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Vapour Lock
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Gratsoy
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PostPosted: Sun Apr 18, 2021 10:57 pm    Post subject: Vapour Lock Reply with quote

There's a few threads on this but wanted to start my own and go in-depth on my problems/troubleshooting.

The Car:
- 1967 Beetle with stock 1500 engine
- Has a square-top Pierburg pump.
- Has a fuel filter installed beside Trans
- All soft fuel-lines were replaced already
- In-tank screen "filter" was cleaned (a few years ago)
- Tank was cleaned (again, a few years ago. But it has been stored correctly since and used high quality fuel so I am assuming it's not dirty)
- Only using 94 octane ethanol-free fuel
- Pre-heat flaps on the air cleaner are set to "summer mode" per the manual.
- I do not have a thermostat/fan shroud flaps installed. So the car should be getting full cooling from the shroud.


Typically I only get vapour lock on a nice sunny day. Last year it happened when it was 30 c outside. The car runs fine until I bring it down to idle (usually at a light). My solution at that time was to try and replace the pump - I was using a Brazilian Pierburg at the time.
I swapped my pump with a German Pierburg that was rebuilt by Ed Fall.

Today it locked again - it was about 24 c out and I had run the car to get errands - about 30 minutes and parked/restarted 3 times no issues. Died at a stop light. Before this happened, I had added a full can of Seafoam to an empty tank and filled up with fresh fuel. I usually Seafoam my cars once a year as a preventative maintenance sort of thing (maybe it works, maybe it doesn't).

Tried cooling the pump with water - didnt help. Tried cranking for a while without using the gas pedal in case the float bowl was flooding, no help.

Got the car towed with a flat deck. Soon as it was dropped off it started back up perfectly. I'm guessing because when it was lifted, gravity forced the gas down the line and primed the pump. This car usually starts instantly, often faster than the starter motor can disengage. This was no different, didn't even need to hit the gas, just turn the key and it fired.


At this point I have no clue what to do next - locking at 24 c is very concerning to me as this is a typical mid-day temperature for the summer. It's possible since I ran the tank down to R I "found" some sediment/gunk at the bottom of the tank - but the filter looks fine. And again, after the car was dropped off the tow truck it ran perfectly.
It could be possible that Seafoam evaporates at lower temps than the gas (might be true as its partially Isopropanol) and caused the lock.

At this point I am going to run through this tank on some colder days, then fill up with pure gas and see if that does anything. Open to some input/sugestions. However, not interested in the clothes pin trick, or installing an electric pump
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Matt Wilson
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PostPosted: Mon Apr 19, 2021 7:21 am    Post subject: Re: Vapour Lock Reply with quote

I've had issues like yours in the past, and it's very frustrating.

Maybe your fuel pump rod is getting a little binding when the engine is warm, which could slow down the pump making it skip a little bit. I've had pumps that let the little hinge pin wander fore and aft, binding the pump internally as well.

I've considered putting a little flow-through solid state electric pump under the tank simply as vapor lock override.
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Gratsoy
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PostPosted: Mon Apr 19, 2021 9:06 am    Post subject: Re: Vapour Lock Reply with quote

Matt Wilson wrote:


Maybe your fuel pump rod is getting a little binding when the engine is warm, which could slow down the pump making it skip a little bit. I've had pumps that let the little hinge pin wander fore and aft, binding the pump internally as well.

I've considered putting a little flow-through solid state electric pump under the tank simply as vapor lock override.


I was considering the pump pushrod as well. I might take it out and polish it on the lathe. I don't see what harm could come from doing that.
I might also consider replacing the riser block or even trying to polish the interior of that.

The electric pump would be a last resort - I've definitely thought of that too. I've used that on other cars (not for vapour lock) to assist the mechanical pump during cold starts, etc with good results.
I'd have to take a look if I can get some power wiring down below the tank without drilling holes in the body.
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aircooled
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PostPosted: Mon Apr 19, 2021 9:45 am    Post subject: Re: Vapour Lock Reply with quote

I went down the same road for a while with my stock 67, 1500. Checked everything found nothing. Changed the condenser on the distributor and problem went away. Inside the condenser is a piece of foil which was causing the problem when it would warm up on a 75/80 temp day. Just a suggestion.
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ashman40
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PostPosted: Mon Apr 19, 2021 10:52 am    Post subject: Re: Vapour Lock Reply with quote

Gratsoy wrote:
Tried cooling the pump with water - didnt help.

To me, the above suggests your problem is not vapor lock. I'm not certain you have yet confirmed it is a fuel deliver problem.

Next time it happens... pull to a safe spot so you can work on the engine.

While the engine is hot, remove the mechanical fuel pump and hand rotate the engine thru 2 full rotations. Do you see the pushrod in the pump stand moving up/down? If the rod seems stationary it could mean it is bound/stuck inside the plastic pump stand. It should move freely. This could be caused by heat constricting the case hole the pump stand slips into. Pull the rod from the stand, do you feel resistance? The aftermarket stands have a reputation for being too large OD. When the case heats up the OD of the stand is constricted and pinches the push rod preventing the pump from working. To fix, after the engine cools, remove the stand and sand down the OD of the length that fits into the case. This allows the case to expand without constricting the push rod. Only do this after the engine has cooled. Trying to force the stand out of a hot engine which has a grip on it will likely crack the stand and leave a chunk stuck in the case.

The next test is difficult on your own. Replace the fuel pump and while your engine is hot you want to check if the pump is working. Disconnect the fuel line from the carb inlet and place into a container. Have someone crank the engine. Does fuel flow from the pump outlet? If the lines are empty it could take some seconds to get fuel flowing once again. Don't run the starter more than 20sec at a time and give time for it to cool between attempts. If you have to wait until everything cools down before you can get fuel flowing out the pump you could have a heat related fuel delivery problem.

Next, remove the top of the carb (just 5 screws). Carefully remove the top so you don't rip the gasket. Look at the fuel level in the fuel bowl. The float and inlet valve should maintain the level at 3/4" from the top of the bowl. Any level lower that 3/4" from the top and your fuel system is not providing enough fuel to the carb.
If you have a vapor lock issue or a fuel delivery issue your fuel bowl will have run empty or be low. If you have any fuel in the bowl you should be able to get the engine running even if only for a few seconds. Pull the throttle arm and confirm the accelerator pump is squirting fuel. You engine should start just on the fuel from the accelerator pump even if only for a couple seconds. With a full fuel bowl and fuel being squirt from the accelerator pump, reassemble the carb, crank the engine with the pedal to the floor. If the engine doesn't even attempt to catch, your problem is probable not fuel related, look to the ignition.
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'67 Beetle #1 {project car that never made it to the road Sad }
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KTPhil Premium Member
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PostPosted: Mon Apr 19, 2021 10:58 am    Post subject: Re: Vapour Lock Reply with quote

Are you sure it's a fuel problem? Maybe the coil is overheating and failing? When it fails, did you confirm spark when cranking?
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Gratsoy
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PostPosted: Mon Apr 19, 2021 11:25 am    Post subject: Re: Vapour Lock Reply with quote

ashman40 wrote:
Gratsoy wrote:
Tried cooling the pump with water - didnt help.

To me, the above suggests your problem is not vapor lock. I'm not certain you have yet confirmed it is a fuel deliver problem.

Next time it happens... pull to a safe spot so you can work on the engine.

While the engine is hot, remove the mechanical fuel pump and hand rotate the engine thru 2 full rotations. Do you see the pushrod in the pump stand moving up/down? If the rod seems stationary it could mean it is bound/stuck inside the plastic pump stand. It should move freely. This could be caused by heat constricting the case hole the pump stand slips into. Pull the rod from the stand, do you feel resistance? The aftermarket stands have a reputation for being too large OD. When the case heats up the OD of the stand is constricted and pinches the push rod preventing the pump from working. To fix, after the engine cools, remove the stand and sand down the OD of the length that fits into the case. This allows the case to expand without constricting the push rod. Only do this after the engine has cooled. Trying to force the stand out of a hot engine which has a grip on it will likely crack the stand and leave a chunk stuck in the case.

The next test is difficult on your own. Replace the fuel pump and while your engine is hot you want to check if the pump is working. Disconnect the fuel line from the carb inlet and place into a container. Have someone crank the engine. Does fuel flow from the pump outlet? If the lines are empty it could take some seconds to get fuel flowing once again. Don't run the starter more than 20sec at a time and give time for it to cool between attempts. If you have to wait until everything cools down before you can get fuel flowing out the pump you could have a heat related fuel delivery problem.

Next, remove the top of the carb (just 5 screws). Carefully remove the top so you don't rip the gasket. Look at the fuel level in the fuel bowl. The float and inlet valve should maintain the level at 3/4" from the top of the bowl. Any level lower that 3/4" from the top and your fuel system is not providing enough fuel to the carb.
If you have a vapor lock issue or a fuel delivery issue your fuel bowl will have run empty or be low. If you have any fuel in the bowl you should be able to get the engine running even if only for a few seconds. Pull the throttle arm and confirm the accelerator pump is squirting fuel. You engine should start just on the fuel from the accelerator pump even if only for a couple seconds. With a full fuel bowl and fuel being squirt from the accelerator pump, reassemble the carb, crank the engine with the pedal to the floor. If the engine doesn't even attempt to catch, your problem is probable not fuel related, look to the ignition.


This is a great write-up, thanks for all the detail.

I am thinking it might not be a vapour lock issue now too, especially considering the water on the pump trick didn't work.
I had previously assumed it was locking just due to heat and it dying once I let off the pedal and let it idle.

KTPhil wrote:
Are you sure it's a fuel problem? Maybe the coil is overheating and failing? When it fails, did you confirm spark when cranking?


To be honest, I hadn't even looked into the ignition system.

It does have an older coil in it - its one of those blue Bosche ones, so could be ready for replacement.
I'll replace the coil and do a Distributor tune-up and see if maybe that does anything. I'll start packing a spark tester with me. Any easy methods to check for spark at the coil on the roadside? I wonder if I can plug a spark tester right into it?
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leowagen
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PostPosted: Mon Apr 19, 2021 3:27 pm    Post subject: Re: Vapour Lock Reply with quote

I had vapor lock in mine nad was a combination of all possible issues in the fuel system.... pump rod binding, pump not preloaded when installed, carb float height was too low, I had a glass bowl filter installed higher than the carb making things even harder... I sorted one by one, and ended up nicely....
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viiking
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PostPosted: Mon Apr 19, 2021 7:12 pm    Post subject: Re: Vapour Lock Reply with quote

I also vote for the fuel pump pushrod binding. Sanding down the outside of the bakelite piece should be a mandatory thing for all VW engines. The phenomenon is not well known by most people. It may NOT be the only problem, but it should be done to remove that one from the equation.

The other thing to check is to remove the gas cap when the engine has died and see if that solves the problem. An unvented tank will pull a vacuum and prevent the fuel pump from delivering.

Re the use of Seafoam and the issue about isopropanol boiling off. It does not. When an impurity (read Seafoam) is added into a solvent (read fuel. gas etc) the boiling point of the whole solution actually increases i.e. reduces the propensity to boil /evaporate. It is the same principle when using an anti-freeze solution. Not only does the impurity (the antifreeze) added into water increase the boiling point of water, but it also reduces the freezing point of the solution as well. Hence why you use it. Read up on Boiling Point Elevation and Freezing Point Depression if you are interested.
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petrol punk
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PostPosted: Thu Apr 22, 2021 1:40 pm    Post subject: Re: Vapour Lock Reply with quote

Do you have J tubes? I had issues with vapor lock on my 36hp. Originally it would vapor lock just driving down the road on a hot day. I swapped the J tubes out with heater boxes and that helped, they acted as a heat shield for the fuel line going over the #3 header. I still had issues with vapor lock when I turned it off and came back after 5 minutes, the heat coming off the #3 header seemed to heat the fuel line enough to boil the gas in it. I threw fiberglass sparkplug insulation material and insulated the entire metal fuel line from the filter by the trans all the way to the fuel pump. Since then it has only vapor locked when it was 113*, except once recently. I'm going to throw another, larger diameter spark plug insulation over the existing insulation to double insulate my fuel line between the pump and pan outlet. I think this is the issue because the fuel pump is sucking gas from the tank, which causes a pressure drop and a corresponding drop in the boiling temperature. Combine that with summer temps of 110*+ and hot air coming off the road and the #3 exhaust header and you have a recipe for vapor lock.
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PostPosted: Thu Apr 22, 2021 2:01 pm    Post subject: Re: Vapour Lock Reply with quote

Check for loose/dirty wire connectors at coil, I had one on the positive side that would expand & work loose when warm. Check for spark.

Check to see if fuel flows, could be pump push rod, pump, even dirt in a main jet.

Open gas cap, if it hisses as you open it, it’s possible that the vent is blocked & with enough vacuum, the fuel won’t flow. I suspect this was dogging a previous owner since they not only removed the gasket on the fuel cap but also drilled a hole in the filler neck.
Image may have been reduced in size. Click image to view fullscreen.


But all the vent hoses were plugged.
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PostPosted: Fri Apr 23, 2021 12:42 am    Post subject: Re: Vapour Lock Reply with quote

Check to see if your fuel line is resting on the manifold. That heats up the fuel in the line and can cause vapor lock. I’ve had that happen to me on a road trip and someone gave me a wooden clothespin which I clipped on the line to keep it off the manifold until I could deal with it properly. Worked.
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