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What is Vapor Lock?

Owners of older cars can add newer low-pressure electric fuel pumps near their tanks to help prevent vapor lock.
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  • Written By: Katharine Swan
  • Edited By: O. Wallace
  • Last Modified Date: 14 August 2014
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Although vapor lock is not often seen in modern cars, it was a common problem with the carbureted cars of past decades. This problem causes a car to stop running when the fuel lines overheat; cars sitting on the side of the road with their hoods raised used to be a common sight, particularly on hot days.

Vapor locking primarily happens in cars with carbureted engines, but since electronic fuel injection replaced carburetors in the 1980s, most car owners don’t have to deal with this issue anymore. Carburetors and electronic fuel injection are two different methods of delivering the proper amounts of fuel to the engine. Fuel injection is more advanced, and requires a computer to tell the injectors how much gasoline to squirt into the engine. A carburetor, on the other hand, is a mechanical device that uses the engine’s natural vacuum, allowing specific amounts of fuel to be sucked into the combustion chambers. The amount of fuel that is delivered to the engine can be changed using simple mechanical adjustments on the carburetor.

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Most carbureted engines have a mechanical in-line fuel pump, which means that the simple mechanical pump is placed on the fuel line, usually next to the engine. The fuel pump’s closeness to the engine means that the engine’s heat causes the fuel in the line to become very hot. When the fuel becomes too hot, it turns into a vapor, just as water begins to turn to steam when it boils. This process is hastened by the vacuum created in the line as the fuel is sucked into the engine.

When the fuel turns into vapor, the mechanical fuel pump can no longer move it along the lines. As a result, some or all of the fuel stops getting into the combustion chambers, and the car either begins to run very roughly or dies completely. If the driver attempts to restart the car, it will probably not start, or will continue having problems.

Vapor lock does not usually happen in fuel-injected engines for several reasons. First of all, most electric fuel pumps are located at or in the fuel tank, which is usually located too far away from the engine to be affected by its heat, so the gasoline is not likely to turn to vapor there. As a result, the fuel pump can push the fuel along without any problems.

Another reason why this problem does not often happen in fuel-injected engines is because the fuel lines are usually pressurized. The high pressure that the fuel is under prevents it from turning into vapor quite as easily, unlike the carbureted system that actually produces negative pressure, also known as vacuum, in the lines.

More efficient engine cooling systems also contribute to the decreased likelihood of vapor lock occurring in modern cars. The cooling fans in older cars usually ran off of the momentum of the engine, which meant that when cars were at a disadvantage when they sat idling in traffic for long periods of time. The lack of motion meant less air flowing through the engine compartment, and the fan — stuck at the idling speed of the engine — was unable to cool the engine sufficiently. As a result, the engines in older cars tend to run hotter in heavy traffic.

Modern cars, on the other hand, typically have electric cooling fans. These fans are linked to a sensor that detects the engine’s temperature, and tells the fan whether to speed up or slow down. As a result, the internal combustion engines in modern cars are less likely to overheat than those in older models.

All of the problems that cause vapor lock are easily corrected, even on older cars. Cars can often be retrofitted with the newer technology that reduces the likelihood of the fuel lines overheating. Even on a carbureted engine, for example, a low-pressure electric fuel pump can be installed near the fuel tank, which will keep the fuel moving along the lines even if it starts to heat up in the engine compartment.

Another easy remedy in older cars is to install an electric fan in the engine compartment. Many auto parts stores carry aftermarket cooling fans, which can be installed on any car. Some cooling fans can be connected to the existing engine temperature sensor, mimicking the behavior of the cooling fans in modern cars.

Vapor lock can also be prevented in any car by protecting the fuel lines from the heat of the engine. This can be done by installing heat shields between the engine and the fuel lines, which will divert most of the heat away from the lines. Fuel lines can also be insulated to prevent the fuel from getting to the temperatures at it will vaporize.

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anon359936
Post 40

My hot rod Corvette has low gasket pressure in the electric fuel pump. It has a 350 small block carb engine and smells like it's getting too much fuel. Sometimes the car runs for 30 minutes or so then starts to die, like a vapor lock or flooding the carb. I opened the hood and the inline glass fuel filter might be empty or full of fuel, but the car has trouble starting when this happens.

Also, after running and shutting off all by its self like it has no fuel. Then after 30 minutes or so, I'll try to start it and it's fine until it's been running 30 minutes or longer and then it happens again.

Does it need a pressure regulator or a heat shield to protect the fuel line or does the carb needle value need to be changed or reset to keep the engine from flooding? Please answer as soon as possible.

anon337115
Post 39

Regarding ignition modules and other electronic components: many, many times as electronic components heat up, if they have a flaw or have grown old they will misbehave. If you drive a car a short distance and you park it, the module or other electronic components get heat soaked and then they don't work if you come back out to the car in 15 or 20 minutes.

I had an Oldsmobile and it developed a very tiny engine miss, but it drove fine most of the time, but don't mess with something if it's not broken. One day I drove 15 minutes up to the store, came out 20 minutes later and could not get the car to start to save my soul. Long story short: in this situation the heat from the engine basically heat soaked that module in the 15 or 20 minutes I was in the store. The expansion caused on the electronic components affected their micro circuitry. If I had I stayed in the store probably 40 minutes, the module in the engine would have been cooling down and it would function well enough to start the car as it always did.

So, on modern cars, something you always have to think about is whether the electronic components have heated up. They might heat up enough to cause trouble as you're going down the highway, or they might only overheat after the vehicle has been parked and there's no air flowing and cooling happening. So, on modern vehicles with all of the new microelectronics and micro circuitry, you have to consider if a component had an opportunity to overheat. It might happen going down the highway, or it might only happen after you've been parked for a short while.

I had a computer that did the same thing to me. I looked for about six malfunctions for me to figure it out, but the motherboard probably had some kind of a hairline fracture and circuitry but it only showed up after about 30 minutes of use with that motherboard when the heat expanded the micro circuitry enough to affect electronics circuits.

The moral of the story is to buy older cars without computers. I bought a 62 Ford Thunderbird last year. I told my friends when I went looking to buy an older car I wanted three things: window vents, floor vents and no damn computer!

anon337099
Post 38

So far, it has only happened once, but I shut off the engine and when I went back three minutes later, it failed to start. The engine cranked like it wanted to start, but would not fire. After several tries, for some unknown reason, I took the gas cap off then replaced it and the engine started and has run flawlessly ever since.

Can anyone give me some guidance as to why this happened by taking off the gas cap? I have a 2003 ford ranger edge 4.0 engine with 64,000 original miles.

anon332534
Post 37

My 1979 Lincoln Mark V won't start after sitting for a while, 15 20 minutes or so. It acts like it's out of gas. Any suggestions? Oceanside, California.

anon278314
Post 36

I have a 1990 Ford Escort LX, and my boyfriend needed to go to the airport. We drove 37 miles (in North Carolina, in the summer) towards the airport and the car subtly jerked a couple times, then refused to continue to accelerate and slowly coasted to a stop. Once I had managed to pull it to the side of the highway, the car shut off but the lights were still on. So I put it in park.

We got this car in January, and it has worked perfectly up until now (July). We have tried figuring out what is wrong with it, and no one seemed to know. They said the transmission was going, the alternator was acting up, or it was just old -- until I stumbled onto this site. Now, after reading this and realizing our car has many similar symptoms, I think I'm going to go to a mechanic and say "it's vapor locking, fix it right this time."

anon255349
Post 35

I have a 2000 Windstar with a vapor lock issue. It has happened only a few times and usually on unusually warm days in early spring or after an extended cool spell then a sudden warm up in the summer. If I let it sit a while, it starts and runs fine. It happened again yesterday and when replacing a suspected fuel filter had excess vapor blow off for quite awhile. What can I do to alleviate this problem?

anon231758
Post 34

They used to make a device called a cool can. You filled it with ice and the fuel runs through a woven coil of metal lines inside the can proximal to the ice cooling the fuel. The result? better fuel atomizing in the combustion chamber, better fuel economy etc., etc. Problem solved.

anon231012
Post 33

I have a 1999 silverado 5.3 LS. Sometimes I go to to town, shut it off, come out, and it won't start. If I let it sit one minute, it will start and run great.

anon211633
Post 32

I have a 1990 cadillac deville stalls at intersections, dies on me. It restarts hours later what could it be? I need it to run.

anon201233
Post 31

I have a 95 Nissan Quest. In hot weather after running a while it will quit. If I undo the fuel line under the hood hot fuel and "fuel steam" (vapor) will blow out. I cycle the pump (turn the key on then off) - or momentarily "bump" the starter, until I get a flow of fuel, then I put the line back on and she starts right up. If the conditions persist it will stop again in 15 minutes or so. It is more likely to happen when the tank is below a third of a tank. I found that the whole tank gets hot so the pump starts trying to pull vapor. Because the head pressure still in the line the pump can't clear the vapor. Others talk about sitting for 20 minutes. This lets the line pressure leak down until the pump can push the bubble out get going again.

Since I haven't solved this yet, my guesses are: 1) I many need to replace the after market pump with a OEM quality, 2) There may not be enough air flow through the engine compartment so the fuel gets hot and returns to the tank. Once, while traveling on the interstate in 100 degree weather, it began regularly quitting on me, I popped the hood and left it up, on the safety latch, and I had no more trouble. 3) Insure the radiator and AC coils are not choked with bugs so air is coming through (the car never over heats) but I notice the A/C condenser coil has a high fin count and looks like it's flow restrictive 4) Insulate the lines under the hood. 5) I notice that when I open the fuel cap I never get a pressure release which most cars have. That may indicate a problem in the vent lines or cap.

One last thought: Other things can make a car quit when hot, like a flaky ignition module, so just because it quit and restated in 20 minutes doesn't mean you have vapor lock. Pete, Tennessee

anon162281
Post 30

i have a 98 ford Taurus doing exactly that, but i don't know why. can anyone please explain what's going on?

anon153026
Post 29

We just experienced this on a 2008 Mercury Mariner. We thought it was a bad fuel pump, we replaced it, still had the problem. The vehicle finally through a code so we got a code reader and it said bad O2 sensors, so we replaced them, still no change. As a last resort we replaced the fuel pressure regulator, no change. It would start up for about 5 seconds then die. I was calling a friend to get a trailer to tow it to the dealership and he mentioned taking the gas cap off and starting it in case it was vapor lock, took a few seconds to adjust, but after that it purred like a kitten.

anon150812
Post 28

This happened on my 1983 dodge ram. Ran fine but when I shut it off after it warmed up starting it was a pain. I replaced the whole ignition system, fuel pump and lines to no avail. Then I noticed my engine runs a mark below half on the temp gauge normally. But when I shut it off for more than 15 minutes and start it the gauge read far past half. It was heat soaking while turned off since the belt driven fan wasn't on. Simple fix in the end. I insulated my mechanical fuel pump and the line going to the carburetor. Then I installed a 1/2 inch fiberglass spacer gasket between the carb and manifold. Never had the problem again.

anon111771
Post 27

@anon50100: You are absolutely right. Newer car owners, stop thinking with your head up your butt. Most crank/no-start conditions on newer cars are due to crank position sensor failure or ignition module failure. Look there and not to the near zero possibility that you are vapor locking on a 1980 or newer automobile.

anon106733
Post 26

For a quick fix for a car stalled by vapor locking, I can recall my dad having a couple of grapefruit in the car. When the car stalled due to vapor locking he would cut a grapefruit in half and hold it on the carburetor. This always got him going again more quickly.

anon106732
Post 25

Vapor locking on older cars such as our model a Fords are notorious for vapor blocking with the present gasoline containing ethanol. If we used regular gasoline (without ethanol) the cars would run like they should. Model a Fords were originally designed to use gasoline containing ethanol.

Henry Ford realized that the farmers who grew corn were his biggest customers. The cars were also may be used unleaded gasoline and will run best on the cheapest gasoline available. The reason being modern gasoline has many additives that can create problems.

The solution for many model A. owners to prevent vapor locking with the model a this to add a bit of diesel fuel to the tank of gas. With each fill up of 10 gallons (the capacity of a model a Ford gasoline tank) some people get by with a pint of diesel to 10 gallons of gasoline. Others use up to a gallon of diesel to 10 gallons or less of gasoline.

A little experimentation by using less and less diesel until the car runs best will determine the amount of diesel but each fellow. July and August are notoriously known as vapor lock months.

Before using diesel in your more modern car you should investigate as to whether this can harm performance or damage your fuel system.

anon104276
Post 24

I had a 62 ford falcon while living in California. Putting wooden clothes pins on her gas line stopped my problem. Now I have a 78 chevy silverado and am starting to have the same problem.

I have replaced a few parts, but am starting to think that some clothes pins are in order. It has been getting over 100 this summer in Indiana

anon101373
Post 23

I have an 85 camaro and this sounds exactly like what happens to me. My only solution is to quickly shift to neutral and rev to 3000 or so and then its fine for like hours. Must find a fuel line heat tape insulator maybe or a remote electric fuel pump away from the heat.

anon94925
Post 22

How about on boats? I experienced vapor lock with twin 500 EFI engines and it was about 93 degrees out and we couldn't get the boat to restart. I assume this is similar to cars.

anon93727
Post 21

I have a 2002 grand am that experienced vapor lock. It was 32 degree heat today so I think that was the cause. I took off the gas tank cap and left it off while starting and it started like a charm. I wonder how many people have been ripped off by mechanics due to this simple problem. It does happen on newer cars on occasion and I have proof.

anon91417
Post 20

I have an 98 Olds Intrigue and after not being able to start it earlier i had my dad come look at it for me. It got above 100 degrees today and he thinks it may have been vapor lock. How rare is it to have that occur?

anon86596
Post 19

I have a restored 1949 Oldsmobile with rebuilt 303 V8. I seem to have vapor lock at extreme hot conditions. I have an in line fuel filter just before the fuel pump. When the vapor lock occurs the fuel filter is empty. I open the hood and wait 10 to 15 minutes and after cranking awhile fuel gets back and starts right up. Why does the filter become empty if vapor lock occurs after the fuel pump?

anon85879
Post 18

Vapor lock is being caused by the bad fuel that is currently being sold everywhere. There is nothing wrong with your cars; the fuel has a very low boiling point.

If you look at the fuel bowl vent you will see vapor and gas percolating out of the vent tube when you are having this trouble. Another way to prove this is to use some type of race gas. race gas has no ethanol or alcohol. it does not start to boil after only a few minutes in hot traffic. I have been having the same trouble here in Connecticut in all my carbed cars and trucks. my buddies also are having the same fuel related trouble.

The only thing that helped at all, besides non ethanol gas is a return-style fuel regulator. this allowed fuel to be pumped back to the tank when in traffic instead of getting cooked just sitting in the hot fuel line. We need better gas now.

anon82849
Post 17

What always flustered me was the claim - and I've seen it work - that putting wooden clothespins on the fuel line prevented the vapor-lock problem. How does that work? Can it be that the wooden clothespins actually dissipate the heat?

anon76970
Post 16

I have a 1946 Willis Jeep CJ2A with the Flat Head 43hp motor. I just replaced the carb and installed a new vented gas cap. my fuel line that runs next to the exhaust is insulated. It starts ok when cold but if you shut it off even before it gets hot, it will not restart.

The new parts did not fix the problem. It acts as if it is flooded. Would low cylinder head pressure cause this, or valve problems?

It is restored to military configurations.

anon60742
Post 15

I have a 1968 AMC Ambassador SST with a 290 2bbl carburetor, ac, automatic. ps, pb. It runs great until I drive over 45 min. Then it will sputter out and will not fire for over two hours. like it is starving for fuel. So the vapor lock seems to be the problem, although it just started happening, and I have a year old fuel pump. It was a cheap fuel pump. Should I replace that first with a better one?

anon53409
Post 14

I have a 1997 Jeep Wrangler. It starts fine when cold, and when i turn it off and try to start it again within 30 minutes it runs very rough or won't start at all. The only way it will start is if i pour water on the fuel line inside the engine.

If it's not vapor lock, then what else could it be? i just replaced the in tank fuel pump three weeks ago. thanks for your thoughts,

anon50100
Post 13

To all with the 'newer' cars (made after the 1970's): Unless your fuel delivery system has been radically modified, I'd look elsewhere. Vapor lock is very, very uncommon in late model vehicles (post 1970's), and especially rare in fuel injected cars. I'd check the distributor (e.g. crank angle sensor). Also, check the ignition module for failure due to heat; and faulty intake air temp sensors (located inside of, and as a part of, the MAF sensor) and faulty cylinder head temp sensor, just to name a few. There are specific methods to pull the service engine DTC's ("check engine" diagnostic trouble codes) to narrow down the faulty part; and then specific procedures to test each part's operation. There are many, many more possibilities, but I just about guarantee that if your car is less than 25 years old, fuel vapor lock is not the problem. -- DD

anon46157
Post 12

I have a 1999 Jeep Grand Cherokee with 103,000 miles on it. Recently I have been experiencing these same symptoms! Each time the weather was very hot and humid. I let it sit for about 30 minutes and it started up with no problems. Maybe I need some kind of fan to cool the engine. Will check with my mechanic. Thanks for the info-- your site was very helpful.

russelll
Post 11

I have a 1998 Pontiac Bonneville. I have never had a problem with the car until they just put new gas lines on the car. The car starts, but I drive it for a short time and shut it off and it won't start again for an hour or more. The car turns over but won't start,like it's not getting gas. What might be causing this?

anon44103
Post 9

we have a 2000 grand am. It starts easily and runs fine,but when i accelerate and come to a stop it dies. Can someone please suggest something for us to correct the problem. thank you TX

anon43534
Post 8

i have a 1994 explorer having the syntoms

of vapor lock. it dies after an hour of heavy

traffic. it dies and after 30 minutes it works

like nothing happened. is this vapor lock? yuma arizona

anon39862
Post 7

I'm having the same problem the 1998 Ford Expedition is having but i'm driving a 1991 Nissan Quest. I just put a new fuel pump in and filter.

anon35042
Post 6

anon17212: One thing to check would be the carb needle valve. Make sure it is in good shape and shutting off the fuel completely when the bowl is full. If there is any slight leakage, you will get fuel seeping past into the carb throat after you shut the engine off from the residual line pressure from the mechanical fuel pump, thus flooding the engine for a while.

anon33202
Post 5

This happened recently on my 1999 Ford Expedition driving on a hot day. Fuel was probably near 1/4 of a tank and I had been driving in a desert like area for over an hour in cruise control. Gave the symptoms of a dead battery and no power from accelerator. After the vehicle sat for a while and cooled and added gasoline was able to restart. Could this come from bad gas/dirty fuel injectors/low fuel and a hot day even though this vehicle is fuel-injected? I never had this problem before- vehicle has 146K on it.

cack
Post 2

I have a 1998 Ford Taurus with 36,000 miles. In the last couple of months my car has stop running while I was driving, after about 5 to 10 mins, it would restart like nothing happened. I'm thinking either my fuel cap is venting or vaporlock. Did anybody have this problem?

anon17212
Post 1

I have a 69 Pontiac Firebird. After running for a substantial period of time and shutting off the engine it will not restart. I have to wait for 30 to 45 minutes before it will restart. What causes this phenomenon? How can I remedy this problem? It's been happening for years. Thanks for your time and thoughts. Alvin B. Sherron Los Angeles, CA

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