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A common problem with older cars is vapor lock, which can often prevent a car from restarting after a hot soak. That is, difficulty in restarting a car with a fully-warm engine after it has been shut down for a short time (like after refueling).  To clarify, Renton Technical College's automotive glossary defined hot soak to be:

  1. Condition that may occur when an engine is stopped for a period of time after a hot hard run, causing the gasoline to evaporate out of the carburetor.
  2. A period of time where localized combustion heat dissipates through the engine.
  3. Allowing a vehicle to sit at a high temperature until all internal and external components stabilize.
  4. Federal test procedure for new vehicle certification of overall emission output.  This test is normally done in a sealed chamber and all emissions coming from the vehicle are measured.  These include exhaust, evaporative. tires, paint, and interior component emissions, in other words, all emissions given off from a vehicle.

The vapor pressure of a liquid depends on its temperature and gasoline requires a low boiling point in order for it to easily vaporize once it is atomized by the carburetor. Petroleum companies vary the volatility of their gasolines according to the seasons as a trade-off between good vaporization and vapor lock.  Gasoline volatility is measured by its Reid Vapor Pressure (RVP).  Modern gasolines are comprised of a variety of hydrocarbon components, each with their own boiling points.

Ethanol-free gasoline (ie, E0) is still available but usually as a high octane (91+) fuel.  See Ethanol-free gas stations in the U.S. and Canada. Try a couple of tanks of E0 to see if this improves your car's hot weather performance.  Using high-octane fuel when not necessary to control engine knock (pinging) can add a significant cost to running your vehicle and there are other solutions that work as well or better at a lower long-term cost.

The fuel systems in older cars commonly use a mechanical fuel pump mounted on the engine that is driven by a lever actuated by an eccentric cam on the camshaft.  Unless the fuel line is in close proximity to the exhaust system, gasoline gains very little heat as it moves from the gas tank to the fuel pump. From the mechanical fuel pump to the carburetor, the fuel line will absorb some heat from the hot environment surrounding the engine.

Mechanical fuel pumps are generally diaphragm pumps where the lever moves a diaphragm and an internal spring controls the mechanical pump's outlet pressure.  Check valves at the pump's inlet and outlet control the direction of flow and the pressure within the pump chamber alternates between suction and pressure with every stroke. However, at low or no-flow, the temperature of the fuel in and around the fuel pump can easily increase to the point where the fuel flashes into vapor on the suction stroke.  Mechanical pumps do not pump vapor very well so carburetors can experience fuel starvation until the fuel condenses back into liquid.  That is why a car that won't start when hot will easily start after given some time to cool down.  This process can be accelerated by hosing down the fuel pump with cool water.

The carburetor can also become hot enough to boil gasoline.  Without a system to vent and recover vapor from the carburetor, this manifests itself as a progressively worse idle (along with the stink of increased hydrocarbon emissions) while idling in traffic or additional cranking to refill the carburetor after a hot soak.  There is nothing to be gained from removing the evaporative emission controls (charcoal canister & associated plumbing) and will actually cost the vehicle owner in terms of wasted fuel.

The advent of fuel injection has largely eliminated vapor lock from modern vehicles.  The fuel pump is located in the gas tank and gasoline is pressurized until it is injected into the engine.  To minimize or prevent vapor lock in older carbureted cars, it is important to minimize the heat gain of the fuel until it leaves the carburetor as an atomized liquid.

A heat-related no-start condition can be caused by either no-fuel or no-spark and both can resolve themselves once the engine cools down.  Make sure that you're fixing the right problem.  Electronic ignition modules and ignition coils generate heat and can overheat in hot underhood conditions.  Fixed dwell systems (like points & condenser, Mopar Electronic Ignition, Ford DuraSpark, etc) have coil saturation currents that are highest at idle and decrease with increasing RPM.  Variable dwell systems like (GM's HEI) use electronic circuitry to only supply enough current to saturate the coil and their coil saturation currents increase with increasing RPM. An HEI system uses 5.5 amps to saturate the coil and, at low RPM, this can generate around 17 Watts of heat in the module.  See HEI Versus Standard Ignition & Customising A Reluctor-HEI System To A TR4A & Fighting Vapor Lock.

The quickest way to check if you have a vapor lock problem is to see if there is any fuel in the carburetor.  Take the air cleaner lid off and pump the throttle a few times.  If you don't see a good stream shooting out of the accelerator pump nozzles, then you likely have a vapor lock problem.

If you have fuel with a no-start condition, there is a possibility that the engine has flooded due to the greater rate of vaporized gasoline entering the carburetor or being produced in the carburetor.  Flooded engines may be cleared of excess fuel by opening the throttle valves to wide open (full throttle) while cranking.  Never pump the gas pedal while trying to clear a flooded engine.  Once started, you can rev the engine slightly (enough to get the engine to run smoother).  If it can idle without stalling, start driving again.

To verify that you have a good spark, you can pull one of the spark plugs wires off of the spark plug.  Either install a spark tester or hold the spark plug terminal a 1/4" to 3/8" away from the plug.  While your helper cranks over the engine, look for a spark but be very careful not to get shocked.  Also make sure that the air cleaner assembly is installed on the carburetor to minimize any possibility of the spark igniting any venting gasoline fumes.

If you've determined that the problem is fuel related, continue on to the next page.  Otherwise, you need to figure out whether the coil or ignition module has overheated.  It is important to keep your ignition module cool, which was sometimes a problem with some HEI systems.  HEI modules are often installed inside of large-cap distributors and dissipate the HEI module's heat into the distributor body.  Make sure that there is adequate thermal paste between the HEI module and its mounting surface.  Moving the HEI module from inside the distributor to a cooler location under hood and preferably on a heat sink will also help.

Spark Tester ACDelco 10474610
GM OEM Ignition Heat Sink
Heat Sink Thermal Compound

At the carburetor, heat from the intake manifold can vaporize the gasoline in the fuel bowl(s), thereby exacerbating vapor lock.  Fuel boiling in the carburetor is known as percolation.  See the following YouTube videos:

It is difficult enough to pump fuel with a hot fuel pump but a hot carburetor will take more time to fill with enough fuel to run.  Even if the carburetor has not completely emptied, the rich fuel mixture from the vented vaporized fuel could also cause a flooded engine condition.

One solution is to add insulation between the carburetor and intake manifold.  This can be done by adding thicker gasket or an insulating spacer and this usually doesn't affect the throttle linkage or other connections at the carburetor.  Adding a carb spacer adds volume to the intake manifold plenum, which tends to shift torque to a higher RPM and are best used with single plane intake manifolds.  Street-driven vehicles need to maximize low RPM torque and V8 engines are often equipped with dual plane intake manifolds.  Use a 4-hole with your dual plane intake to improve bottom-end torque. Insulating heat dissipator gaskets are easily available from aftermarket sources.  If you can't find a commercially-available spacer for your carburetor, you can also make one yourself.  Some people have even successfully used materials such as wood but wood is porous and can absorb gasoline.  I would use an phenolic material of suitable thickness to solve a hot carburetor issue.

Some engines (like the Chrysler slant six) run the fuel line from the fuel pump to the carburetor in front of the engine, which exposes the fuel line to the hot air blowing through the radiator.  Some vehicle owners have had success improving their hot temperature operation by rerouting the fuel line to reduce this exposure.  See Fuel Line Mod Improves Starting & Driveability.  A simple and cheap way to add a heat shield to the fuel is slip some split flex loom over the fuel line between the fuel pump and carburetor.

1/4" Fuel Line
5/16" Fuel Line
3/8" Fuel Line

Vehicle manufacturers have worked to combat vaporization (and hydrocarbon emissions) in the fuel lines with additional vent lines to the gas tank and the charcoal canister.  Adding vent lines to cars that were never originally equipped with them adds unnecessary complexity to the fuel system and many have found this to be ineffective at controlling vapor lock.

Some people have added a return fuel line to the tank to prevent percolation of the fuel in the supply line to the carburetor.  This will work in the short term but serves to heat the fuel in the gas tank.  Besides causing unnecessary evaporative hydrocarbon emissions at the gas tank, this can result in a longer cool-down time if the bulk temperature of the fuel in the tank becomes too hot.  In addition to hot fuel recirculated back from the engine, the gas tank is also picking up thermal radiation from the hot pavement below.

I do NOT recommend disabling the intake manifold heat system (heat riser) system to keep the intake manifold cooler.  Street-driven engines need intake manifold heat to work properly and blocking the cross-over passages in the V8 will also disable the EGR system, which is necessary to minimize NOx emissions and engine knock and will affect the carburetor's tuning.  See Intake Manifold Heat.

More insulation is better and thicker spacer between the carburetor and intake manifold usually improves engine performance.  Excessive insulation can cause clearance issues between the air cleaner assembly and the hood and/or linkage binding issues dues to the new linkage geometry.  Check your hood clearance by putting some putty (or kids' play dough) on the air cleaner lid and closing the hood.

4bbl Carburetor Insulation

Holley/AFB Square Bore 4bbl Quadrajet Spreadbore 4bbl
Mr Gasket PN 98 (1/2" thick)
Mr Gasket PN 86B (1/2" thick)
Canton Racing PN 85-152 (1/2" thick)
Canton Racing PN 85-250 (1" thick)

2bbl Carburetor Insulation

Holley 2bbl Rochester 2bbl
Canton Racing PN 85-040 (1" thick)
Canton Racing PN 85-032 (1/2" thick)

Gasoline will pick up some heat around the fuel pump.  Some of the heat may come from radiation heat transfer due the gas line's close proximity to the exhaust system and some convective heat transfer may come from hot air blowing across the gas line from the radiator.  It is much easier and more effective to keep heat out of the fuel by insulating the fuel line than it is to keep the heat inside the exhaust system with thermal wraps or ceramic coatings.  The insulation also serves to block thermal radiation from reaching the gas line.

A significant amount of heat entering the fuel will come from conductive heat transfer from the mechanical fuel pump.  The mechanical fuel pump is bolted to the engine block, which will be around the temperature of the water inside the block.  When the fuel flow slows (eg, idling in heavy traffic) or stops (when refueling), less heat will be carried away by the fuel through the fuel pump and the body of the fuel pump will increase and eventually approach the engine block's temperature.  Because gasoline is composed of a variety of components with their own volatities, each component will start to boil off as the fuel pump's temperature reaches its respective boiling point.  The boiling fuel could overwhelm the carburetor's float(s) and cause over-rich operation or flooding.

The following 9:57 minute video shows how cooling the fuel pump restores its fluid flow:

A spacer would reduce heat transfer to the fuel pump but could also change the geometry of lever mechanism.

V8 Fuel Pump Spacer SBC Fuel Pump Spacer
Canton Racing PN 85-010
Canton Racing PN 85-000

While reducing the amount of heat gained by the fuel is helpful, the easiest and most reliable way to keep fuel from vaporizing in the fuel lines is to increase the fuel's pressure.  This means either replacing the mechanical fuel pump completely with an electric fuel pump or adding an electric fuel pump to supplement the mechanical pump.  In either case, an electric fuel pump should be located near the fuel tank and must have at least the flow rating of the mechanical pump and no greater output pressure than that required by the carburetor.

Many aftermarket universal electric fuel pumps are rotary vane style which is  positive displacement design and does not flow-through very well.  Rotary vane pumps can "dead head" (ie, pump with very little flow) The P4070 pump has an internal regulator, and therefore can bypass internally when reaching max pressure.  Rotary vane pumps are self-priming and can lift fuel on the suction side.  The vanes are lubricated by the fuel so it is not good to run the fuel tank too low.

Solenoid-style pumps are another common aftermarket fuel pump and these are generally a flow-through design. These pumps have an internal piston that produce a pulsating flow, somewhat like a mechanical pump and the internal valving allows the pumps to produce flow the necessary flow. Although solenoid pumps are also self-priming, they're not made to draw fuel and rely on tank pressure (via gravity and atmospheric pressure) to siphon fuel from the tank.

A fuel pressure regulator may also required if the pump's output pressure is higher than the carburetor's required fuel pressure. This is usually required on racing applications where the high volume fuel pump puts out much more pressure than the carburetor requires.

Most pumps have threaded connections where hose barb nozzles are screwed in.  For non-racing applications, these threads are typically NPT (National Pipe Thread) or NPSF (National Pipe Straight– Fuel) or NPTF (National Pipe Taper – Fuel).  NPSF and NPTF are also known as "Dryseal" threads and are designed to be used without PTFE tape thread sealant.  NPT threads require either pipe dope or PTFE tape to seal the threads from leaking.  Check which threads are used on your pump.

With any electric pump, it would be wise to install an oil pressure safety switch that stops the fuel pump when the engine stops.  Fuel pump manufacturers also generally require filters or strainers upstream of the pump to protect the pumps from wear and not having one could void your warranty.  Use the manufacturer's recommended micron rating for the pump filter and this may be coarser (larger) than what' required for the carburetor.  Carter recommends 60-100 micron filters for their pumps.  Carter reports that the most common cause for failures are wrong application or incorrect mounting.  Make sure you get the right pump and follow the manufacturer's installation instructions.

Oil Pressure Safety Switches

Carter A68301
Airtex OS75
Edelbrock 72214
Holley 12-810
ACDelco D1847A

Fuel Pump Filters

Mr Gasket 1242G
(40 micron)
5/16" hose inlet x 1/8" NPT outlet
Holley 12-431
(74 micron)
5/16" hose inlet x 1/8" NPT outlet
Holley 12-432
(74 micron)
3/8" hose inlet x 1/8" NPT outlet
Airtex G17
(300 micron)
1/8" NPT inlet & outlet

Replacing the mechanical fuel pump with an electric fuel pump is commonly done in high performance applications for improved fuel delivery and should eliminate vapor lock completely. This prevents fuel vaporization in the fuel pump from conductive heat transfer.  After removing the mechanical pump, the fuel pump's boss on the engine block must be capped-off and a tubing splice to connect the two sections of gas line must be added.  The electric pump should be mounted as close as possible to the tank and as low as possible to allow gravity to prime the electric electric pump.  Follow the installation instructions of the electric fuel pump's manufacturer.

Often, the electrical systems in older vehicles were designed to only provide enough power to the limited requirements of the car.  An electric fuel pump can draw several amps continuously, which can be a problem at idle due to the relatively low output (compared with modern cars) of the old generators and alternators at low rotational speeds.  Reducing the generator/alternator pulley diameter can help to increase low RPM power output.  Some cars can be easily upgraded to higher-output modern alternators but this upgrade requires that the electrical system also be upgraded to handle the additional current.  Not taking the modern alternator's higher output into account risks electrical fires due to excessive current.

Electric fuel pumps generate noise and some pumps are louder than others.  Unless you're building a race car, be sure to install the noise insulation parts that go with your particular fuel pump.

Look up the required fuel pressure required for your carburetor.  If the pump you want to use has a higher output pressure, you need to add pressure regulator to prevent the pump from flooding your carburetor.

For a stand-alone electric fuel pump, you can use either a rotary vane or a solenoid pump.  I recommend rotary vane pumps in this case because they have better lift and self priming capability.

AC Delco EP247
(6-8 psi, 72 gph)
Carter P4070
(4-8 psi, 50 gph)
Spectra Premium SP1130
(4 - 5.75 PSI; 72 gph)
Earl's 128011ERL
7 psi, 97 gph

Adding an electric pump upstream to prime a mechanical pump as required is also fairly straightforward. This can control vapor lock on hot days in heavy traffic without needing to put an unnecessary continuous load on your charging system.  Using the electric pump sporadically during starting should pose minimal issues on the electrical system.  As with mechanical fuel pump replacement, the electric pump should be mounted as close as possible to the tank and as low as possible to allow gravity to prime the electric electric pump.  Follow the installation instructions of the electric fuel pump's manufacturer.

The electric fuel pump setup in this case must be capable of flow-through operation.  That is, it should pose a minimal flow restriction so that fuel will easily flow from the gas tank to the mechanical fuel pump when the electric pump is not running.

The Carter P4070 is a rotary vane electric fuel pump and requires the use of a bypass around the pump with a fuel check valve (PN 169-1002). (crack open pressure of 1 psi).  For a new installation, the Carter P4602RV includes a pump, check valve, and additional fittings.

Airtex has a similar electric pump & bypass setup for RV applications.  Airtex E8120) is a rotary vane pump that includes a fuel check valve (Airtex CV8000) but Airtex has confirmed that the CV8000 has a crack open pressure of 1-5 lbs (psi?), which may be excessive for fuel to readily flow to the mechanical pump.  Airtex recommends their solenoid pumps, Airtex E8016S and Airtex E8012S for priming mechanical fuel pumps in 12V automotive applications.

Fuel Check Valves

Carter 1/4" NPT
Airtex 1/4" NPT
Dorman 5/16" Hose
Dorman 3/8" Hose

Rotary Vane Pump with Bypass

Carter P4602RV
(9 psi, 50 gph)
Airtex E8120
(5-9 psi, 40 gph)

Solenoid Pumps

Airtex E8016S
(2.5-4.5 psi, 30 gph)
Carter P74021
(3.5-5.5 psi, 15 gph)
Mr. Gasket 42S
(2-3.5 psi, 28 gph)
Edelbrock 17303
(2-3.5 psi, 30 gph)
Airtex E8012S
(5-9 psi, 30 gph)
Carter P74019
(5.5-9 psi, 20 gph)
Mr. Gasket 12S
(4-7 psi, 35 gph)
Edelbrock 17301
(4-7 psi, 38 gph)

Because the electric fuel pump is only required to combat vapor lock, the operation of the electric pump may be controlled by the driver with a simple toggle switch that is powered by the Ignition Run circuit.  The fuel pump's noise should indicate with the pump is running.

Another method to control the operation of the fuel pump is to wire it with a relay controlled by the Ignition Start circuit.  This ensures that the electric fuel pump operates whenever the engine is started.

I first became aware of the use of an electric fuel pump to overcome vapor lock decades ago when my friend in the ACCCC (Antique & Classic Car Club of Canada) told me that he has an electric pump on a toggle switch for his 32 Plymouth.  Whenever he has trouble starting his car after a hot soak, he flips the switch to run the electric pump and he is then able to easily start his car.  The pumps he uses AC Model 6VER11 (6V) and 12V2401 (12V), which now appear to be long obsolete.  These pumps are solenoid operated and use a couple of check valves to allow flow-through operation.

The same things that work with newer carbureted cars (let's call them 12V cars) also work with older carbureted cars (let's them 6V cars). They include:

  • insulating the fuel lines & shielding them from thermal radiation from the exhaust system
  • adding insulation to the carburetor base
  • adding an electric fuel pump

Insulating the carburetor from intake manifold heat will help hard hot starting because it will help to keep fuel in the carburetor bowl and to minimize poor idling from over-rich fuel mixtures.  You could stack some carburetor base gaskets together but too thick of a gasket pack might not seal very well and you might need sheet metal spacers interspaced between the gaskets to add stability.  Commercially available phenolic spacers for specific carburetors are not readily available for this limited market and they would have to be fabricated.  Some people have successfully used plywood as a spacer.  Measure how much hood clearance you have with putty or play dough before starting fabrication.

As with newer cars, do not disable the intake manifold heat control system (heat riser).  A hot spot under the carburetor is necessary for good drivability even if you live in Furnace Creek during the summer.  Your car may work OK without intake manifold heat but it works better WITH heat.  Keep the intake manifold hot and the carburetor cool.

If you're going to build your own phenolic carb spacer, sheets of phenolic resin are available in a variety of thicknesses.

1/4" Thick
1/2" Thick
7/8" Thick
1" Thick

Electric fuel pumps are available for 6V applications but the selection is much smaller than for 12V applications.  Most 6 volt pumps are solenoid rather than rotary vane (like the Carter P4259) so these pumps put a smaller electrical load on old generators.

Airtex E8902
2.5-4.5 psi, 30 gph
Airtex E8011
5-8 psi, 30 gph
MACs Auto Parts 32-49430
4-6.5 psi
Carter P4259
6 psi, 72 gph

I wrote this article after encountering vapor lock issues after reinstalling the mechanical fuel pump when I did the HEI Ignition Upgrade of my 1965 Barracuda.  With this car having a slant six engine, the fuel pump is located on the opposite side of the engine as the exhaust system so there is no heat gain from a hot exhaust manifold or exhaust pipe.

I had had enough of the electric fuel pump that I installed in 1988.  It was noisy at idle and put an unnecessary 5 amp load on my [30-35 amp?] alternator, which was especially noticeable at night while stopped at a traffic light.  While the P4070 was my only fuel pump, I never experienced any vapor lock issues.

Since the Carter P4070 electric fuel pump was already in place, I decided to keep it to prevent any hot-start issues by running it only during starting.  This required keeping the start and run circuits separate and using a relay operated by the start signal.  However, with the P4070 being a rotary vane pump, there was too much of a restriction for the mechanical fuel pump, which caused fuel starvation in the carburetor.

Because of the fuel starvation problem, I removed the electric fuel pump as I didn't recall the car ever really having any hot start issues a few decades ago.  After then filling up the tank for the first time in 2018, I was dismayed that the car stalled shortly after pulling out of the gas station.  It would not restart, even after being towed home.  My first thought was that my newly rebuilt fuel pump had failed.

I did get it to restart the next day and had an uneventful 30 minute drive around town but experienced some fuel starvation while accelerating onto a highway.  However, after pulling back into the garage and idling in the garage, the car once again stalled and would not restart even after a few hours.  It looks like the mechanical fuel pump gets vapor locked during a short hot-soak and I can foresee hot-weather idling issues with the use of the mechanical fuel pump.

Switching to premium fuel (ie, ethanol-free) might minimize this issue but this seems to me to be waste of money as my compression ratio is too to take advantage of it.  The simplest solution for me for now is to re-install my electric fuel pump and my car once-again has no hot-start issues.