First, if you have not already read my previous discussions about Fuel Pressure Regulators (Stop Blaming The Regulator!) and Checking Fuel Pump Electrical Operation (Electron Flow = Fuel Flow), I would recommend giving those a look first. The first one covers some common misconceptions about fuel pressure regulators, and the second one covers some quick tests to make sure your pump is not suffering from a low voltage condition. The following discussion and diagnostic procedures will assume that you have already confirmed that you don't have any leaks at the regulator, and that your pump is receiving proper voltage.If you are diagnosing fuel pressure issues that appear at idle, using the liquid filled gauge in the regulator may be enough. If your issues appear or get worse under load, it is best to have some kind of fuel pressure gauge that you can see while driving the truck. This can be done with a temporarily installed "mechanics gauge with a hose", with an aftermarket in-cab electric gauge or with a pressure sensor connected to a monitor screen like an Edge CTS. If you can't see fuel pressure under load, you are going to have a hard time diagnosing the problem and confirming that it has been fixed.
Final Regulator Check - Follow if you have Low Pressure at Idle
Locate a flexible/rubber section of the RETURN line (the line coming off the bottom port of the regulator). With the engine running, slowly pinch the return hose closed (do NOT pinch S.S. Braided Teflon hose!) and watch your gauge to see if the pressure increases. This is extremely rare, but if the pressure increases when pinching the return hose, something is preventing the regulator from controlling fuel flow to the return port and the regulator needs to be taken apart and carefully inspected. This could be as simple as a piece of debris keeping the regulator cracked open, or damage to the diaphragm ball or its seat (I've only seen this one time in 19 years).
Final Regulator Check - Follow if you have Low Pressure Under Load
Just like the test above, you need to pinch a flexible/rubber section of the return line (do NOT pinch S.S. Braided Teflon hose!). The difference is, you need to pay attention to what the idle pressure goes up to and also make sure that you secure the pinching tool so it doesn't fall off while driving. You should already be familiar with what your fuel pressure is at idle and what it drops to under load. Since you have adequate pressure at idle, you only want to pinch the return hose closed enough to increase the pressure about 10psi from where you have the regulator set, this ensures that your pinched return has become the "regulator" in the system. Once you've pinched the hose and verified that the idle psi increased, secure the pinching tool and drive the truck. If you are still getting a similar amount of pressure drop after pinching the return line, the regulator is not your problem. If you are unsure, remove the hose pinching tool and adjust the regulator up by the same 10psi at idle and drive the truck again. If the pressures are the same as they were with the pinching tool, the regulator is working. In a nutshell, you would only suspect the regulator to be a problem if there is a drastic difference in results between the 2 different methods of increasing pressure (adjusting vs pinching).
Pinched Return Summary
The whole point of the above 2 tests was to eliminate the regulator as a cause of your low fuel pressure issue, once and for all. In both cases, pinching the hose moves the pressure management function from the regulator to the pinch point. If performing this test does NOT solve the problem, you know that the regulator is NOT the cause, a lack of fuel flow is. Further diagnosis will be required.
FASS and AirDog Pumps and the Pinch Test
If you are running a standalone FASS or AirDog pump (not feeding another pump), you need to take into consideration the fact that your pump has a pressure regulator. Having 2 regulators in the system can create an issue where they compete with each other for control of the overall pressure. Generally speaking, you want the FASS/AirDog pump to have its regulator set to about 10psi more than your desired pressure at the engine, so that the regulator on the engine ends up with control of final system pressure. Many of those pumps come pre-set at 55psi, so if you are trying to get 60-65psi with a Regulated Return, you will have a hard time getting there right out of the box. Each brand and model has to be looked at differently, see below for some steps to make sure that the regulator in your FASS or AirDog pump is working as needed.
EDIT (10/31/24) : Check out THIS ARTICLE for some more information about FASS and AirDog pumps that may be helpful to understanding why you may not have the pressure you desire. This article was written as a comparison between the pumps, but the flow bench data and problem resolution discussion coincide with the diagnostic steps below.
Grey Titanium Series FASS Pumps : These pumps came from FASS with a 55psi spring in the return port. We offer a Regulator Upgrade Kit for this pump, so that it will work properly with a regulated return. If you want to see if your FASS pump is causing your low pressure situation, you can repeat the hose pinch test on the Return hose from the FASS. First, adjust the engine regulator clockwise 2 full turns beyond where the gauge needle stops moving (likely around 55psi). Now, pinch the return hose on the FASS slowly until you get to 70-75psi at the engine. Last, back the engine regulator down to 65psi. Now, with the FASS return hose still pinched, drive the truck. If the pressure holds at 65psi, you need a regulator kit for your FASS. If you try the pinch test, or the updated regulator kit, and you still can't get high enough pressure from your FASS, you will need to continue with additional diagnosis as there could be an issue elsewhere in the fuel supply system, or the FASS pump itself may not be working properly.
Black Titanium Signature Series FASS Pumps : There are 2 different versions of the regulator on these pumps. The early ones still used the spring and ball. Later pumps use a spring and an aluminum poppet. In either case, these pumps should make around 70psi out of the box, so you should be able to get a regulated return dialed in to 60-65psi without modification. If you are having problems getting the fuel pressure into the 60-65psi range, or if it will not hold the set pressure under load, you can use the pinch test to see if the FASS regulator is causing the problem. First, adjust the engine regulator clockwise 2 full turns beyond where the gauge needle stops moving. Now, pinch the return hose on the FASS slowly until you get to 70-75psi at the engine. Last, back the engine regulator down to 65psi. Now drive the truck, if the pressure holds at 65psi, you need our Regulator Upgrade Kit. If you try the pinch test, or the updated regulator kit, and you still can't get high enough pressure from your FASS, you will need to continue with additional diagnosis as there could be an issue elsewhere in the fuel supply system, or the FASS pump itself may not be working properly.
AirDog II/4G Pumps : These pumps have an internal regulator that is pre-set to 55psi and needs to be adjusted. First, adjust the engine regulator clockwise 2 full turns beyond where the gauge needle stops moving. Next, loosen the jam nut and adjust the regulator on the AirDog pump until the pressure at the engine is 70-75psi. Finally, adjust the engine regulator down to the desired 60-65psi. It needs to be noted that the adjustable regulator on the AirDog II/4G pumps has internal o-rings that can develop leaks over time. When these o-rings leak, they allow fuel to flow from the pressure side of the pump to the suction side of the pump, essentially creating an internal flow loop that causes lower pressure output. If you are having trouble getting the base output pressure from the AirDog to 70-75psi, or having trouble maintaining proper pressure under load after adjusting the regulators, you may need to pull the regulator out of the AirDog pump and rebuild it with all new o-rings. If, after both rebuilding the regulator and readjusting both regulators, you are still having low fuel pressure issues (most likely just under load), you will need to continue with additional diagnosis as there could be an issue elsewhere in the fuel supply system, or the AirDog pump itself may not be working properly.
AirDog II/5G Pumps : These pumps have an integrated diaphragm style regulator that needs to be adjusted. First, adjust the engine regulator clockwise 2 full turns beyond where the gauge needle stops moving. Next, loosen the jam nut and adjust the regulator on the AirDog pump until the pressure at the engine is 75psi (if the pressure stops climbing before 75psi, the engine regulator needs more clockwise turns). Finally, once the AirDog regulator has been adjusted to 75psi, adjust the engine regulator down to the desired 60-65psi and lock down both jam nuts. If, after both rebuilding the regulator and readjusting both regulators, you are still having low fuel pressure issues (most likely just under load), you will need to continue with additional diagnosis as there could be an issue elsewhere in the fuel supply system, or the AirDog pump itself may not be working properly.
In the next article, I will get into other causes for fuel flow problems that lead to low fuel pressure.
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.