Vapor Lock

Why it happens and how to control it.

Updated October 2004

It is funny how you forget the pain once you solve the problem.  I am leaving the article below for future reference but understand that I solved my fuel problems and it was not any of what was described below.  My entire problem was my old fuel pump.

Every fuel problem I had was solved when I converted to the Weldon 2025 fuel pump

(let me say that again)

Every fuel problem I had was solved when I converted to the Weldon 2025 fuel pump

You see the design of my old pump was the problem.  Do yourself a favor and BUY A WELDON PUMP!

I do find it very ironic that after 3 years and many thousands of dollars of fighting my fuel overheating problems that I so quickly forgot that it ever happened.  I should have updated this page long ago.  The only reason I remembered is someone who was viewing this page asked me for advice on a fuel cooler.  Note:  I have removed any reference to my old fuel pump in the article below.  If you want to know email me, or better yet just buy the Weldon pump because that is what I am going to tell you anyway.....


If you run a large fuel pump and have a return type fuel system (most common for performance fuel injected cars) then you have probably experienced vapor lock from heat in your fuel system.  When I first started building my car I talked with the people at Xxxxxxxxx and Weldon and asked what they would recommend for an 800+ hp car that would be driven for extended periods of time and on a regular basis.  Both companies said that fuel heat problems would pose a problem but neither prepared me for the amount of trouble I was going to have.  Both explained that when you pump a large volume of fuel it heats up faster and causes vapor lock.  Both explained that the only way to help prevent this was to slow down the pump when the extra volume wasn't needed and to insulate the fuel lines.

Armed with this knowledge I built my fuel system.  I decided on (and am still very happy with) the xxxxxxxx  xxxxxxxxx fuel pump.  This pump has allowed me to produce 904 hp and I am confident it can take me even further.  In extended street driving I have had other issues.  Issues that I was warned about but that posed what seemed to be insurmountable obstacles and had me re-thinking my high-horsepower daily driver.  Further in this article I will explain the steps I went through to overcome the problem and let you know how each worked.  Many of the items I changed did not help at all, telling you what they were will help keep you from making the same change and wasting your time and money.

First, why does vapor lock occur?

Fuel vaporizes (turns from liquid to vapor) in a much different manor than most liquids (like water for example).  Everyone knows that water boils at 212 degrees.  Before 212 degrees water is a liquid, after 212 degrees it is a gas.  The only things that can alter these numbers are pressure (or vacuum) and the dilution of the water with other chemicals (antifreeze).  By pressurizing water you increase its boiling (vaporization) point.  Putting water in a vacuum will lower its boiling point (that is why water boils at a lower temperature in Denver). 

Fuel needs to vaporize to enter the combustion chamber and mix properly with oxygen.  To ensure fuel vaporizes several different additives are used that either increase or decrease the temperature at which fuel starts to turn to vapor.  Because of these additives fuel may start to turn to vapor at one temperature but portions will stay liquid until a higher temperature.  The vaporization point of fuel is referred to as its Reid Vapor Pressure (RVP).  Here is the definition of RVP from a race fuel web site:

The pressure of the fuel. The tendency of the gasoline to evaporate. Too high of a RVP may cause the fuel to boil or evaporate in the pump, lines or carburetor at a given temperature which can lead to "vapor lock". Too low of a RVP and the fuel will not vaporize and could cause difficulty starting the engine when cold. Most Racing Fuels have an RVP in the range of 5 to 7.

As you can tell by reading this a "winter mix" fuel from a cold climate area has a higher RVP and will turn to vapor at a lower temperature than normal pump gas.  Most race fuel has a lower RVP to help prevent vapor lock and cylinder head temperatures are typically higher in a race engine.  This also explains why it is harder to start your car on a cold morning with race gas in the tank (the fuel doesn't want to turn to vapor).  I won't get any deeper into RVP, there is a ton of other information on the web that you can find if you are interested.  What I have found are the following temperatures that some common fuels boil and cause vapor lock (all temperatures taken using my low temp gauge).

92 Octane "Winter" gas 135 degrees Fahrenheit
92 Octane pump gas 150 degrees Fahrenheit
100 Octane unleaded race gas 165 degrees Fahrenheit

Second, why does fuel get hot?

The next question is why does the fuel get hot just because a bigger pump is being used?  Before we get into that lets look at where the heat comes from in any fuel system.

Radiant heat:  On a hot day if I leave my car in the parking lot (blacktop) all day my fuel tank is often 115-120 degrees before I even start the engine.  If you had winter gas you would only have another 15 degrees before it boiled!  I would imagine winter gas in a car parked in the sun in Arizona may just boil in the tank without the car even running!  If you have ever measured track temperatures at the race track you will know what I mean.  It is not the cars driving that heat up the track, it is the sun!

Radiant heat (2):  Don't forget about the heat from your exhaust and transmission.  Run your fuel lines too close and you will increase the heat in the fuel system.

Pump heat:  Pumping fuel takes energy and causes heat.  The more work your pump does the more heat it will generate.

Compression heat:  Compressing your fuel from zero lbs of pressure to 40lbs (most fuel injected engine fuel pressure setting) will generate heat.  The process of pressurizing any gas or liquid introduces heat into the compressed media (fuel).

Engine heat (most significant):  Your engine is running at probably 180-200 degrees.  This is already over the boiling point of any of the fuel listed.  The only reason it doesn't boil in the fuel rail is it is under pressure (40 lbs typically).  As the fuel enters the fuel rail it is heated until it exits the rail.  Fuel conducts heat very well (that is how it was engineered, so it will heat quickly and vaporize as it enters the combustion chamber) so assume it reaches near the engine temperature as it exits the rail.

So your still asking why the larger fuel pump heats the fuel faster.  Take a look at the volume my fuel pump is pushing versus a typical 190lph aftermarket performance pump.  The math to convert is as follows.  1 Gallon of fuel weighs 6.1 lbs and 1 gallon of fuel = 3.79 liters.

Pump Liters Per Hour Pounds Per Hour Gallons Per Hour
Walbro 190lph 190 305 50
Xxxxxxxxx xxxxxxxxx 561 900 148
xxxxxxxxx with Fuel Controller 390 630 103

Now picture a fuel tank with 10 gallons of fuel in it.  With the Walbro pump you will cycle the same fuel through the engine 5 times per hour (50 gph / 10 gallons) or every 12 minutes.  With the Xxxxxxxxx pump you will cycle the same fuel every 14.8 times or every 4 minutes.  Since the most significant item in the heating of the fuel is the time it spends in the fuel rail you can see why the extra pump volume is significant in generating heat.  Add to this the fact that the same fuel is being compressed 3 times as often and the pump is doing 3 times the work (and introducing the associated heat) and you have a fuel heat problem.

As you can see from the table the xxxxxxxxx pump using the xxxxxx  Fuel Controller does reduce the volume but even at the reduced volume it is over twice what the other "performance" pump was pushing.  This is significant and will be explained later. 

Steps I took to solve the problem:

The first change I made was to replace the fuel tank with a fuel cell.  I will admit that curing the heat problem was not the only reason, I also wanted the fuel cell for the extra fuel capacity and the "cool factor" of having a racing fuel cell. 


---- in work - previous information below ---









Of course when they told me this I didn't believe them.  I mean I have read about all these tuner cars that generate horsepower numbers like this and they "say" they drive them regularly on the street right?  Well, I now understand that most of these tuners do not expect the cars to be driven regularly or for extended periods of time or they make modifications to get the horsepower numbers they advertise then de-tune the fuel system to be street driven.

My fuel pump (xxxxx) pumps in the neighborhood of 2 gallons/minute.  My tank holds 14 gallons.  That means the fuel cycles through the system every 7 minutes with a full tank.  The pumping process generates heat and heat is absorbed in the fuel rail on the engine.

So, how do xxxxxx and Weldon tell you to solve the problem?  There are actually a couple of ways they recommend.  Then of course there is my way! (I can't ever do something the way you are supposed to....). 

1 - Both companies offer a step down regulator that causes your pump to push less fuel through the system at low RPM situations.  Both companies say this will help but not solve the problem.  Even if you reduce the pump output by 25% you are still pushing serious fuel through the system.  I do have an Xxxxxxxxx fuel controller on order but it won't be here for a while and that is only part of the way to solve the problem.

2 - Both companies recommend running multiple smaller pumps and only turning on the second one at load conditions.  I am sure this would work but it would be expensive (another pump, filter, a backflow valve, etc.) and would take up more space that I don't have.

3 - Both companies recommend always keeping your fuel tank full or always more than 10 gallons.  This is great but with a 14 gallon tank that means I can drive between gas stations and that is about it!

So, how am I going to fix it?  Really 3 things.  One I already mentioned and is the pump controller.  The other get a little more creative!

Remember the water tank I have in my trunk to hold water for the intercooler?  Why not mount a cooler inside the tank and run the return fuel through it?  If you think about it the fuel is generating heat only at low load conditions since very little fuel is being used and large amounts are being cycled through the system.  This is the same time that the intercooler is not really doing anything.  When you floor it you generate heat in the intake track but your fuel system is no longer getting hot since you are using the fuel.  With the giant Setrab heat exchanger I have in the front grill I should never see a point where my water tanks are above ambient temperature.

What I have done is mount a transmission cooler that had 1/2" NPT fittings inside my tank and install bulkhead connections through the lid and through the trunk floor.  My fuel return will now enter through the trunk bulkhead connectors, flow through the cooler, and then return through the trunk into the fuel tank.  This alone should provide more fuel cooling than any of the other modifications I have planned!

So you are looking at the pictures and asking what the center red connector in the water tank lid is?  What I found was my water tank was generating a lot of pressure because of the sheer amount of water the pump pushes.  My water pump actually cycles 7 gallons/minute (more than the fuel pump) and although it doesn't get hot the flow of water ends up pressurizing the water tank.  This red fitting is just a fuel tank tip valve designed to vent a fuel system.  I have it vented to the outside via another bulkhead connector in the trunk.

So, what is the other plan for solving the problem?  I am having Fuel Safe build a custom 22 gallon fuel cell just for my car.  Fuel Safe already builds a 22 gallon Mustang fuel cell but it has a stock-type fuel pickup and isn't really designed to flow the amount of fuel my system needs.  This new fuel cell (pictures will be posted when I get it) will have a -12 rear sump and an oversized return.  It will be fully bladdered and have an oversized fuel trap door system to contain the fuel in the pickup area.

In the end the answer to any problem is always "spend more money".  Isn't that always the truth!