No Fuel Pressure with Engine Off (Pumps Off)
It is normal (and desired) for our fuel systems to depressurize when the pumps shut off. Depending on the installation details and conditions, the fuel pressure may drop to zero immediately, or it may hold a lower pressure initially, and then eventually bleed off. Regardless, the fuel pressure is expected to be zero with the pump(s) not running.
However, a properly functioning fuel system should immediately pressurize when the fuel pump(s) is turned on. More explicitly, you should have fuel pressure while the starter motor is turning over the motor.
Unlike your OE fuel system, our fuel systems are designed for significantly higher output because of the following:
- Our fuel system have large, flexible lines...even the slightest movement of the lines will cause pressure to drop.
- Our fuel systems often have a large, external checkvalve, which has a slower reaction time than a smaller, low capacity checkvalve.
- Our fuel system typically operates with more fuel heat, which starts to immediately cool in the fuel line, leading to the fuel pressure to drop quickly.
- Our fuel pressure regulators are designed with a very soft spring that minimizes pressure changes over large bypass flowrates. Consequently, this results in very low valve seat pressure, which is very difficult to seal at the low pressures that fuel systems operate.
What You Should Expect
- Fuel pressure should drop to zero after shutting off the engine. (immediately or slowly)
- Fuel pressure should immediately "snap" to operating pressure when the prime cycle is started.
- Fuel pressure should drop to zero after running a prime cycle, but not starting the engine.
- Fuel pressure should be at operating pressure (base pressure) the entire time the starter motor is turning over the motor.
When You Have a Problem:
- Fuel pressure slowly rises to operating pressure when initiating the prime cycle or an engine start.
- No fuel pressure while the starter motor is turning over the engine.
- The starter motor is turning over the motor, the fuel pressure is normal, but no engine start.
Solutions to Problems:
- Slow rise of fuel pressure indicates a fuel supply problem, not a regulator problem. Examples of a supply problem are failed/damaged fuel pump(s), clogged filter(s), kinked/pinched feed line, and electrical degradation. These are normally coupled with fuel supply deficiencies when the motor is under load as well.
- If no fuel pressure is available while the starter motor is turning over the engine, the first source is to ensure the fuel pump is actually commanded to be activated while the starter motor is turning. This is typically related to coordination of the tune file with the appropriate wire on the wiring harness. (used to close the fuel pump relays)\
- If there is good fuel pressure while the starter motor is turning the engine, but the engine fails to start, there are several potential causes to investigate.
- Check that the other engine systems are online to run the motor, such as ignition and injectors.
- Check the hot and cold start parameters if the vehicle has been recently converted to run on ethanol.
- Recalibration may be required for drastic seasonal changes or altitude changes, if the car was originally tuned in a different climate or altitude.
- Check torque values on any recently changed/moved induction component, such as intake, throttle body, etc.
- Fuel systems that have the regulator before the rails, such as our L1(E) and L3 systems require the engine be able to start on fuel vapor since putting the regulator before the rails. While these systems provide very clean/minimal fuel line routing, they are unable to purge the [expected] fuel vapor out of the rails for engine start. Generally speaking, these configurations are well suited for gasoline, but can be more problematic on E85. The first solution is to typically ask the tuner to get more aggressive with fueling for engine start, and if that is exhausted, consider rerouting the fuel lines to a L2 or L4 configuration where the regulator is after the fuel rails.
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