|Trouble Code||Fault Location||Probable Cause|
|P0094||Fuel system leak -small leak detected||Wiring, fuel pressure sensor, mechanical fault|
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What Does Code P0094 Mean?
OBD II fault code P0094 is defined as “Fuel System Leak Detected Small Leak”, and is set when the PCM (Powertrain Control Module) detects an unexpected decrease in the fuel pressure in the high-pressure part of the fuel injection system. Note that code P0094 is most commonly encountered in diesel injection systems.
Diesel engines require very high fuel pressures to work efficiently, and any drop in the fuel pressure will set the code, and illuminate the CHECK ENGINE light. In many cases, this code will cause the PCM to command a fail-safe or limp mode, which mode reduces power, and may prevent gear shifts. In some cases though, the engine may be shut down completely.
The image below shows a leaking diesel injector. This type of leak is typical of code P0094, although there are many other possible sites on a diesel engine at which fuel may leak.
What are the common causes of code P0094 ?
Common causes of this code include the following-
- Burnt, damaged, shorted, or otherwise damaged wiring and/or connectors.
Ruptured fuel lines; two common causes of steel fuel lines failing are rubbing, or chafing against engine components, and weaknesses induced by vibration in lines that are not properly secured.
Split or otherwise damaged fuel rails.
- Component failure in high pressure pumps.
- Loose connections in high pressure fuel lines.
Damage to return lines. Although the return pressure is only a fraction of the working pressure, the volume of fuel delivered by the pump is too small to compensate for the volume of fuel lost through a leak.
Defective fuel pressure sensor or pressure regulator; however, unless the fuel pressure sensor and/or fuel pressure regulator is actually leaking fuel, these components have their own dedicated fault codes meaning that a failure of one, or both of these components is unlikely to set code P0094.
A defective PCM may cause the code to be stored, but this is a rare event. Therefore, the code must be investigated thoroughly before any controller is replaced.
NOTE: Intermittent leaks in high pressure diesel injection systems are rare. One a leak path has opened, the high pressure in the system will keep it open, which will continually set the code until the problem has been resolved.
What are the symptoms of code P0094 ?
The symptoms of code P0094 can appear suddenly, and may be dramatic in some cases. Common symptoms may include the following-
- Sudden loss of power as the engine enters limp mode
- Inability to accelerate, and the transmission may refuse to shift
- The engine may shut off unexpectedly, with the accompanying loss of power steering and vacuum assistance for the brakes
- Smoke may be visible from under the hood
- Depending on the site of the leak, engine fires may result
- The strong smell of diesel fuel may be present
- Depending on the size and site of the leak, the engine may not start.
How do you troubleshoot code P0094 ?
WARNING: While gasoline engines generally work with fuel pressures of well under 100 psi, the fuel pressure on diesel engines could be anywhere from 4 000 psi to as high as 30 000 psi. Therefore, extreme care must be exercised when working on diesel injection systems, since serious personal injury or even death can result from following incorrect testing procedures.
Diagnosing and repairing diesel fuel systems requires proper training and more than a working knowledge of the system- thus; do NOT attempt this repair if you are not knowledgeable, or feel uncomfortable working on high pressure fuel injection systems.
NOTE #1: A repair manual for the system being worked on, as well as a dedicated diesel fuel pressure gauge is required to diagnose and test any diesel fuel injection system.
NOTE #2: Although defective fuel pressure regulators can cause a drop in fuel pressure (including intermittent drops) this type of issue has its own dedicated trouble code(s), such as P1280 and others that may be make and model specific.
NOTE: Make sure the engine is cold and that there is no risk of fire before starting the diagnostic procedure. Depending on the site of the leak, fuel may have sprayed all over the engine compartment, and there may be pooled fuel present on the engine. To eliminate the risk of fire, be sure to remove all visible liquid fuel before starting the diagnostic/repair procedure.
Record all fault codes present, as well as all available freeze frame data, but only after the vehicle has been made safe to work on.
If the site of the leak is not immediately apparent, start the diagnostic process by thoroughly inspecting all wiring associated with the fuel injection system, since burnt, shorted, or damaged wiring and connectors can also set code P0094. Repair all wiring issues found, clear the code(s), and test the vehicle to see if the code returns.
If no obvious wiring issues are found, perform resistance, reference voltage, and ground checks on all wiring in the system. Pay particular attention to the reference voltage on the fuel pressure sensor and/or regulator; on most applications, the reference voltage will be 5 volts, but be sure to consult the manual to confirm this value.
Make the repairs required to ensure that all input voltages, reference voltages, and resistance/ continuity values fall within the manufacturer’s specification. Note that if damaged wiring is found, the better option is always to replace the entire (relevant) harness, as opposed to making repairs that could cause continuity/resistance issues later on.
NOTE: Be sure to disconnect the control system from the PCM before starting continuity checks to prevent damage to the controller. Consult the manual to determine the location, function, routing, and color-coding of each wire in the control system to prevent accidental short circuits.
If all electrical values fall within the manufacturer’s specifications, clear all codes and rescan the system to see if the fault returns. However, this may require that the engine be cranked, which offers an excellent opportunity to try and spot the site of the leak.
If the leak is not spotted immediately, enlist the help of an assistant to crank the engine. Likely sites of leaks are failed/cracked injectors (rare, but not impossible) loose connections in high pressure fuel lines, split fuel rails, and failed seals on regulators and pressure sensors. Note that while leaks on pressure pumps are relatively rare, they do happen, so check the pump for outward signs of leaks as well.
TIP: Since many engine compartments are so crowded, it is sometimes almost impossible to spot leaks in fuel systems. A great tool to have is a small mirror with a moveable head mounted on a telescopic handle. The smaller the mirror, the easier it is too get it behind fixed fuel lines to inspect parts of fuel lines that cannot be inspected in any other way.
If the leak is found, make suitable repairs, but resist the temptation to repair high pressure fuel lines; the better option is always to replace failed and/or suspect lines with OEM parts. Also be sure to tighten all connections in fuel lines properly to prevent a recurrence of the problem.
Note that high pressure fuel lines are secured to the engine for a reason; vibrations of the engine can cause steel fuel lines to weaken at certain spots, so make sure that all retaining devices are in place, and that all securing bolts/screws are in place and tightened properly to prevent vibration- induced leaks in fuel lines.
NOTE: Small leaks in diesel injection systems will rarely squirt a jet of liquid fuel; the extremely high pressures involved will more likely expel the fuel in a fine mist that might look like smoke. If what appears to be smoke is found, hold a sheet of paper against it, and move it toward the origin of the mist/smoke to narrow down the search field until the exact site of the leak is identified. The paper will get progressively wetter with fuel the closer to the site of the leak it gets.
After repairs had been made, clear all codes and retest the system to see if the code returns.
If the wiring checks out OK, and no leaks are found on the fuel lines but the code has returned, it is entirely possible that the pressure pump is leaking fuel into the engine. However, internal fuel leaks are unlikely to be visible. Since the pressure pump is lubricated by engine oil, the leaking fuel is likely to mix with the pump’s lubricating oil, which effectively renders the leak invisible.
This is a relatively rare event but it does happen: if it established that there are no leaks on the visible parts of the high pressure system, the pump is the only remaining source of the leak. If this is suspected, the best option is to refer the vehicle to a diesel injection specialist. This type of repair requires specialist knowledge, training, and equipment. Do NOT attempt DIY repairs on a high pressure diesel pump.
Once all repairs had been made, clear all codes and consult the manual on the correct procedure to test the fuel pressure in the system with the engine running. However, take extreme care to follow the directions in the manual exactly- serious personal injury can result from mistakes during this test.
Compare the test result with the values stated in the manual, which if all repairs had been successful, should match the stated values exactly, or be very close to stated values.
Codes Related to P0094
P0093 – Relates to “Fuel System Leak Detected Large Leak”