But It's A Dry Heat?NOTE: Articles in this series were written in an order that might make "skipping around" confusing. If you didn't start at the beginning, you might want to CLICK HERE.

Fuel temperature seems to pop up as a topic of discussion among those doing 7.3L or 6.0L fuel system modifications every few years. Most recently, a manufacturer and proponent of the "4-Corner Feed" style of fuel system has been pushing "Lower Fuel Temperatures" as a selling point for their product (check out THIS ARTICLE for a discussion about why that claim is misleading at best). The data discussed below was all collected back in 2005 and 2006, using the best option we had at the time for data collection, Dual Digital Readout temperature gauges from Spa Technique with the probes in the measured fluid streams for accuracy. While almost 20 years has passed since the data was collected, very little has changed with regard to the conclusions regarding fuel temperatures and regulated return fuel systems.

NOTE: This data was collected in a daily driven 2001 7.3L "Street Truck", and the discussion here is geared toward the same type of vehicles. There may be very valid reasons for adding fuel coolers to competition vehicles that run extremely small fuel cells, particularly metal fuel cells that are exposed to direct sunlight (rather than being hidden under the floor of the bed of the truck).

(3) Spa Technique Dual Digital Temperature Gauges (6 Digital Readings Total)
• Probe 1 - FUEL TANK - Installed in a T-Fitting to Measure Fuel Leaving the Tank
• Probe 2 - FUEL FILTER - Installed in OEM Fuel Filter Bowl
• Probe 3 - REGULATOR - Installed in the Gauge Port of the Aftermarket Fuel Pressure Regulator
• Probe 4 - FUEL RETURN - Installed in a T-Fitting to Measure Fuel Returning to the Tank
• Probe 5 - COOLANT - Installed in Water Pump to Measure Coolant Leaving the Engine
• Probe 6 - AMBIENT - Installed in Grille to Measure Outside Air Temperature while Driving

The goal was to be able to measure the temperature of the fuel at the most likely points of change in the system. First, we measured the fuel leaving the fuel tank, which would give us the "Beginning Fuel Temperature" and tell us whether the use of the Regulated Return system was increasing the fuel tank temperature over time. The next reading was at the OEM fuel filter bowl, which would tell us if the trip from the tank through the engine bay and OEM fuel filter canister was affecting the temperature of the fuel. The third reading was at the fuel pressure regulator, telling us what the temperature increase was after passing the fuel through the fuel rails that are cast into the cylinder heads. The final fuel temperature reading was in the return line, just before the fuel dumped back into the tank. For reference, we also monitored the engine coolant temperature and the outside (ambient) temperature using the same gauges and probes. The testing was performed in July, August and September in Phoenix, Arizona. The hottest test day was about 115°F Ambient, and the coolest was around 90°F Ambient, with 12 out of 15 days being over 100°F.

Data was collected manually, using a stopwatch to collect at predetermined intervals, depending on the day and the length of the drive. We have data ranging from 1 hour of drive time to 3 hours of constant drive time, over several different days in each month.

Below are some charts of the data that I collected. Both charts are the same data, it is just sorted differently. The top chart is the data in the order it was collected (first data point on the left, last data point on the right). The second chart is the same data, but sorted by drive time (lower driving time on the left, higher driving time on the right). There are several data points for each drive time as follows: (4) 1 hour, (4) 1.5 hour, (3) 2 hour, (3) 2.5 hour and (1) 3 hour. I present the data sorted both ways so that you can visually see the same patterns I do, and why I came to the conclusions that I did.



• Fuel Tank to OEM Fuel Filter Bowl : 22.4°F
• OEM Fuel Filter to Aftermarket Regulator : 22.9°F
• Aftermarket Regulator to Fuel Return : -21.2°F

In 11 of 15 data points, the fuel gained more temperature from the fuel tank to the OEM fuel filter bowl than it did from the fuel filter through the engine to the regulator. It is pretty clear that this is because the fuel is spending significantly more time in the fuel lines and OEM fuel filter bowl than it is between the fuel filter and the regulator, even though the fuel rails in the heads will be the hottest path the fuel comes in contact with. Additionally, in 9 of 15 data points, the fuel temperature dropped more from the regulator to the return port at the tank than it increased from the fuel filter to the regulator, suggesting that the amount of time the fuel spent in the OEM metal fuel lines along the frame was sufficient enough to dissipate more heat than the fuel rails added to the fuel.

NOTE : It is entirely possible that the lowest data point in the orange line in the above charts (117°F) was not recorded correctly. It stands out as being "off" (because it is drastically different than the yellow line it follows) in both charts, and if it is adjusted to even 5°F below the fuel filter average temperature, the overall averages change to 24°F/21°F/21°F. This also makes the figures in the above paragraph 12 of 15 and 10 of 15 respectively. I left the charts showing the data as recorded for the sake of integrity, but I'm certain that one point is not correct.

We also installed a fuel cooler (chart below) in the return line (after the return probe, so the only temperature reading impacted by the cooler would be the tank temperature) and collected some data to see what difference was made. The results were pretty surprising. It should be noted that the ambient temperatures during our testing of the cooler were only 90-95°F.


Fuel Tank Temperatures - 120.6°F : 117.4°F : -3.2°F
Fuel Filter Temparatures - 143.0°F : 143.4°F : 0.4°F
Regulator Temperatures - 165.9°F : 166.3°F : 0.4°F
Fuel Return Temperatures - 144.7°F : 138.0 : -6.7°F
Ambient Temperature - 101.4°F : 93.6°F : -7.8°F
Coolant Temperature - 190.1°F : 190.5°F : 0.4°F
Underhood Temperature - 161.0°F : 160.6°F : -0.4°F

What we managed to prove, in an average street truck with an OEM fuel tank, was that the installation of a fuel cooler after a regulated return has virtually no impact on the overall temperature of the fuel in the tank, and is not worth the expense. The temperatures of the fuel at the filter and regulator (as well as the coolant and underhood temperatures) were virtually unchanged and the fuel return temperature was lower by almost the same difference that the ambient temperature was lower (further proving that the OEM metal fuel lines are dissipating fuel heat on the way back to the tank).

I've looked at all of this data a lot of different ways, and a few things are ALWAYS true:

The REGULATOR (grey) fuel temperature (after the fuel has passed through the heads) tracks almost perfectly with UNDERHOOD (green) temperature. The regulator consistently averages about 5°F warmer than the underhood air temperature.

Generally speaking, EVERY TEMPERATURE (except coolant, which is managed by a thermostat) tracks very closely with the bottom AMBIENT (dk blue) temperature line. Not perfectly, but enough to be clear that ambient temperature makes a noticeable difference in fuel temperatures throughout the system, with or without a fuel cooler.

The length of the drive cycle does NOT play any significant role in the temperature of the fuel in an OEM (or similar) fuel tank in the OEM location (see my note above about competition vehicles being different). It is clear that between the fuel return line on the frame and even the fuel tank itself, the flow of ambient air while driving is more than enough to cool the components and the fuel within them.

So, does a regulated return fuel system that passes fuel through the heads and back to the tank through a bypass fuel pressure regulator increase the temperature of the fuel...of course it does! Does this translate into higher fuel tank temperatures, higher overall fuel temperatures or any sort of fuel temperature related problem...NO! Should you run a fuel cooler with a regulated return fuel system...only if you like spending money unnecessarily while adding additional possible leak points to your fuel system. Is there any merit to the fuel temperature claims made by the proponents of the "4-Corner Feed" style fuel systems...ABSOLUTELY NOT, and don't forget to check MY OTHER ARTICLE for information about what they are neglecting to tell you about fuel temperatures in their design. It is worth noting that the same issue that exists with fuel temperatures and the "4-Corner Feed" also exists with the OEM deadhead style fuel system, adding yet another benefit to the list of reasons to install a Regulated Return style fuel system instead.

Hopefully this clears up any confusion about 7.3L/6.0L Regulated Return style fuel systems and their impact on fuel temperatures.

Author: Dennis Schroeder - Co-Owner of Strictly Diesel

Dennis has been Designing, Building and Supporting Aftermarket Fuel Systems for 7.3L and 6.0L Powerstrokes since 2001.