I’ve been thinking about this for a couple of years trying to make sense of it. Here’s my theory on the issue: This condition is less likely due to a singular problem, more likely points to a combination of factors that lead to excess vapor pressure, and collectively contributes to the EVAP system not being able to keep up with and properly manage excess vapor pressure. Let me qualify that I am no expert in thermodynamics. I think the combination of some or all of these various factors impact the fuel vapor issue.
Vehicle Age: Let's be honest, even well maintained 15-20+ year vehicle platforms are not going to be in optimal mechanical condition which contributes to this problem. Further I think when you operate off road, at high altitude, in summer climate; you have to take this into account and ensure you have good mechanical service awareness of your truck and make the appropriate adjustments to mitigate this issue. Below is a compilation of my thoughts on what to be aware of, and where to look for potential issues.
Fuel: Regional blend differences, tank level, fuel quality (adds or lack there of), octane rating, seasonal blend vs summer blend, ethanol content, butane content: more on that below. Note: Fuel blends are different between many different metro markets across the US based on regional pollution requirements which might help explain why this seems more random to those that do and those that dont experience this issue.
Altitude: Higher altitude contributes to increased vapor pressure
High ambient air temps 85~90f+
Excess engine/under hood temps and radiant heat increasing fuel temperature due to:
a.) poorly maintained cooling system: (dirt and debris in radiator, poorly functioning fan clutch, non functioning radiator cap, failing thermostat, low coolant);
b.) lean condition: ( vacuum leaks, exhaust leaks, clogged cats etc)
c.) failing fuel pump: overheating fuel via cavitation, running fuel on low frequently
d.) failing or failed fuel pump resistor: unable to compensate fuel pump speed and over heating fuel
e.) failing or failed fuel pressure regulator: unable to process return fuel flow adequately
Poorly functioning EVAP System
a.) saturated charcoal canister
b.) poorly functioning switching valves/solenoids
c.) partially clogged pressure switching valve
d.) loose fitting or cracked vacuum lines
e.) partially clogged fuel cut off valve
f.) Pinched or kinked vacuum hose
Off Camber Driving at Altitude with 3/4 to full fuel tank in hot weather: my thought that running off camber at various angles, sloshing gas, warm conditions where fuel and vapors expand fuel inside tank the EVAP line gets fuel or condensation in it and cannot process vapor effectively.
FUEL (dynamics) as a key contributor:
I’ve “plagiarized “ some content from various places to articulate my theory:
Region, seasonality of fuel, climate combined with altitude play a big factor in this vapor pressure issue. This doesn't explain away occurrences in late summer but in later spring/early in summer ( June 1) especially with transition from winter blended fuel to summer blended fuels. Especially in 2020 with substantially less fuel sold due to Covid this spring, the transition from winter to summer blend at the pump may be slow due to turnover at both gas stations and in supplier inventory.
What is winter vs summer blend:
“Winter blended fuel contains higher concentrations of Butane which is highly volatile- and vaporizes at lower temperatures, in warm conditions it will cause fuel to boil at lower ambient temperatures .”
Consider FUEL VOLATILITY. “The ease with which gasoline vaporizes is called volatility. High volatility gasoline vaporizes very quickly. A low volatility gasoline vaporizes very slowly. Higher octane fuel has lower volatility, and lower octane fuel has higher volatility.”
“Gasoline is composed of a variety of ingredients that evaporate at different temperatures. The faster vaporizing components evaporate at lower temperatures then the slower vaporizing components. Established standards for gasoline volatility (ability to vaporize). “
"Gasoline with lower evaporation temperatures (more highly volatile) starts easier, warms up better and contribute less to deposits but would have more fuel losses and would be more likely to vapor lock. If the gasoline has too low a boiling temperature it may boil in fuel pumps, fuel lines or in carburetors at high operating temperatures. The vapor causes a decrease in fuel flow to the engine resulting in loss of power, rough engine operation or complete shut down of the engine”.
“Gasoline with higher evaporation temperatures (decreased volatility) wouldn’t have the losses or vapor lock problems but would start harder and not warm up as easily with increased deposits and dilution of engine oil. Exhaust emissions could also increase. If the gasoline doesn’t vaporize sufficiently it will be hard to start a cold engine, cause poor warm up and acceleration, and unequal fuel distribution to the cylinders. To minimize the effects of these driving conditions gasoline specifications change from season to season and take into account geographic considerations. To minimize the effects of volatility on the formation of smog there are further limitations during the summer months.”
“A key aspect of gasoline stability is vapor pressure. This is determined by how much pressure builds up inside a sealed fuel container when the fuel is heated to 100°F. A higher vapor pressure suggests a higher concentration of low boiling point hydrocarbons that vaporize under 100°F. Pump fuels with high (12 pounds/square inch, psi) vapor pressures are used in cold weather to prevent engine starting issues due to low temperatures. Pump fuels are limited to 7.8-9psi maximum in warm weather depending on county and state (www.epa.gov). If stored in a vapor tight container the vapor pressure can be maintained for long periods of time. Fuel exposed to the atmosphere can lose light components within a couple of days. Over time as vapor pressure decreases the fuel can become stale. Stale fuel doesn’t evaporate as easily and can cause rough engine idle and hard starting. Butane is a volatile gasoline component used to tailor vapor pressure in accordance with seasonal needs. Cold weather fuel has higher concentrations of butane. Butane has a high blending octane value which helps manufacturers hit their octane targets. The main downfall of butane is that it boils at 32°F. If the fuel tank is vented to atmosphere the butane can start to evaporate out unless the daily temperatures are below freezing. This makes cold weather fuel more susceptible to vapor pressure loss and octane decreases.”
“The higher the temperature, the higher the altitude, the more likely it is that Gasoline vapors build, This tendency is higher in hotter climate and in higher altitude. Ethanol blending affects the vapor pressure. Adding 10% of Ethanol to Gasoline – as is currently done in the USA - drastically reduces the V/L ratio temperature. Therefore a critical vapor liquid ratio, which can cause vapor lock, will be reached at lower temperatures with Gasoline-Ethanol blends than with regular Gasoline “
“Winter specification fuel will not perform well during the summer months and visa verse it is important to ensure adequate inventory turnover to avoid non-compliant situations as the seasonal requirements change.”
Take what you will from my thoughts on this- or add to it. I believe that a properly maintained engine and properly functioning EVAP system should be able to handle the majority of user conditions for city and highway general offroad use driving from sea level to 10,000+ feet anywhere in the country. We know Ethanol attracts moisture, one theory I have is that as fuel in tank heats up and expands, warm moist vapor and condensation propagates and collects at the top of the tank where the cut off valve is and where the outbound evap line from tank to charcoal canister runs- I think condensation and or sloshing fuel when running trail gets into this line making it difficult for vapor to pass, and what does pass moisture saturates the charcoal in the canister making it difficult for the canister process vapor and keep up.. I have experienced some of these issues at 1,000 feet and 12,000. I noticed as I vented the gas cap very warm and moist vapor came out of the tank- for more than a minute. Since then I’ve made substantial improvements/updates to my cooling system, fuel system and engine so I will report back after my high country trip next month to say if any of it helped and if my theory holds any water.
If you want to test your EVAP system there is a very detailed procedure listed in the Emission Control section of the FSM. (attached)
