PTFE flexible fuel line

[sab]

Many years ago, I was on a trail somewhere above 11,000ft. Out of nowhere, I started experiencing what seemed like the vapor lock we occasionally get on a hot restart on a hot day. The engine was stumbling. I kept going and noticed that the stumbling progressively disappeared as the elevation decreased until everything was normal again.

Sometime later, I discovered that the tailpipe had been shoved into the gas tank skid with some obvious signs that the plastic tank had some heat deformation.

Thinking back to the recent odd high elevation engine stumbling, I wondered if the hot exhaust had been warming the fuel enough that it would boil at the fuel rail at high elevation where vapor pressure is lower. I fixed the tailpipe and that specific problem never occurred again.

That's a more complicated situation to unwrap than it seems. The pressure that the fuel sees in the pressurized system is not the pressure you'd measure on a typical gauge. In the engineering world, we talk about psia and psig, not psi. Those terms stand for:

• PSIA = Pounds per Square Inch, Absolute
• PSIG = Pounds per Square Inch, Gauge

The psi that we're all familiar with is almost always psig. The difference between the two is the the local atmospheric pressure (uncorrected for altitude - that's another topic altogether.) So, in your case, you are considering altitude effects on the boiling point in the fuel rail. As most know, "normal" atmospheric pressure at sea level is about 14.7 psi. That's one of the odd cases were psi is not psig; rather, it's psia. At 11,000', the atmospheric pressure is about 9.9 psia. So, in the fuel rail, the difference in pressure, which is affecting the boiling point, is the difference in atmospheric pressure. In the 2003 TJ, fuel pressure target is 49.2 psi. At sea level, the absolute pressure in the fuel rail is 49.2 psia +14.7 psia = 63.9 psia. At 11,000', the absolute pressure in the fuel rail is 49.2 psia + 9.9 psia = 59.1 psia. So, the drop in absolute pressure is 63.9 psia - 59.1 psia = 4.8 psia, which happens to be the difference in the atmospheric pressure (14.7 psia - 9.9 psia).

I don't know how much the boiling point of gasoline changes with about a 5 psia drop in pressure. I couldn't find any information on that.

But here's the really big question: Does the fuel pressure regulator regulate to 49.2 psig or psia? I have no idea, having never seen how one works. If the later, than this is a moot argument because there was no pressure difference at altitude in terms of psia.

Also, I have no experience with the "hot restart on a hot day" problem. If it's from heat soak in the fuel rail, I would expect that as soon as you start pumping fuel again by running the engine, the fuel temp would drop because you're replacing the heated fuel with cooler fuel from the tank. Wouldn't that mean that as you drove down the mountain, in addition to losing altitude, you'd be lowering the fuel temperature. So what looks like a causal relationship isn't?

 

[jjvw]

That's a more complicated situation to unwrap than it seems. The pressure that the fuel sees in the pressurized system is not the pressure you'd measure on a typical gauge. In the engineering world, we talk about psia and psig, not psi. Those terms stand for:

• PSIA = Pounds per Square Inch, Absolute
• PSIG = Pounds per Square Inch, Gauge

The psi that we're all familiar with is almost always psig. The difference between the two is the the local atmospheric pressure (uncorrected for altitude - that's another topic altogether.) So, in your case, you are considering altitude effects on the boiling point in the fuel rail. As most know, "normal" atmospheric pressure at sea level is about 14.7 psi. That's one of the odd cases were psi is not psig; rather, it's psia. At 11,000', the atmospheric pressure is about 9.9 psia. So, in the fuel rail, the difference in pressure, which is affecting the boiling point, is the difference in atmospheric pressure. In the 2003 TJ, fuel pressure target is 49.2 psi. At sea level, the absolute pressure in the fuel rail is 49.2 psia +14.7 psia = 63.9 psia. At 11,000', the absolute pressure in the fuel rail is 49.2 psia + 9.9 psia = 59.1 psia. So, the drop in absolute pressure is 63.9 psia - 59.1 psia = 4.8 psia, which happens to be the difference in the atmospheric pressure (14.7 psia - 9.9 psia).

I don't know how much the boiling point of gasoline changes with about a 5 psia drop in pressure. I couldn't find any information on that.

But here's the really big question: Does the fuel pressure regulator regulate to 49.2 psig or psia? I have no idea, having never seen how one works. If the later, than this is a moot argument because there was no pressure difference at altitude in terms of psia.

Also, I have no experience with the "hot restart on a hot day" problem. If it's from heat soak in the fuel rail, I would expect that as soon as you start pumping fuel again by running the engine, the fuel temp would drop because you're replacing the heated fuel with cooler fuel from the tank. Wouldn't that mean that as you drove down the mountain, in addition to losing altitude, you'd be lowering the fuel temperature. So what looks like a causal relationship isn't?

The stumbling hot restart on a hot day is quite common. And it does quickly go away as cooler fuel is introduced into the fuel rail. Mine has largely gone away as more engine bay venting has been added over the years.

All I know about my moment with a stumbling engine at high altitude is that it never happened before or since. It occurred while driving and it disapated as the altitude decreased. And the behavior was the same as a hot restart on a hot day. And the only meaningful thing I came up with was the tailpipe heating the gas tank.

Fair or not as it relates to adding a return fuel line, my takeaway is that we should not be heating the fuel in the fuel tank.

 

[Steel City 06]

Interestingly, if you look at HP Tuners for our ECMs, our ECMs have compensation charts for fuel injector mass versus pulse width for fuel line pressure loss as a function of flow rate and for predicted fuel temperature.

Theoretically if we change either of those characteristics (either by up sizing the fuel line to reduce pressure drop as a function of flow rate, or by adding a return system to change fuel temperature) the ECM would not be able to estimate the fuel mass delivered in a particular injector cycle quite as accurately.

Granted, we can change those compensation charts, but getting the data to do that accurately would be a rather time-consuming task.

 

[macleanflood]

Is this just something to learn for fun, or is there a problem to solve with a hypothesis to prove?

Yes. =)

Granted, we can change those compensation charts, but getting the data to do that accurately would be a rather time-consuming task.

Another argument for replacing the PCM. Haltech would let you run a flex fuel sensor too, letting you adjust fuel tables based on alcohol content.

Haltech would also allow data logging so we could scientifically approach the topic and decide if it even needs a solution.

-Mac

 

[mrblaine]

The stumbling hot restart on a hot day is quite common. And it does quickly go away as cooler fuel is introduced into the fuel rail. Mine has largely gone away as more engine bay venting has been added over the years.

All I know about my moment with a stumbling engine at high altitude is that it never happened before or since. It occurred while driving and it disapated as the altitude decreased. And the behavior was the same as a hot restart on a hot day. And the only meaningful thing I came up with was the tailpipe heating the gas tank.

Fair or not as it relates to adding a return fuel line, my takeaway is that we should not be heating the fuel in the fuel tank.

We get a lot of brake issues from local folks wheeling in Big Bear. Typically they stem from wheeling when it is hot and their brake pedal goes to the floor when they least need it to, usually nose down trying not to crash and burn. I've had all of them do a fluid flush with a higher wet boiling point brake fluid and the problem never returns. My theory is they are overdue for a fluid change and it has absorbed too much water out of the air. Heat and elevation have a detrimental effect and they get a very temporary vapor lock that goes away as soon as they get anything cooler and bring the pressure up by pumping the brakes to get pedal back.

 

[jjvw]

Yes. =)
...
-Mac

Which means you are just screwing around looking for stuff to mess with.

 

[macleanflood]

Which means you are just screwing around looking for stuff to mess with.

Yes! =)

-Mac