavy Metal]

I'll jump in on this one. I've had ignition coils fail when they get hot. I tore apart carb's for months
when one day i decided to try something different. Put in a new coil and the problem solved.
And it acted just like I was starving for fuel. Also if you still have the points and condenser i would change those too.
You may want to switch to a updated magnetic distributor as well, this eliminates the 50yr old technology.
Just another option to try for this issue.

 

[jpaz]

I had a vapor lock issue last summer. But I had remade the steel line going from the fuel pump to the carb and got it to close to the front of the block. So I remade a different line making sure that I got it away from the block and no more problems. Just make sure that line isn’t up against the front of the block.

 

[Stanglover]

I'll jump in here too. No experience with 73's or stock carbs, but I would check a couple of things off the list of possibilities.
1) loose those gear clamps off the fuel lines. Install new Fuel Injector (ethanol resistant) rubber throughout where needed with correct fuel line clamps. That will ensure no leaks at those joints.
2) check for an insulator gasket under the carb.
3) As mentioned, check to see if fuel is squirting when you operate the throttle shaft. Check for fuel in the fuel bowl. Check for fuel in the carb fuel line by cranking the motor (no ignition on of course and done with extreme care) I use a bump switch for doing this.
If all the checks are good, then I too would start thinking ignition issues. A bad coil is a definite possibility.

 

[DSwan]

Had the same issue on my 73 with the 351c motor. I ended up insulating the fuel line going from the pump up through the power steering pump bracket where it was close to the engine block. I then used an insulator between the cast manifold and carb as stated by stanglover, no more dying issues.

 

[machcrazy]

I'm fairly sure I'm getting vapor lock: just running along normally, sometimes while moving sometimes stopped at a light, it's as if suddenly there is NO gas and the engine dies. One time only while in motion I was able to slip into neutral and restart, but only that once. Turns over real nice, but never fires. A no-start situation for roughly the next two hours. Then, voila! As if nothing ever happened.

As I understand, some say it's just air, some say it's gasoline vapors, but either way a bubble of vapor gets in the fuel line and prevents any gasoline getting to the engine. and it's usually (always?) because of the fuel line overheating.

One solution is to get an electric fuel pump mounted at the tank so that the fuel line is always under pressure, instead of the fuel being sucked from a long line starting at the tank by the engine-mounted fuel pump. Another solution is to insulate the fuel line at the point it's picking up the extra heat.

I'm having trouble finding that spot. I had a new tank installed, and I've had a new full exhaust system installed. It sort of looks like the fuel line may be too close to the exhaust shortly after it exits the tank in the photo below, but that's about a two inch distance from pipe to pipe where my finger is, and it's a rubber hose (which should? provide some insulation itself?) Past that point the solid fuel line goes up and over the rear wheel assembly then forward to the engine. I can't see any other point where the fuel line comes at all close to the exhaust.

Thoughts/advise/witty remarks? Thanks in advance.

View attachment 59369

Have you converted to electronic ignition? Getting rid of points and going with pertronix was huge for me. Reccomend a flame thrower coil also.

 

[Stanglover]

Have you converted to electronic ignition? Getting rid of points and going with pertronix was huge for me. Reccomend a flame thrower coil also.

Indeed, HUGE improvement. Personally, I installed the Pertronix Ignitor II and matching coil. P III is a POS in my experience. Don't forget if you go this route, it needs a full 12V. Simple way is to use a relay (so I'm told!!) . Another alternative is a DuraSpark system.

 

[Stanglover]

Had the same issue on my 73 with the 351c motor. I ended up insulating the fuel line going from the pump up through the power steering pump bracket where it was close to the engine block. I then used an insulator between the cast manifold and carb as stated by stanglover, no more dying issues.

On my 71 4V, I added a 1" thick fiber spacer plus I had blocked off the heat cross-over passages. I'm not sure if the 73 is the same manifold system as it has an EGR.

 

[RC92234]

Wow - what a great bunch of answers and possibilities. A few I can answer now:
- Yes, I converted to electronic points, the Pertronix 1.
- Replaced the coil (not the Flamethrower) at the same time, the yellow top clone.
- Both short sections of rubber hose are new, ethanol-resistant hoses.
- The return-to-tank pump is new to me; was this in use for '73s? I know there is a vapor capture canister, but that's about it.
- There is no spacer between carb and manifold other than the EGR plate (but isn't the vapor lock occurring in the fuel line?)
- I don't know that it takes exactly two hours for the problem to clear up; just happens that's been the break in between dying and getting towed home, or leaving it pushed into a parking space and returning.

As to all the suggestions for diagnostics at the time of failure, always happens away from home, in the middle of doing something else. Diagnostics at that point are not an option. By the time it gets home, it starts right up so I'm not sure those suggested diagnostics would be valid two hours later.

I can easily put the old coil back in and test. It worked fine, the upgrade was purely vanity. The fuel line passes behind the power steering pump and bracket so I will get a better eye on what's happening there. (The p/s pump is now failing after having had it rebuilt a year ago so it needs to be pulled in any case)

Thanks, y'all

 

[RC92234]

Had the same issue on my 73 with the 351c motor. I ended up insulating the fuel line going from the pump up through the power steering pump bracket where it was close to the engine block. I then used an insulator between the cast manifold and carb as stated by stanglover, no more dying issues.

What did you use for insulation?

 

[Stanglover]

What did you use for insulation?

You may be able to buy a 1/4" thick spacer at most of our vendors. You may also need the gasket that has a metal insert between two layers of gasket material. This is to cover the heat passages and stop the gasket/insulator from burning. Again I can't say for sure on the 73 as it's a spread bore (I think). Check NPD's website.